S 2 BIOLOGY NOTES-4.pdf

Biology and Health Sciences, S2
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Biology and Health Sciences, S2

Key competences at the end of S2

➢ At the end of S2, each and every one should be able to:

• Classify animals into their main groups based on external features. (Unit 1)
• Explain the concepts applied in environmental biology. (Unit 2)
• Demonstrate and explain different processes of movement of water and ions in and out
of a cell. (Unit 3)
• Analyze and interpret the process of active transport and its significance to living
organisms. (Unit 4)
• Carry out chemical tests on a variety of foods to identify the nature of food substances.
(Unit 5)
• Explain the role of enzymes in living organisms and how they are affected by temperature
and pH. (Unit 6)
• Explain the process of photosynthesis and how various environmental factors affect the
rate at which photosynthesis occurs. (Unit 7)
• Explain the process of uptake and transport of xylem sap, transpiration and translocation
and their roles in plants. (Unit 8)
• Demonstrate and explain gaseous exchange in humans and plants. (Unit 9)
• Describe the structure and function of excretory organs and suggest good practices for
healthy kidneys. (Unit 10)
• Describe types of joints and relate their structure to their functions. ( Unit 11)
• Identify symptoms of common infectious diseases namely cholera, malaria, Ebola and
HIV/AIDS. (Unit 12)
• Describe natural and artificial methods that fight against the infection. (Unit 13)
• Explain safe sex and sexual behaviours. (Unit 14)
• Apply knowledge of pregnancy prevention in sexual and reproductive decisions. (Unit 15)
• Apply knowledge of sexually transmitted diseases and HIV transmission, prevention and
treatment in sexual decision making. (Unit 16)

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Unit 1: Classification of kingdom Animalia.

Learning objectives: After studying this topic, to be able to:
❖ Identify the common features of chordates and state the characteristics of all
animals
❖ Explain the economic importance of arthropods to humans.
❖ Distinguish different groups of animals using observable features.

1.1 Introduction

Based on structural similarities and differences and the way the organisms get their nutrition,
there are 5 main groups of living organisms, called kingdoms. They don’t include virus since it
doesn’t obey some characteristics of life. The five kingdoms are: Monera, Protista (Protoctista),
Fungi, Plantae, and Animalia.

The main features for these kingdoms are:
Kingdom monera: prokaryotic and unicellular
Kingdom Protoctista: eukaryotic cell and unicellular
Kingdom fungi: eukaryotic cells, unicellular or multicellular without specialized tissues
Kingdom plantae: multicellular eukaryotic cells, autotrophic
Kingdom animalia: multicellular eukaryotic with differentiated tissues and organs,
heterotrophic.
The kingdom Animalia is composed of a number of phyla and classes:
Phylum Chordata, with Pisces, Amphibia, Reptilia, Aves and Mammalia as classes
Phylum Arthropoda, with 5 classes: Insecta, Arachnida, Crustacea, Diplopoda and Chilopoda
Other phyla are Platyhelminthes, Nematoda, Annelida, Mollusca, Coelenterata (Cnidarians),
Porifera/ sponges, Echinodermata.

Note:
✓ The phylum Arthropoda is the largest one in the animal kingdom. Organisms in this
phylum inhabit land, water and soil. They have become very successful in their environment due
to the possession of proof exoskeleton.
✓ Phylum porifera comprising sponges are the simplest animals since they lack the nervous
system.
General characteristics of animals (Kingdom Animalia)

• Animals are multicellular organisms
• Animals are heterotrophic organisms: they depend on other organisms for food
• They have eukaryotic cells.
• Cells do not have cell walls
• cells that are differentiated to form tissues and organs
• Cells do not have chloroplasts and cannot photosynthesize
• Communication is by the nervous system
• Most animals reproduce sexually with the diploid stage usually dominating the life cycle

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• Most animal are able to move their whole body from one place to another

1.2 Phylum Chordata
The name chordate refers to the notochord: a firm, flexible rod of tissue located in the dorsal
part of the body, backbone that enables to stand upright. At some stage of the development, all
chordates have a notochord, as well as a dorsal nerve cord, pharyngeal pouches and a post
anal tail.
In most vertebrates, the notochord becomes the vertebral column that surrounds the spinal cord.

1.2.1 Main characteristics of chordates
• Presence of a chord like structure called notochord which supports the body in lower
chordates while in higher chordates (vertebrates), it is present only during embryonic
stages. It is replaced by the vertebral column.
• Presence of a vertebral column which forms part of an integral skeleton. Vertebral
column is a bony structure made up of vertebrae. It protects the spinal cord.
• Presence of a nervous system with a brain connected to a hollow nerve tube or a single
tubular nerve cord
• Bilateral symmetry: the body can be divided along one plane of symmetry into two equal
halves that are mirror images of each other.
1.2.2 Classification of chordates

The phylum chordate is divided into three subphyla: VERTEBRATA, CEPHALOCHORDATA and
UROCHORDATA.
Members of the subphylum vertebrata, vertebrates, constitute more than 95 % of all
chordate species.

Organisms in the phylum Chordata are subdivided into 5 different classes:
Pisces: Pisces derives from Latin word Piscis, meaning fish. They exhibit external fertilization,
where eggs are first laid by the females then the male sheds sperms over them.

Biology and Health Sciences, S2

Amphibia: the term comes from latin word amphi, meaning dual or two. Organisms in this class
can live both on land and in water. Most adult amphibians live on land but they go back to the
water for breeding purpose. They undergo External fertilization
Reptilia: The term comes from a Latin word, reptilis, meaning crawl. Animals within this class
move by creeping or crawling. They undergo internal fertilization. The male introduces sperms
into the female body. The eggs laid thereafter are covered with a shell.
Aves: This term comes from Latin word, aves, which means bird. They carry out internal
fertilization.
Mammalia: From a Latin word mammals meaning mammal: milk secreting organ of female
mammals. They exhibit internal fertilization.

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Biology and Health Sciences, S2
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A comparative summary of the chordate classes

CLASS PISCES/
FISHES
CLASS
AMPHIBIA
(amphibians)
CLASS REPTILIA
(reptiles)
CLASS AVES
(birds)
CLASS
MAMMALIA
(mammals)
Body covered
by
scales
Soft moist skin
can be used for
gaseous
exchange
to supplement
lungs
Dry scaly skin
with horny
scales
Skin bears
feathers, legs
have
scales
Skin bears hair
with two types
of glands,
sebaceous and
sweat
Paired pectoral
and pelvic fins
Two pairs
pentadactyl
limbs
Two pairs
pentadactyl
limbs usually
present
Two pairs
pentadactyl
limbs,
front pair form
wings
Two pairs
pentadactyl
limbs
No
metamorphosis
Metamorphosis
from larva to
adult in life
cycle
No
metamorphosis
No
metamorphosi
s
No
metamorphosis
No external ear No external ear No external ear No external
ear
External ear (in
addition to
middle and
inner ear)

Respiration by
gills

Respiration by
gills (tadpoles),
lungs and skin
(adults)

Respiration by
lungs

Respiration by
lungs

Respiration by
lungs
Eggs produced,
external
fertilization
Eggs produced,
external
fertilization;
Adults must
return to water
for
reproduction
Fertilized yolk
eggs laid on land
or eggs retained
until hatching.
Eggs have a
leathery skin.
Internal
fertilization
As reptiles but
eggs in
calcacerous
shells,
internal
fertilization
Embryo
develops in
mother. Mother
has mammary
glands which
produce milk for
the newborn.
Internal
fertilization.
Ectothermic Ectothermic Ectothermic Endothermic Endothermic
e.g. Tilapia,
herring
e.g. Frogs,
toads
e.g. Snakes,
crocodiles,
tortoises
e.g. Eagle,
doves,
chickens
e.g. Human,
dogs, lions

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Adaptation techniques of some chordates to their environments
Adaptation of fish to aquatic environment

Fig.: External features of a fish

The following features enable fish to survive in water:
✓ Gills: Fish use gills to breathe under water. Most fish have to swim constantly. This
enables water to pass through gills to allow for gaseous exchange.
✓ Streamlined body: The body of the fish is well suited to its particular habitat. A
streamlined body allows water to pass easily over them, reducing friction (resistance) as
they swim.
✓ Fins and tails: They allow fish to move through water. The tail propels the fish while fins
guide their movement in water by controlling their direction and balance.
✓ Lateral lines: They allow fish to detect vibrations in water, alerting them of predators.
✓ Huge number of eggs: A large number of eggs ensures that at least some will survive as a
lot eggs laid by fish are eaten by predators while others are washed away by water
currents.

Adaptation of birds to their environment
✓ The forelimbs of birds are modified to form wings for flight. The sternum of pectoral
girdle is expanded for attachment of flight muscles.
✓ Flight birds are light in weight. Their bones are air-filled and therefore have low density
to enable easy flight.
✓ Birds lay eggs with hard calcareous shells to avoid drying out.
✓ The beaks of birds are modified for different modes of feeding:
❖ Seed eaters: have short thick conical bills for cracking seeds; e.g sparrows
❖ Flesh eating birds: have sharp curved beaks for tearing meat; e.g hawks
❖ Nectar eating birds: have long slender beaks to probe the flowers; e.g humming
birds

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❖ Insect eaters: have thin pointed beaks; e.g bee-eater
❖ Filters feeders: have serrated beaks to filter food from muddy water; e.g duck,
flamingoes.
❖ Water plant eaters: have flat beaks to strain small plants and animals from the
water; e.g ducks.
✓ Birds have different types of feet to adapt them to their different environments. These
include:
❖ Feet for grasping they are large and curved to grasp a prey tightly; e.g kingfisher
❖ Feet for scratching: they have nail-like toes to scratch the soil to search food; e.g
chickens.
❖ Swimming birds: they webbed feet used like paddles; e.g ducks
❖ Perching feet: they have a long back toe to grab and perch tightly on a tree
branch or back; e.g robin
❖ Feet for running: they have three toes to enable stability when running; e.g
ostrich

1.3 Phylum Arthropoda

An arthropod is an invertebrate animal having an exoskeleton (external skeleton), a segmented
body, and jointed appendages.
Arthropods are members of the phylum Arthropoda (from Greek árthron, "joint", and podós
"leg", which together mean "jointed leg"), and include the insects, arachnids, crustaceans, and
others.
Arthropods are characterized by their jointed limbs and cuticles, which are mainly made of chitin;
the cuticles of crustaceans are also bio mineralized with calcium carbonate. The rigid cuticle
inhibits growth, so arthropods replace it periodically by molting. The arthropod body plan
consists of repeated segments, each with a pair of appendages.
Arthropods have adapted to life on land, at sea, and in the air. As plant pollinators, nutrient
recyclers, and prey for other animals, they are essential members of the web of life.

Characteristics of arthropods
The main features of arthropods are as follows:
❖ They are segmented and bilaterally animals. The bilateral symmetry means that an animal
can be cut into two similar halves in only one way, each half being a mirror image of the
other.
❖ They have an exoskeleton made of chitin which protects internal organs against damage.
The exoskeleton does not grow, but usually shed in a process known as molting to allow
the organism to grow.
❖ They have an open circulatory system.
❖ They have a fluid filled body cavity called haemocoel, for example in earthworms
❖ Each segment has a pair of jointed appendages.
❖ Reproduction usually involves larval or immature forms

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Classification of arthropods

The Arthropoda is composed of 5 classes: Insect, Arachnida, Crustacea, Diplopoda and
Chilopoda. Their main characteristics are summarized as follows:

Table: Classes of the phylum arthropoda and their characteristics

CLASS
CRUSTACEA
(crustaceans:
hard shiny
coat,
carapace)
CLASS
INSECTA
(insects)
CLASS
CHILOPODA
(centipedes)
Few segments
÷ millipedes
CLASS
DIPLOPODA
(millipedes)
CLASS
ARACHNIDA
(arachnids)
Mainly aquatic Mainly
terrestrial
Mainly
terrestrial
Terrestrial Terrestrial
2 body parts :
cephalothorax
and abdomen
3 body parts :
head, thorax
and abdomen
2 body parts :
head and
segmented
trunk
2 body parts :
head and
segmented
trunk
2 body parts :
cephalothora
x and
abdomen
-Two pairs of
antennae ;
-Appendages
modified form
legs for
walking,
feeding,
protection and
swimming
-A pair of
compound
eyes
One pair of
long
antennae;
one pair of
large
compound
eyes ;
1 or 2 pairs of
wings on
thorax
One pair of
antennae ;
Compound or
simple eyes,
or no eyes
One pair of
antennae ;
Simple or
compound
eyes or
absence of
eyes in some
cases
-No antennae
but a pair of
pedipalps;
-8 simple
eyes;
-A pair of
poison glands
called
chelicerae
Larval forms
occurs
Life cycle
commonly
involves
metamorpho
sis with a
larval stage
No larval form No larval
form
No larval
form
Typical gas
exchange by
gills,
outgrowths of
the body wall
or limbs
No gill in adults.
Gaseous
exchange by
tracheae or
spiracles.
Gaseous
exchange by
tracheae; breathe
through spiracles.
Gaseous
exchange by
tracheae or
through
spiracles
Gaseous
exchange by
book lungs or
tracheae

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Eg: crayfish,
lobsters,
barnacles,
woodlice
Eg: Flies,
grasshoppers,
bees, earwings
Mainly
carnivorous. They
use poisonous
claws to kill prey.
Eg: Lithobius
(centipede)
Mainly
herbivorous
Eg: Iulus
(millipede)
Eg: Scorpion,
ticks, spider

The exoskeleton: The exoskeleton of insects prevents the water loss and adapts the insect to live
in many areas. It also protects them against predators. The disadvantage is that it is heavy so big
insects cannot fly for a long time. Another disadvantage is that it doesn’t allow growth so it must
be shed periodically during molting. During this time, the animal is extremely vulnerable to
predators and in the case of terrestrial arthropods, susceptible to desiccation. For these reasons,
arthropods usually remain in hiding from the time they begin to molt until their new exoskeleton
has hardened.

Fig: Arthropods
Economic importance of Arthropods
❖ Some arthropods are useful to other organisms in many ways:
-Butterflies and bees act as pollinators of flowering plants: Approximately two-thirds of all
flowering plants are pollinated by insects, and soil and leaf-mold arthropods, which include
insects, mites, myriapods,
-Bees make honey,
-Lobsters and prawns are used as food.
-Small planktonic crustaceans, such as copepods, water fleas, and krill, are a major link in the
food chain between the photosynthetic phytoplankton and the larger carnivores, such as many
fish and whales.

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-some crustaceans (pill bugs), play an important role in the formation of humus from
decomposed leaf litter and wood.

❖ However, on the other side, some arthropods are harmful to other living organisms.
Medically, arthropods are more significant as carriers of diseases such as malaria, yellow fever,
dengue fever, and elephantiasis (via mosquitos), African sleeping sickness (via tsetse flies),
typhus fever (via lice), bubonic plague (via fleas), and Rocky Mountain spotted fever and Lyme
disease (via ticks). Many diseases of domesticated animals are also transmitted by arthropods.
-Ticks transmit diseases in animals, bed bugs are parasites for humans,
-Mosquitos transmit malaria
-Tsetse flies transmit trypanosomiasis in humans and Nagana in cattle
-Aphids destroy crops such as maize, coffee and cassava in the fields while weevils destroy stored
grains.
❖ Some arthropods cause harm and injury to human beings as well. Some are poisonous it
they bit, for example spiders, wasps, centipede and crabs.

Adaptations for some Arthropods
In order to survive in their environment, arthropods have developed different modes of
adaptations. The main are the following:
➢ Have antennae for feeling
➢ Have membranous wings for flight
➢ Have proboscis for sucking fluids
➢ Have jointed legs for locomotion
➢ They lay many eggs for the survival of species
➢ Have compound eyes for seeing
➢ Have claws for grasping on objects
➢ Have halteres for balancing during flight
➢ Presence of proof exoskeleton to survive in varied environment
Other phyla belonging to the kingdom Animalia
Kingdom Animalia comprises also other lower level organisms that belong to different phyla,
namely: Platyhelminthes, Nematoda, Annelida, Mollusca, Coelenterata (Cnidarians), Porifera/
sponges, Echinodermata.

Phylum Platyhelminthes (FLATWORMS)
Main characteristics

▪ They are triploblastic (embryo organized in three layers: ectoderm, mesoderm and
endoderm)
▪ They have dorso-ventrally flattened bodies with bilateral symmetry

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▪ They are unsegmented
▪ They have a mouth but no anus
▪ They are mainly hermaphrodites
▪ They usually have larval stages
Examples of flatworms: taenia, blood flukes and planaria
Fig: A planaria and a taenia worms

Phylum Nematoda
Main characteristics
❖ They are triploblastic
❖ They are bilaterally symmetrical.
❖ They are elongated and round with pointed ends, with some cephalization at the interior.
❖ They have unsegmented body with thick elastic cuticle
❖ They have a true alimentary canal with a mouth and anus.
❖ There are two separate sexes.
❖ Some are free-living, but many are plant and animal parasites.
Examples of nematodes: ascaris, filarial, trichinella

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Phylum Annelida
Main characteristics

-They are triploblastic
-They are coelomate animals. They have a fluid-filled cavity within which the digestive system is
suspended.
-They are bilaterally symmetrical.
-They are segmented and the segments are separated from each other by septa.
-Most species have bristles (chaetae/setae) made of chitin on each segment which they use in
locomotion
-They have an alimentary canal with oral and anal opening
Examples of annelids: earthworms, leeches
Fig: Annelids

Phylum Mollusca
Main characteristics

-They have soft and unsegmented body.
-Have complete digestive tract and nervous system.
-Some have protective shell made of calcium carbonate.
-Most have muscular foot adapted for movement, attachment to surfaces and food capture
-They carry out respiration by gills.
-Have sense organs for smell and taste called tentacles on which eyes are located
-The dorsal part of the body wall is a mantle (or pallium) which secretes calcareous spicules,
plates or shells. It overlaps the body with enough spare room to form a mantle cavity.
-The anus and genitals open into the mantle cavity.
-There are two pairs of main nerve cords.
Examples of molluscs: snail, slug, octopus, squid…

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Phylum Coelenterata (Cnidarians)
Main characteristics

• They are diploblastic.
• There is some tissue differentiation and have tentacles
• There is a single body opening for digestion and excretion.
• They have a sac-like body cavity that also acts as a gut
• They have a radial symmetry.
• There are sedentary polyp forms which may be solitary or live in colonies and medusa
forms which are free swimming and solitary.
• They reproduce asexually by budding (An outgrowth (bud) that becomes detached from
the body of the parent and develops into a new individual).
• They reproduce sexually by sex organs which form when needed.

Examples of cnidarians include hydra, jellyfish

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Fig: Cnidarians

Phylum Porifera (Sponges)
Main characteristics

• Sponges have no true tissues. They are sessile. They attach themselves on rocks or dead
corals
• Adults do not show locomotion
• Body has two layers of cells separated by a jelly like layer called the mesophyl
• Sponges are filter feeders
• Sponges are porous animals: they have a simple body made of a cavity or interconnected
cavities opening to the outside through pores
• The food enters by the pores and wastes leave by the osculum.
• Sponges are asymmetrical animals or radially symmetrical animals
• Sponges are all marine. Flagellated cells on the body cavities create water currents
• Asexual reproduction takes place by budding and by regeneration
• They lack a nervous system
Examples: sponges

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Fig: Sponges

Phylum Echinodermata
Main characteristics

• The skin has calcareous exoskeleton and spines
• The body of the larva has bilateral symmetry while adult body has a five-way radial
symmetry ( pentaradiate)
• They have a water-vascular system which is a network of water-filled canals inside their
body.
• They have many small movable extension of the water vascular system called tube-feet,
which aid in movement, respiration and excretion.
• The mouth is located on the lower side (oral) while the anus is located on the upper
(aboral) side

Examples: starfish, sand dollars, sea cucumbers, sea urchins

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The scientific study of these relationships between
organisms and their environment is called ecology,
which seeks to explain the connection between
plants and animals and their surroundings and
provides information about the benefits of the
ecosystem and how we can use the earth’s
resources efficiently.
Unit 2: Introduction to environmental biology

Learning objectives: After studying this topic, to be able to:
❖ Define terms used in ecology
❖ Construct and interpret simple food chains, food webs, pyramids of biomass and
numbers
❖ Appreciate the interdependence of living organisms and the role of green plants
Organisms depend on others for survival:
❖ Examples of interdependence among living organisms:
i. Plants are the main sources of food for animals and humans.
ii. Animals, such as birds depend on plants for shelter.
iii. Plants need animals such as insects for pollination.
❖ Examples of interdependence between living things and non-living things:
i. Plants need sunlight and carbon dioxide for photosynthesis.
ii. Humans and animals need oxygen in the air for respiration.
❖ Living things and non-living things interact with one another to create a balanced
ecosystem.

Definition of basic concepts of ecology

• Ecosystem: A stable unit of nature consisting of all communities interacting with each
other and their surrounding physical environment; interacting populations of organisms
with biotic and abiotic factors. We distinguish aquatic ecosystem (marine and freshwater
ecosystems) from terrestrial ecosystems. E.g pond, grassland, desert.
• Ecology: the study of relationship between living organisms and their relationship with
the environment.
• Population: total number of organisms of the same species living together in a particular
area at a particular time.

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• Predator: an animal that hunts and eats other animals.
• Producer: All green plants which manufacture their own food by the process of
photosynthesis
• Species: Organisms that have common characteristics and that can interbreed to produce
fertile offspring.
• A community: A combination of different species of organisms living together in a
particular area.
• Habitat: An area with the right amount of food, water and shelter in which an organism
lives.
• Biosphere: The part of the earth and its atmosphere capable of supporting life. It is an
area where organisms live, including the ground and the air.
• Biodiversity: The variety of life in the world or in a particular habitat or ecosystem
• Biome: A large naturally occurring community of animals ( fauna) and plants ( flora)
occupying a major habitat
• Biomass: The total mass of organisms in a given area or volume
• Biotic factors: living components of an ecosystem. These may be plants, animals, fungi,
and any other living things
• Abiotic factors: These are non-living components of an ecosystem. They include pH,
sunlight intensity, temperature, soil, wind, water, and air.
• Edaphic factors: All non-living components of an ecosystem both chemical and physical.
They are related to abiotic factors
• Carrying capacity: The maximum size of a population that a given area can support
without straining
• Niche: The position an organism occupies in a habitat. It includes where it lives, its role
and feeding habits. For example, a garden spider is a predator that hunts for prey among
plants, while an oak tree grows to dominate a forest canopy turning sunlight into food.
• Environment: A combination of all factors that affect the life of an organism.
• Commensalism: A symbiotic relationship where one species benefits and the other is
neither harmed nor helped
• Parasitism: symbiotic relationship in which one species benefits (parasite) and the other
is harmed (host)
• Mutualism: symbiotic relationship where there is beneficial to both species

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Biospher

Biotic and abiotic factors

An ecosystem is a natural unit consisting of all the living organisms in an area functioning
together with all the non-living physical factors of the environment. For example, a large body of
fresh water could be considered as an ecosystem, and so could a small piece of dead wood. Both
contain a community of species that interact with one another and with the abiotic components
of their environment.
The environmental conditions that affect a community are temperature, light, pH, wind, salinity,
atmospheric pressure,….these non-living physical factors affect the type of organisms living in an
ecosystem and their distribution. They are called abiotic factors.
Light: The sun is the main source of light energy the plants need and use to make their own food
for the survival of other many organisms.
Temperature: Its variation in an ecosystem affects the type and distribution of organisms found
in it.
Population
Individual

Communities

landscape

Biosphere e

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Atmospheric pressure: This is the pressure exerted by the air in the atmosphere. It affects the
amount of oxygen in the air. At sea level, we find high air pressure. Different organisms will
occupy different altitudes based on their endurances.
Salinity: This refers to the degree of salt concentration in water, a factor mainly found in aquatic
habitats and determines the types of organisms to live there. Some organisms prefer salty
environments whereas others live in fresh water.
Humidity: It is the amount of water vapor in the atmosphere. It influences evaporation and
transpiration rates.
pH: This is the degree of acidity or alkalinity in a given environment. It affects the type and
distribution of organisms found in it.
Wind: Wind is the moving air in different directions and speeds. By this, it influences
environmental factors such as temperature and humidity.

2.2 Energy flow in the ecosystems

The primary source of energy is the sun. Energy enters ecosystems in form of sunlight. Part of it
is then converted into chemical energy by the green plants through the process of
photosynthesis.
Within an ecosystem, organisms can be classified as autotrophs, producers, heterotrophs,
consumers and decomposers, according to their relationships in terms of sources of food.
• Autotrophs: organisms that can make their own food though photosynthesis. They are
also known as producers. E.g green plants
• Heterotrophs: organisms that feed on plants or others organisms since they cannot
manufacture their own food
• Consumers are organisms that obtain food from other organisms because they cannot
make their own food.
• Herbivores: consumers that feed on plants directly
• Carnivores: consumers that feed on herbivores. They are grouped into first level, second
level up to the topmost level
• Primary consumers are consumers that feed directly on plants and are herbivorous or
omnivorous animals.
• Secondary consumers are consumers that that feed directly on primary consumers and
are carnivorous animals.
• Tertiary consumers are consumers that that feed directly on secondary consumers and
are carnivorous or omnivorous animals.
• Omnivorous: Organisms that can feed on both plants and animals
• Decomposers are organisms that break own the tissues of dead organisms into simpler
substances, allowing the recycling of nutrients. They can be saprophytic or detritivores.

Saprophytic: decomposers that act on dead remains of organisms in all the other levels,
e.g: fungi and bacteria break down dead plants and animals into compounds of carbon
and nitrogen which are released into the soil to be used by plants and animals for growth.

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Detritivores: Decomposers that derive their energy from non-living organic material such
as faeces and fallen leaves.

In ecology, a trophic level refers to the position an organism occupies in a food chain. Producers
occupy the first level followed by consumers on the second level, beginning from herbivores up
to the top carnivores.

Table: Organisms in different trophic levels

Trophic level Composition of organisms
Producers First trophic level • Green plants
• Photosynthetic bacteria
• Algae
• Phytoplankton (all are autotrophic)

Consumers
Second trophic level
Primary consumers
• Plant-eating organisms
• Depend on plant materials
• Include filter feeders in water,
herbivores
Third trophic level
Secondary consumers
Small carnivores like insectivores insects,
spider, tadpoles, hydra, small fishes, lizards,
frogs, toads, chameleon,…
Fourth trophic level
Tertiary consumers
Large carnivores such as lions, leopards,
cheetah, hyena, sharks, owls,…
Fifth trophic level
Quaternary consumers
Carnivorous that eats tertiary consumers.
E.g: the hawk eats owls
Decomposers • All are saprophytic or detritivores
• Most are fungi and bacteria
• They can only get food from dead materials or non-living organic
materials

2.3 Food chain

It is a linear representation of how organisms eat each other before they are eaten in return. It is
a simple linear pathway through which energy and materials are transferred from one species to
another in an ecosystem. In general, food chains show how energy and materials flow from
producers to consumers.
Examples of food chains include:
Plant caterpillar chameleon snake mongoose
Sun algae tadpole dragon fly frog turtle
Plant insect mouse owl

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2.4 Food web

A food web is a complex series of interconnected food chains. It shows feeding relationships
between various species of organisms in a given community and is composed of all possible food
chains in a given ecosystem.
Food chains tend to be overly simplistic representations of what really happens in nature. Most
organisms consume multiple species and are, in turn, consumed by multiple other species. A
food web represents these more complex interactions.

Fig: Food webs
Note: -The arrow means “eaten by”
-If any of these organisms dies, saprophytes
decompose it to obtain food nutrients.
-Humans may fall into second, third, and
fourth trophic levels of food chains or webs. They
eat producers such as grain, primary consumers
such as cows, and tertiary consumers such as
salmon.

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2.5 How Energy flows in the ecosystem

Within the ecosystem, not all energy from the producers is transferred to the consumers. Less
energy enters higher trophic levels: as we go up in the trophic levels, energy goes decreasing.
Only about 10 percent of the total energy stored in organisms at one trophic level is actually
transferred to organisms at the next trophic level. The rest of the energy is used for metabolic
processes or lost to the environment as heat.
Several factors account for the loss of energy as one moves from one trophic level to the next.
They include:
*Respiration
*Part of the energy is lost as undigested food matter
*The rest is lost as excretory products
✓ These above factors explain why there are rarely more than four or five trophic levels.
Thus, an ecosystem cannot survive without the constant input of energy from the sun.
2.6 Ecological pyramids

Ecological pyramids are diagrams that show how important factors in an ecosystem such as
energy, biomass and population size change at each trophic level.
Within these pyramids, producers are at the bottom, whereas the highest trophic levels are
placed at the top. The size of the portion of the diagram associated with each trophic level shows
the amount of factor in consideration.
Three types of pyramids are used in ecology:
❖ Pyramids of numbers, based on counting the number of organisms at each
trophic level;
❖ Pyramids of biomass, which note the weight (usually dry weight) of organisms at
each trophic level;
❖ Pyramids of energy, which monitor the energy content of the organisms at each
trophic level.

Energy pyramids are considered the most important since they deal directly with the
fundamentals of food chains, the flow of energy.
The amount of energy at different trophic levels can be represented by an energy pyramid like
the one below.

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2.6.1 Pyramid of biomass

One important ecological consequence of decreasing energy transfers through a food web can be
represented in a biomass pyramid, in which each tier represents the standing crop biomass (the
total dry weight of all organisms) in a trophic level. Biomass pyramids generally narrow sharply
from producers at the base to top-level carnivores at the apex because energy transfers between
trophic levels are so inefficient.
It is always hard difficult to construct a pyramid of biomass because of the following reasons:
✓ Measuring biomass often means death of the organism
✓ Organisms may belong to more than one trophic level in an ecosystem. It is therefore not
accurate to represent it with only one bar.
Note: When making a pyramid of biomass, you must use a scale as the dry weight at any trophic
level is expressed in terms of mass/square unit area, e.g: kg/km
2
, g/m
2
.

Unusual pyramid of numbers

Unit 3: Passive movement of substances across the cell membrane
Learning objectives: After studying this topic, to be able to:
❖ Define diffusion and osmosis and describe their importance
❖ Explain turgor pressure and appreciate the importance of turgidity in plant cells
❖ Investigate diffusion and osmosis through experiment

All living cells are surrounded by a very thin membrane, the cell surface membrane whose main
function it to control the exchange of materials such as nutrients and waste products between
the cell and its environment. Inside cells, regulation of transport across the membranes of
organelles is also vital.
This pyramid shows the total energy stored in
organisms at each trophic level in an ecosystem.
Starting with primary consumers, each trophic level
in the food chain has only 10 percent of the energy
of the level below it. Once again, the pyramid
makes it clear why there can be only a limited
number of trophic levels in a food chain or web.
A: The producer is a single plant such as a tree.
B: The producer is a single plant which is infested
with parasites (primary consumers) and the latter are
parasitized by further parasites.
C: A large number of producers are eaten by a single
primary consumer which is infested with parasites.
D: Normal pyramid for comparison.

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Substances are always moving into and out of the cells. The way substances move into and out of
the cells depends on certain properties of the substances, size of the molecules and the type of
substance.
There are three main physiological processes by which substances move in and out of cells,
namely: diffusion, osmosis and active transport.

3.1 Diffusion of gases and solutes

Diffusion is the movement of molecules from an area of high concentration of the molecules to
an area with a lower concentration. Particles move by their kinetic energy ( = energy possessed
by an object because of its movement *⅟2 mv
2
]).
The difference in the concentrations of the molecules in the two areas is called the
concentration gradient. Diffusion will continue until this gradient has been eliminated. Since
diffusion moves materials from an area of higher concentration to the lower, it is described as
moving solutes "down the concentration gradient". The end result of diffusion is an equal
concentration, or equilibrium, of molecules on both sides of the membrane.

3.1.1 The importance of diffusion to organisms
Diffusion is a purely physical process which does not need energy from the cell to drive it. It is a
very important process for all organisms. Lots of substances move in and out of cells by diffusion.
It is involved in digestion, absorption, respiration, removal of waste products,…
For example:
• Plant root hairs absorb water, some mineral salts and oxygen from the soil by diffusion.
• Cells and unicellular organisms, such as Amoeba, take in oxygen and get rid of waste
products by diffusion.
• Digested food, for example sugars and amino acids, enter blood from the gut by diffusion.
Once dissolved in the blood, these substances are transported around the body in solution and
can diffuse out of the blood into cells where they are needed.
• Diffusion is involved in exchange of gases in stomata, skin of frogs, gills of fish and in the
lung of mammals
• Waste products of metabolism, for example poisonous compounds containing nitrogen,
diffuse out of animal cells and into the blood. They are then transported to an animal’s kidney
where they are removed from the blood and excreted in urine.

3.1.2 Factors that affect the rate of diffusion

The time taken by particles to move within an available space until they are evenly distributed is
influenced by a number of factors:
- Size of particles: Small particles move faster than large ones so small particles will diffuse faster
than large ones.
- Temperature: Increasing the temperature makes particles in a solution or a gas move faster. So
increasing the temperature will increase the rate of diffusion, because with high temperature of
particles, their kinetic energy increases and the particles move faster

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- Concentration gradient: The bigger the difference in concentration between two points, that is
the steeper the concentration gradient which causes the diffusion rate to be faster. When the
concentration gradient is low, diffusion rate is also low.
- Surface area: For a molecule to get into an organism’s body it has to pass through a cell
membrane. The bigger the area of the cell membrane available for diffusion, more of the
substance diffuses across it than when it is small.
Note: A larger surface area to volume ratio does not increase the speed of diffusion of the
particles. It simply enables more particles to diffuse across it in a given time.
Small organisms such as Amoeba have a greater surface area compared to volume than larger
organisms. Diffusion of substances into and out of smaller organisms is faster than in larger
organisms: they can absorb oxygen and other materials more rapidly than large ones. They can
also excrete waste products at a faster rate than large organisms.
In larger organisms diffusion of substances into their bodies would be slow and take long time.
Their bodies are equipped with a complex system of transport, the blood circulatory system.

-Diffusion distance: The distance the particles have to travel to be evenly distributed within the
available space influences the rate of diffusion. Is small distances, the diffusion is faster. In a thick
membrane the diffusion takes a long time while it takes less time for molecules to diffuse across
a thin membrane.

3.1.3 Facilitated diffusion: This process is used for molecules that cannot diffuse rapidly through
cell membranes, even when there is a concentration gradient across the membrane. In facilitated
diffusion, the movement of these kinds of molecules from an area of higher concentration to an
area of lower concentration is assisted by specific proteins known as carrier proteins. Because
the molecules are moving down their concentration gradient, facilitated diffusion is passive
transport. The cell does not have to supply additional energy to make it happen.

3.2 Osmosis
Osmosis is the movement of water molecules from a region of high water potential (dilute
solution) to a region of low water potential (concentrated solution) through a partially
permeable membrane.
Since the concentration is defined in terms of solute concentration and not in terms of water
content, water molecules diffuse from less concentrated solution (=fewer solutes, more water)
to a more concentrated solution (=more solute, less water).

Types of solutions
When a solid is dissolved in water, we get a solution. The solid that is dissolved in the solution is
called the solute. The liquid that dissolves the solid is known as the solvent.
Solution= solute + solvent
The concentration of a solution depends on the amount of solute dissolved. A dilute solution has
more water molecules whereas a concentrated solution has more solute molecules than water
molecules.
Special terms are used to describe solutions of different concentrations: isotonic, hypotonic
hypertonic, water potential and osmotic pressure.

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Term Meaning
Isotonic Iso means equal. When 2 solutions of equal concentration are separated by
a semi-permeable membrane, the net movement of water out of a cell
balances water movement into the cell.
Hypotonic Hypo means under. When two solutions with different concentration are
separated by a semi-permeable membrane, the dilute solution is said to be
hypotonic to the more concentrated solution. There is a net movement of
water from the hypotonic solution to the concentrated solution.
If a solution has a lower concentration of solutes than the cytoplasm of a
cell, water molecules move into the cytoplasm. The cytoplasm expands as
water moves in.
Hypertonic Hyper means above. A hypertonic solution has a higher concentration of
solutes than the cytoplasm of the cell. In this situation, water molecules
move from the cytoplasm which is now the area of higher water
concentration to the area outside a cell. The cell shrinks as its cytoplasm is
losing water.
Water
potential
This is a measure of the concentration of free water molecules in a solution.
A solution with more free water molecules has a higher water potential. It
has a higher tendency to lose water molecules.
Osmotic
pressure
This is a force generated by a solution causing it to draw water molecules
from another solution separated from it by a partially permeable
membrane.

Water relations in plant cells

The movement of water into and out of the cell and the effects that such movements have on
the cell are described as water relations in the cell.
Living cells are surrounded by a fluid medium which may be isotonic, hypertonic or hypotonic to
the cell content.
If the fluid and the cell are isotonic, there will be no net movement of water into and out of the
cell.
If the external fluid is hypertonic to the cell contents, then, water leaves the cell.
If it is hypotonic, then water enters the cell.

a) Plant cells in hypotonic solutions

Plant cells a have a large vacuole containing cell sap which contains salt and sugar molecules.
When surrounded by a hypotonic solution, water molecules enter into the vacuole by osmosis.
The vacuole swells. Due to intake of more water, turgor pressure (pressure exerted by the cell
contents against the cell wall due to intake of water by osmosis) increases and the cell wall

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exerts a pressure that is equal to turgor pressure on the protoplasm (cell membrane and
cytoplasm), a pressure called wall pressure.
With turgor pressure the cell becomes stiff, firm or erect, turgid.
Turgidity in plant cells is important because the stiff cells give support to the soft tissues such as
petals and sepals. Healthy plants are turgid because plants rely on turgidity to maintain rigidity
and stand upright.
Turgor pressure enables soft, non-woody plant stems to remain erect, upright despite the
downward force of gravity.
Note: Animal cells, when placed in hypotonic solutions, because they do not have a cell wall, will
swell and lyse (burst).

b) Plant cells in hypertonic solutions

A plant cell that is surrounded by a hypertonic solution will lose water from the cytoplasm and
then from the vacuole which in turn reduces in size. As the turgor pressure decreases, the cell
membrane and cytoplasm shrink away from the cell wall. The cell wall is undergoing plasmolysis
and the cell is plasmolysed. When put in distilled water, a plasmolysed cell become turgid again
and is deplasmolysed.
Incipient plasmolysis is the point at which plasmolysis just begins.
When the turgor pressure is zero and the cell therefore loses its turgidity, it is a flaccid cell.
When this persists, the plant dies.
The figure below summarises the behaviors of plant cells when placed in solutions of different
concentrations.

Cell wall keeps
plant cell from bursting
Role of osmosis in plants cells
- Uptake of water by roots: The cell sap in root hair cells usually has a higher solute
concentration than water in the soil. Therefore, root hair cells are able to take up water
from the soil by osmosis.
- Opening and closing of stomata: When guard cells gain water, they become turgid. They
change in shape, making the stomata to open:
*Gaseous exchange takes place through the open stomata

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*Water vapour also exits through the open stomata
When guard cells lose water to surrounding cells, their size and shape change, and
stomata close.
- Movement of water from cell to cell in tissues: When a cell takes up water, its solute
concentrations become lower compared to the cell adjacent to it. As a result, water
moves from it to the adjacent cell whose solute concentration is higher.
- Feeding in insectivorous plants: Insectivorous plants such as Pitcher plant prey on insects.
They trap insects when there is a sudden change in their turgor pressure when disturbed
by insect.

Unit 4: Active transport

Learning objectives: After studying this topic, to be able to:
❖ Define active transport
❖ Compare passive and active transport
❖ Appreciate the importance of active transport
❖ Describe hoe carrier proteins move particles across a membrane during active
transport
In contrast to passive diffusion which does not require energy and carries molecules or ions
down a concentration gradient, active transport pumps molecules and ions against a
concentration gradient. In active transport, the particles move across a cell membrane from a
lower concentration to a higher concentration. This process requires the expenditure of energy
since particles move against a concentration gradient.

Difference between passive and active transport

Passive transport Active transport
Occurs from a high concentration of molecules
and ions to low concentration, in order to
maintain equilibrium in cells
Occurs from a low concentration of solute to
high concentration of solute
Does not require cellular energy Requires cellular energy
Involves osmosis and diffusion processes Involves endocytosis and exocytosis processes
Transports anything soluble in water, oxygen
and carbon dioxide
Transports proteins, ions, large cells and
complex sugars

The role of proteins in active transport
The movement of substances across a membrane by active transport requires carrier protein
molecules. They pick up molecules of a substance from one side of the cell membrane and
transport them across. Carrier protein molecules have a binding site for molecules. The
molecules to be transported bind to the carrier protein using energy.

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Role of active transport in living organisms

Active transport is involved in absorption by root hair cells, by cells lining the human small
intestines, in functioning of nerve cells, reabsorption in kidneys,…
1. Through active transport, root hair cells absorb mineral salts and ions from the
surrounding soil.
2. The cells lining the human small intestines continue to absorb food molecules by active
transport even when the concentration of these molecules is higher in the cells than in
the intestinal lumen
3. Nerve cells need sodium ions and potassium ions to function. The nerve cells maintain the
concentrations of these ions by active transport.
4. When urine is first formed in the kidney, it contains useful substances like glucose in
addition to the waste substances. The useful substances are reabsorbed into the blood by
active transport.
5. Active transport is involved in accumulation of substances in the body of some organisms
to offset osmotic imbalances in arid and saline environments.

Factors that affect active transport

a) Concentration of oxygen: Higher oxygen concentration leads to rapid active transports by
protein carriers because cells produce more energy.
b) Concentration of glucose: more glucose available, more energy produced; hence rapid
active transport.
c) Temperature: Active transport works better under normal temperature (37
o
C in humans)
because high temperatures destroy/ denature carrier proteins.
d) Enzyme inhibitors: An enzyme is a substance which speeds up reactions taking place in
the cell. Enzyme inhibitors make an enzyme inactive and prevent active transport from
taking place.
e) pH: Abnormal changes in pH in the environment alter the structure of protein carriers.
This reduces their ability to transport molecules across the cell membrane.

4.2 Endocytosis and Exocytosis

Endocytosis is the process of capturing a substance or particle from outside the cell by engulfing
it with the cell membrane. The membrane folds over the substance and it becomes completely
enclosed by the membrane.
There are two main kinds of endocytosis: phagocytosis and pinocytosis.
• Phagocytosis or cellular eating occurs when the dissolved materials enter the cell. The process
is selective and specific to solid materials engulfed, forming a phagocytic vesicle.
In addition, certain cells in animals use phagocytosis to ingest bacteria and viruses that invade
the body. These cells are known as phagocytes.

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Fig. phagocytosis of bacteria (which ends in release of microbial products)

• Pinocytosis or "cellular drinking". This process is similar to phagocytosis. The only difference is
that cells take in materials in liquid form. There is formation of smaller vesicles (pinocytic vesicle)
compared to phagocytic vesicles.

The table below summarizes the difference between phagocytosis and pinocytosis
Phagocytosis Pinocytosis
Cellular eating Cellular drinking
Taking in materials solid state Taking in materials in liquid
form
Formation of phagocytic
vesicle
Formation of pinocytic vesicle

Exocytosis is the process by which materials are removed from the cells. Materials removed
include solid particles, undigested remains from food vacuoles among others. Within this
process, the molecules or cell secretions accumulate in secretory vesicles, which then moves and
fuses with the plasma membrane. The content of the vesicles is then expelled out of the cell.
Examples of exocytosis:
-Secretion of digestive enzymes by pancreas
-Secretion of milk from mammary glands
-Secretion of mucus by salivary glands

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Unit 5: Identification of food components

Key Unit Competence: To be able to carry out chemical tests on a variety of foods to identify the
nature of food substances.
Learning objectives: After studying this topic, to be able to:
❖ State the chemical reagents used in identification of each of the classes of food
❖ Carry out tests to identify food substances in a given sample
❖ Appreciate the importance of different food classes in one food substance

5.1 Components of food substances
➢ The foods we eat contain different types of nutrients. They contain mainly two classes of
nutrients: organic and inorganic nutrients.
➢ The inorganic nutrients include mineral salts and water.
➢ The organic nutrients include proteins, carbohydrates, lipids and vitamins.
➢ In order to be healthy, it is essential to know the components of the food we eat and take a
balanced diet.
➢ Food testing is the process by which foods and drinks are taken to the laboratory to confirm
the type of food component. It is done to confirm the presence of starch, reducing sugars,
proteins, fats and vitamin C in a food sample.

5.2 Testing for carbohydrates (starch and reducing sugars)
a) Test for starch (polysaccharide)
Starch is the storage form of glucose in plants. It does not have a sweet taste and is insoluble in
water.
With the laboratory test, starch gives a blue-black colour with Iodine in potassium iodine
solution. If there is no starch in the food sample, the brown colour of iodine persists.

b) Test for sugars
Based on their reaction with Benedict’s solution, there are two groups of sugars: reducing sugars
and non-reducing sugars
Reducing sugars: All monosaccharides are reducing sugars, they have sweet taste and are soluble
in water.
When Benedict’s solution is boiled with glucose, an orange precipitate is formed.
A change in colour from blue to green, yellow and finally red indicates the amount of reducing
sugar present (small and high amount respectively).

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➢ Non-reducing sugars: Most non-reducing sugars are disaccharides, meaning that they
contain two single sugar units.
➢ They are unable to reduce the copper ions in Benedict’s solution: this makes the colour of
Benedict’s solution to persist when these sugars are boiled with it.
➢ The most common non- reducing sugar is sucrose.

Tests for non-reducing sugar
To 1cm
3
of the test solution, add 1cm3 of Benedict’s solution and boil. The solution remains blue.
To the mixture, add 1cm3 of dilute HCl and boil for 5 minutes. Cool the mixture, add 1cm3 of
dilute NaOH solution followed by 1cm3 of Benedict’s solution and boil.
Observation: As Benedict’s test.

Basis of test:
Disaccharide can be hydrolysed to its monosaccharide constituents by boiling with dilute
hydrochloric acid. Sucrose is hydrolysed to glucose and fructose, both of which are reducing
sugars and give the reducing sugar result with the Benedict’s test. NaOH is added in order to
neutralize HCl because Benedict’s solution works best in neutral PH.

5.3 Testing for proteins

There are two major ways in which scientists test for the presence of proteins in food:
Millon’s test and Biuret test.
a) Millon’s test: Careful use of Millon’s reagent to test for proteins.
The table below shows the results that may be obtained during Millon’s test

Test procedure Observation Deduction
To 2 cm
3
of the food sample in a
test tube, add Millon’s reagent and
boil.
Coagulated pink mass
appears.
Protein present in a
food sample.
To 2 cm
3
of the food sample in a
test tube, add Millon’s reagent and
boil.
The solution remains
colourless.
Protein absent in the
food sample

b) Biuret test: It uses several reagents that are mixed in equal proportions. Copper (II)
sulfate and sodium make up the reagent.

The normal colour of the
reagent is blue. In the presence
of peptide bonds, the reagent
turns violet.

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5.4 Testing for lipids: Lipids are composed of fats and oils; fats are solid while oils are liquid
at room temperature. Two types of tests can be carried out on a food sample to identify if it
contains lipids: Ethanol emulsion test and translucent paper mark test.

a) Ethanol emulsion test
In this procedure, ethanol is used as the reagent. Water is also added. A white emulsion confirms
the presence of lipids in the food sample.

Procedure: Add 1cm
3
fat or oil to a test tube containing 1cm
3
of absolute ethanol. Dissolve the
lipid by shaking vigorously. Pour the solution into a boiling tube half filled with water. A white
emulsion is formed.

b) The translucent spot test

A translucent spot indicates the presence of oil or fat in a food sample.

5.5 Testing for vitamin C
Presence of vitamin C in a food sample is tested by a chemical DCPIP (Dichlorophenol
indophenol). This chemical is a deep blue reagent in colour. When vitamin C is present in sample,
the blue color colour disappears (decolourised).
The table below shows the results of the test for vitamin C with DCPIP
Test procedure Observation Deduction
a) To 1 cm
3
of DCPIP in a test tube, add the
orange extract solution drop by drop. Do not
shake
The mixture
turns
colourless
Vitamin C present
b) To 1 cm
3
of DCPIP in a test tube, add
distilled water drop by drop. Do not shake
The solution retains
the
blue colour of DCPIP
Vitamin C absent
Food to
test
Chemicals to use Observation Deduction
Starch Iodine solution Formation of a
blue-
black colour
Starch present
The yellowish colour
of
iodine remains
No presence of starch
Reducing
sugars
Benedict’s solution and boiling Formation of an
orange precipitate
Presence of some reducing
sugars (A change in colour
from blue to green, yellow and
finally red indicates the
amount of reducing sugar
present (small
and high amount respectively).

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Persistence of the
colour
of Benedict’s solution
Presence of non-reducing
sugars
Proteins - Millon’s reagent and
boiling
Appearance of a
coagulated pink mass
Protein present in a food
sample.
The solution
remains
colourless.
Protein absent
- Biuret test (Copper (II)
sulfate and sodium
The blue colour of the
reagent turns violet.
Presence of peptide bonds
Lipids -Ethanol and water Appearance of a
white
emulsion
Presence of lipids
Translucent paper Translucent spot Presence of oil or fat in a
food sample
Vitamin C DCPIP (Dichlorophenol
indophenol, a deep blue
reagent in colour)
The mixture
turns
colourless
Vitamin C present. Vitamin C
prevents scurvy, helps in
formation of connective tissue;
promotes healing of wounds;
Strengthening of the blood
vessels
The solution retains
the
blue colour of DCPIP
Vitamin C absent

Experiment to be carried out
Requirements
- Specimen O which is an orange - Specimen M which is milk - Test tubes and test tube rack
- Syringe - Millon’s rreagent - Source of heat -Distilled water iodine solution - DCPIP
- Droppers - A stirrer

Procedure
✓ Cut a transverse section of specimen O and obtain about 10 cm
3
of juice from the
specimen by squeezing into a test tube
✓ Mix the orange juice above with the milk to form a solution. Label the solution Extract A

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✓ Carry out the following tests on this Extract A using the reagents provided and record
your observations in the table below

Experiment Observations Deduction
(i) To 2 cm
3
of extract A in a test tube, add 3
drops of iodine solution

(ii) To 2 cm
3
of extract A in a test tube, add 2
cm
3
of Benedict’s solution and boil

(iii) To 2 cm
3
of extract A in a test tube, add
4 drops of Millon’s reagent and boil gently

(iv) To 1 cm
3
of DCPIP in a test tube, add
extract A, drop by drop

Study questions
a) From your observations, name the food substances present in the extract
b) What is the biological significance of the food substances named in (a) above?

Expected results
Experiment Observations Deduction
(i) To 2 cm
3
of extract A in a test tube, add 3
drops of iodine solution
The yellowish colour
of iodine remains
No starch present
(ii) To 2 cm
3
of extract A in a test tube, add 2
cm
3
of Benedict’s solution and boil
Brick-red solution is
formed
Some reducing
sugars present
(iii) To 2 cm
3
of extract A in a test tube, add
4 drops of Millon’s reagent and boil gently
The changes from
colourless to panic
and finally purple
Proteins present
(iv) To 1 cm
3
of DCPIP in a test tube, add
extract A, drop by drop
Deep blue colour of
DCPIP changes to
colourless
Vitamins present

(a) Food substances present in the extract are: reducing sugars (Lactose), Proteins and
Vitamin C
(b) Lactose: Source of energy. It is converted to glucose and galactose which are broken
down through respiration to provide the body with energy
Proteins: Help in building body tissues and in repair of worn out tissues
Vitamin C: Prevents scurvy, a nutritional deficiency disease associated with lack of
vitamin C in the diet.

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Catalase
Unit 6: Enzymes

Learning objectives: After studying this topic, to be able to:
❖ Define catalyst and enzyme
❖ Explain the factors that affect enzyme activity
❖ Draw and interpret graphs for the rate of enzyme activity
❖ Appreciate the importance and specificity of enzymes in speeding up reactions

➢ Enzymes are biological catalysts that are protein in nature.
➢ They speed up or slow down the rate of chemical reactions in the body without being
used up themselves.
➢ They are proteins responsible for catalysing almost every metabolic reaction occurring in
living organisms.
A catalyst is a substance which speeds up a chemical reaction without itself undergoing any
permanent chemical change.

6.1 Types of enzymes
There are two major types of enzymes by considering their action area:
Intracellular enzymes: these are enzymes that are used within the cells that produce them.
Extracellular enzymes: These are enzymes that are transported from the cells that produce them
to be used in other cells.
There are also different types of enzymes based on the foods they act on for example:
• Peptidases: break down proteins
• Lipases: break down lipids
• Carbohydrase: break down carbohydrates
✓ Catalase, an enzyme found in living tissues, breaks down hydrogen peroxide to water and
oxygen.

2H2O2 (aq) 2H2O (l) + O2 (g)

6.2 Characteristics of enzymes

Enzymes are always proteins, and their characteristics therefore reflect the properties of
proteins. They are characterized by their specificity, sensitivity to temperature and pH change
and working very fast. Their main properties are as follows:

i. Enzymes are protein in nature: all enzymes are made up of proteins
ii) Enzymes are not destroyed by the reaction: they remain unchanged after catalyzing a
reaction. They can be used over and over again in small amounts without being
changed.

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iii) An enzyme catalyse reversible reactions: they can change a substrate into products and
the products back into the original substrate.

A + B C
iv) Enzymes are affected by temperature. Enzymes work better at specific temperature,
called optimum temperature.
Very low temperatures inactivate enzymes and become unable to catalyse reaction. On the
other hand, high temperature, beyond the optimum temperature, denatures enzymes.

v) Enzymes are sensitive to PH. Every enzyme has its own range of PH in which it functions
most efficiently. Some enzymes work best at low pH (acidic medium medium. E.g: Pepsin which
digests proteins in the stomach works best at pH of 2) while others work best at high pH (alkaline
medium). Others, for instance, work best at neutral or slightly alkaline pH. E.g. lipases,
peptidases and amylase

vi) Enzymes are substrate-specific: They can only catalyse one reaction involving a particular
substrate. This is because they have the active site which can only fit to a particular substrate
whose shape complements the active site.
vii) Enzymes lower the activation energy of reactions they catalyze. The activation
energy is the energy necessary for a reaction to get started. Every chemical reaction involves
both bond breaking and bond forming. The reactant molecules must absorb energy from their
surroundings for their bond to break, and energy is released when the new bonds of the product
molecule are formed. Enzymes reduce the activation energy needed to change a substrate into a
product.
viii) Enzymes possess active sites where the reaction takes place. These sites have
specific shapes.
ix) Enzymes work rapidly: They work very fast in converting substrates into products. The
fastest known enzyme is catalase found in both animal and plant tissues
x) Enzymes are efficient: They are required in very small amount and are not used up in
a reaction; they can therefore be used repeatedly.

6.3 Factors that affect enzyme activity
Among these factors, we include temperature, pH, enzyme concentration, substrate
concentration and presence of inhibitor and co-factors.
a) pH: Different enzymes have different pH at which they work better, their optimum pH.
Any change in pH above or below the Optimum will quickly cause a decrease in the rate of
reaction. Extreme pH causes enzymes to denature and permanently lose their function. Most
enzymes in the human body work best at neutral pH (pH 7). Trypsin which digests proteins in the
duodenum works at pH of 9.

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Fig: Effect of the pH on an enzyme-controlled reaction.

b) Temperature
The temperature at which enzymes work best is called optimum temperature. Temperatures
lower than the optimum cause enzymes to be inactive and work at slow rate. Higher
temperatures than the optimum destroy enzymes.
✓ Enzyme activity is higher at high temperature since molecules gain more kinetic energy
✓ Most enzymes begin to denature at 40
o
C
✓ Enzymes of bacteria found in hot springs may begin to denature at temperatures higher
than 80
o
C
Effect of the temperature on an enzyme-controlled reaction

6.4 Mode of enzyme action

Enzymes are very specific in nature: they can catalyse a specific reaction. The shape of the
substrate must complement the shape of the enzyme’s active site. This enables fitting of the
substrate into the enzyme active site and enables the substrate to be changed into products.
When the substrate fits into the active site, an enzyme-substrate complex is formed. This
reaction complex can be explained by a key and lock model as illustrated below:

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Unit 7: Photosynthesis

Learning objectives: After studying this topic, to be able to:
❖ Define and state the word equation for photosynthesis and identify products of
photosynthesis
❖ Recall the location of plastids and chloroplasts in a plant cell
❖ Explain how the internal and external structures of a leaf are adapted for
photosynthesis
❖ Describe the uses and dangers of fertilisers
❖ Carry out experiments to test for starch in green leaves

Photosynthesis is a metabolic process by which green plants make up (synthesise) sugar
(carbohydrates) from carbon dioxide (CO2) and water, using sunlight energy absorbed by
chlorophyll found in chloroplasts.

H2O + CO2 Chlorophyll Glucose (C6H12O6) + oxygen (O2)
Sunlight
Light energy, carbon dioxide, water and chlorophyll are conditions necessary for photosynthesis
to take place.
Chloroplasts are either in palisade mesophyll cells, spongy mesophyll cells and guard cells. Cells
with chloroplasts are referred as photosynthetic cells. The presence of starch in these cells turns
iodine from brown to blue-black colour.

End products of photosynthesis

6H2O (l) + 6CO2 (g) Chlorophyll C6H12O6 (s) + 6O2 (g)
Sunlight
Primary products

✓ Glucose: It is the main product and is used in respiration to release energy. Excess glucose
is stored as starch or oil.
✓ Oxygen: A part of it is used during respiration while the rest is released in the atmosphere
during gaseous exchange.

Secondary products:

• Starch, reserve of glucose stored in different parts of plant. Ex: grain of maize, root of
sweet potato, etc.
• Lipids, glucose is converted into lipids and stored in different parts of plant. Ex: in fruits of
avocado, roots of groundnuts, fruits of fruits etc.
• Proteins, glucose is converted into proteins and stored in different parts of plant. Ex:

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fruits and seeds of bean, soya bean, peas, etc.
Limiting factors of photosynthesis
These factors influence each other and there is a point where the plant cannot photosynthesise
any faster even with further increase in any factor. If one factor is not a limiting factor, other
factors limit the rate.

a) Light intensity: Light is a necessity for photosynthesis to occur. In low light intensity, the
rate of photosynthesis is low, but as light intensity increases, the rate of photosynthesis also
increases. In darkness, plants cannot photosynthesise at all.

b) Carbon dioxide concentration: Increase in carbon dioxide increases the rate of
photosynthesis till the point where other factors limit the rate.
c) Temperature: The rate of photosynthesis is highest at optimum temperature. Further
increase in temperature above optimum results to a decrease in rate of photosynthesis since
enzymes are denatured.
d) Water: When water is in short supply, the stomata close, less CO2 diffuses via the stomata
into the leaf. This reduces the rate of photosynthesis.

Internal structure of the leaf and its adaptation to photosynthesis

The structure of the leaf is well suited for photosynthesis as shown by the diagram below.

Internal structure of the leaf

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The following are some of the adaptations of the leaf to photosynthesis.
Feature Structure Function

External shape
and structure
Most plant leaves have
numerous leaves, most
leaves are thin with a large
surface area and a flat leaf
blade
Allows maximum absorption
of incident light energy
Facilitates inward diffusion.
of carbon dioxide in the
mesophyll cells.
Epidermis Modified parenchyma cells,
with thick external wall
with a waxy cuticle on the
outer surface
Epidermis has epidermal
hairs (extensions of
epidermis), particularly on
young leaves.
Support protection of the
leaf tissue.
The epidermis protects leaf
tissue from invasion by
predators and from
excessive water loss by
evaporation.
Stomata Pores in the leaf epidermis
between two guard cells for
gas exchange
Regulate entry of carbon
dioxide and the loss of
water vapor from the leaf.
Palisade
mesophyll
Modified parenchyma cells
with numerous chloroplasts
in the cytoplasm; cells are
elongated in shape and
packed closely
Power house of the leaf;
chloroplasts are the site of
photosynthesis. Sugars and
other substances are
produced in the
chloroplasts and oxygen is
released.
Spongy
mesophyll
Modified parenchyma cells
with some chloroplasts in
the cytoplasm; cells mostly
more or less spherical or
elongate, with large
interconnecting air
spaces between them
Facilitates gaseous
exchange by diffusion
Veins
containing
vascular
bundles
Network of small vascular
bundles continuous with
those of stem and root.
Vascular bundles contain
xylem vessels and phloem
vessels.
Supports the leaf tissues as
a large, thin, flexible organs
Xylem delivers water to leaf
cells
Phloem transports the
organic products of
photosynthesis
Leaf cuticle and
epidermis
Thin and transparent
structures
Allow easy penetration of
light for photosynthesis

Importance of photosynthesis
All life on earth depends on photosynthesis directly or indirectly for the following reasons

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a) As a source of energy (food): Animals depend directly or indirectly on plants for food.
b) Provides oxygen in air: As a by-product of photosynthesis, this O2 replaces the one which
is continuously used up by living organisms
c) Makes carbon available to plants and animals: The carbon part of CO2 from air is
incorporated into synthesized food and made available to living things
d) Prevents accumulation of carbon dioxide in the air ( air cleaning up): Reduced level of
CO2 in the atmosphere prevents global warming which is the increase in global
temperature caused by increasing levels of CO2 in the atmosphere among other factors.
e) It is responsible for the energy stored in coal and petroleum: Fossils of plants and
animals that lived long ago contain energy in form of fuels such as petroleum and coal.

Mineral requirements for plant growth
For their growth, plants need essential mineral ions such as magnesium and nitrate ions.
Magnesium ions Nitrate ions
Required for the synthesis of chlorophyll Required for protein synthesis;
Needed for activation of enzymes involved
in energy transfer process
Nitrogen is a major constituent of amino
acids, proteins, coenzymes and chlorophyll
Their deficiency affects plant growth and
development: leaves start to change colour
from green to yellow, less photosynthesis
occurs hence poor growth
Their deficiency results in poor synthesis of
chlorophyll: leaves become yellowish, less
photosynthesis occurs hence stunted plant
growth

Uses and dangers of nitrogen fertilisers.
➢ Soils naturally contain many nutrients needed by plants.
➢ These nutrients are nitrogen, phosphorous, calcium and potassium.
➢ They enable plants to grow healthy and produce food. In soils that are nutrient-deficient,
fertilizers are applied.
➢ Fertilizers are nutrients applied to agricultural fields to supplement required elements missing
in the soil to make the ground more suitable for growing plants.
➢ The misuse and overuse of fertilizers has adverse effects to organisms and the environment and
causes depletion of minerals from soils.
➢ Dangerous levels of nitrates in the environment due to overuse of nitrogen fertilizer is linked to
diseases such as diabetes, cancer and Parkinson’s disease, and so on.
➢ Consumption of ground water containing high amounts of nitrate may lead to health problems
in young children.
Harmful effects of nitrogen fertilizers to the environment.
(a) Nitrogen fertilizers aid plants in their growth. However, weeds and non-native plants tend
to grow more readily with additional nitrogen supplies.
(b) Excess nitrogen in the soil creates an imbalance of nutrients.
(c) High amounts of nitrates in the soil results to increased acidity.

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(d) Nitrogen-polluted air, caused by nitrates from automobiles and industrial plants, results
in acidification of the soil when acid rain falls.
(e) Eutrophication: when nitrogen levels in rivers and streams increase, they aid in algae
overgrowth. As algae die and decompose, organic matter in water increases. This process
uses up the available oxygen, causing levels to drop. Without oxygen, fish, crabs and
other aquatic organisms die.
(f) Nitrogen is a soluble substance. It soaks deeply into the soil after a rainstorm or irrigation.
It may reach ground water and nearby wells hence contaminating them.

Unit 8: Transport of water, minerals and organic foods in plants

Learning objectives: After studying this topic, to be able to:
❖ Identify xylem and phloem tissues from transverse sections and state their functions!
❖ Explain the adaptations of plant leaves to controlling water loss!
❖ Explain the adaptations of plants to different environmental conditions!
❖ Describe transpiration and its effects and explain how it takes place!
❖ Appreciate the importance of absorption and transport of water in plants!

Transport is necessary because all cells in living organisms need food substances and oxygen
from their surroundings. They produce waste substances that need to be eliminated from their
bodies.
In unicellular organisms, diffusion alone is enough to transport materials into and out of the body
because they have a greater surface area to volume ratio. In plants, however, diffusion alone is
not enough to transport materials because they are more complex and multicellular. There is a
need of a transport system.

8.1 Transport system in plants
In plants, the transport system is made up of specialized tissues called vascular bundles which
contain two types of tissues: xylem and phloem which are found in the leaves, root and the
stem.
Substances that need to be transported are:
✓ Water required in photosynthesis.
✓ Mineral salts used in various plant processes.
✓ Organic substances manufactured by photosynthesis in leaves.

Transport systems in plants are needed for the following reasons:
• To move substances from where they are absorbed to where they are needed for
example, water and mineral salts are absorbed by roots and transported in the xylem to
other parts of the plant.
• To move substances from where they are produced to where they are needed for
metabolism. For example, sugars and are produced in the leaves, but glucose is needed
by all parts of the plant for respiration and converting to cellulose for making cell walls in
areas for growth. Glucose and amino acids can be moved in phloem as part of the sucrose
molecule.

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• To move substances to a different part of the plant for storage for example, to move
sugars into a potato tuber for storage in the form of starch.

Water and mineral salts are absorbed by roots through root hairs. In both monocot and dicot
roots the vascular bundle occupies a central position.
In the dicotyledonous root, the xylem occupies the centre where it forms a star shape. The
phloem is found in between the two rays of the star. The vascular bundles are arranged in a ring.
Transverse section of dicot and monocot roots

In the monocotyledonous root, the xylem and phloem are arranged to form a ring in which xylem
tissue alternates with the phloem tissue.
Note: Most of the tissues in the root and stem are similar. This is because these tissues are
continuous from the root into the stem. The stem has additional tissue known as pith. Pith is the
central part of the roots of monocots and dicots and the stem of dicots. It is lacking in the stem
of monocots. It is made up of parenchyma cells.

Adaptations of the root hair cells.

(i) They are numerous so as to increase the surface area over which absorption of water
mineral salts occurs
(ii) They are thin and fine so that they can penetrate the spaces in between the soil particles
where water is found

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Root hairs absorb water from the soil by osmosis. Due to concentration gradient between cell
sap in its vacuole and its surrounding, water molecules move from the soil through the semi-
permeable membrane of root hair cells into the cell sap. Once in the roots, water moves from
cell to cell also by osmosis because adjacent cells contain different water quantities.Different
theories to explain the movement of water in plants

a) Capillarity: Capillarity is the interaction between the surfaces of a solid and liquid in
contact. In plants, xylem form narrow tubes through which water moves. Water rises in the
xylem because of strong forces of attraction between the water molecules and the cell walls of
the tubes or xylem vessels.
b) Root pressure: Root pressure is responsible for the rising of water up the xylem. It is
generated by osmotic pressure within the root tissue. It pushes water into and up the xylem
tissue. The existence of root pressure becomes evident when the main stem is cut a short
distance above the soil surface.
Water oozes out of the exposed surface of the remaining intact stem. This water is forced out by
pressure originating from the root system itself. This is why it is called root pressure.

c) Transpiration pull and cohesion tension theory
Leaves contain small pores called stomata. Stomata continuously lose water vapour. Since water
molecules stick together, a pull is created known as transpiration pull. Transpiration pull is as a
result of water being lost at the leaves. It allows lost water to be compensated for.
The adhesive forces between water molecules and the walls of the xylem vessels allow water to
climb up. Cohesive forces allow a stream of water which is unbroken to ascend up the plant. The
continuous movement of water in columns in the root xylem, stem xylem and leaf xylem, to the
air spaces in the leaf is referred to as the transpiration stream.

8.2 Active uptake of mineral salts

Soil water contains dissolved mineral ions such as potassium, magnesium, nitrates and
phosphates. These mineral salts are taken up by plants in ion form (e.g calcium as Ca
2+
,
magnesium as Mg
2+
), against their concentration gradient i.e by active transport which requires
the expenditure of energy. The energy is obtained from respiration.
Mineral salts find their way into xylem vessels, where they move together with water in form of
solution up the plant.

Transport of water takes place within the xylem vessels. Xylem is composed of a system of
interconnected tubes which run all the way from the roots to leaves. In leaves they are in form of
veins. Xylems contain two types of modified cells namely tracheids and vessel elements. Xylems
are hollow tubes which act like pipes allowing water and dissolved minerals to flow through
them. The cell walls in xylem vessels contain a substance called lignin. Lignin strengthens the
cells and gives them structural support.

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Phloem tissues transport food substances in the plant.
They degenerate partially leaving spaces behind called
sieve pores through which dissolved substances pass from
one cell to another. The sieve tubes have companion cells
which control the activities of the sieve tube.
✓ They have sieve pores which allow dissolved food
substances to pass
✓ Companion cells have organelles to supply energy
and other chemicals needed in food transportation
✓ Cytoplasmic filaments along which substances
stream from one tube to another.

Transpiration

➢ Transpiration is the evaporation of water from the plant surface mainly through the leaf.
➢ Leaves contain small pores called stomata on their surfaces, which open and close.
➢ This allows the exchange of respiratory gases as well as the loss of water in form of vapour.
➢ It is this loss of water vapor from the leaf that is called transpiration.
➢ Much of the water that plants take up through their roots is lost to the atmosphere by evaporation.
➢ Through transpiration, plants are able to maintain a steady supply of water since the lost water has to
be compensated for.
Importance of transpiration in plants.
➢ Water is important for the survival of a plant. Yet, transpiration is responsible for a large loss of
water from a plant.
➢ Too much transpiration can be disadvantageous for the plant since it can lead to disruption of
the different biological processes.
➢ Also, excessive water loss through transpiration can result in a decrease in cell turgidity, and
consequent loss of mechanical support in the plant.

✓ Hallow tubes enables water to move easily
through lumen.
✓ Lignified walls: lignin strengthens wall and
prevents the xylem vessel from collapsing and
thus provides mechanical support to the plant.
✓ Being very narrow: helps water to move up the
xylem vessel by means of capillary action.
✓ Tracheids and vessel elements allow in carrying
water.

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However, transpiration is useful to the plant because it brings about:
i. Transport of water: due to transpiration pull, water is raised and rapidly distributed to all parts
of the plants, where it performs various functions of the plant.

ii. Cooling of the plant: the heat which is generated within the plant during cell respiration and
that which the plant receives directly from the sun (solar radiation) would cause the plant to
reach high temperatures, thereby denaturing enzymes and killing the plant. Transpiration brings
about the general cooling down of the plant since it draws heat (latent heat of vaporization) out
of the plant itself as water evaporates from the leaves.

iii. Distribution of minerals: The movement of water and ions in the xylem vessels also brings
about the distribution of mineral ions to all parts of the plant, especially to meristems where
rapid growth takes place.

Sites for transpiration (types of transpiration)

Transpiration may occur from the following three sites:
❖ Stomata (stomatal transpiration): by evaporation of water from cells and diffusion of the
water vapor through stomata, the pores found in the epidermis of leaves and green
stems. (About 90 % of the water is lost via this way.)
❖ Cuticle (cuticular transpiration): This is the loss of water through the cuticle in
herbaceous stems. Leaf surfaces and stems are normally covered with a waxy substance
called cuticle. It accounts for up to 10% of the total water loss by the plant, varying with
thickness of cuticle.
❖ Lenticels (lenticular transpiration): This is the loss of water through the lenticels found in
woody stems. Because of the limited distribution of lenticels, this type of transpiration
accounts for less than 1% of the total loss of water by a plant. However, this is the main
method of water loss from deciduous trees after leaf fall.

Factors that affect the rate of transpiration

The rate of water loss can be affected by many things. Some of these are to do with the plant,
others are environmental. Anything that increases the gradient between the water vapour
potential inside the leaf and that outside the leaf will increase water loss.

Feature that affects
the rate of water loss
How it affects water loss
Number of leaves A plant with more leaves has a large surface area over which water vapor can be
lost.

Number, size and
position of stomata

If the leaves have many large stomata, then water vapor is lost more quickly. If
the stomata are on the lower surface, water vapor loss is slower.

Presence of cuticle

A waxy cuticle reduces evaporation from the leaf surface

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Light In light, the stomata open to allow gaseous exchange for photosynthesis. The
rate of transpiration is high when there
is high light intensity because the stomata open more.
Stomata close in darkness, therefore at night very minimal amounts of water is
lost.

Water availability

If there is little water in the soil, then the plant cannot replace the water that is
lost. Water loss in plants is reduced when stomata are closed or when the plants
shed their leaves.
Temperature Lot of heat in the atmosphere causes faster evaporation and therefore the rate of
transpiration is high.
A higher temperature will increase the rate transpiration in three ways. It will:
- Increase the rate of evaporation from the cell surfaces so that the water
vapor potential in the leaf rises.
- Increase the rate of diffusion through the stomata because the water
molecules have more kinetic energy.
- Decrease the relative water vapor potential in the air, allowing more rapid
diffusion of molecules out of the leaf.
Relative humidity Humidity is the amount of water vapour in the air. When humidity is very high,
air around the plant becomes saturated with water vapour and transpiration
reduces or even stops. However, when the air is dry that is humidity is very low,
there is a high water vapour gradient between the inside of the leaves and the
surrounding environment.
Therefore the rate of transpiration is high.

Air movement or wind

Wind carries with it moisture that has evaporated from the leaf surface creating
room for more to occupy. The faster the air moves, the faster the rate of
transpiration. Plants will lose a lot of water during windy conditions as compared
to calm conditions.

Potometer: measure of the rate of transpiration

➢ Transpiration is measured using an instrument known as a potometer.
➢ This works on the principle that the amount of water lost is equal to the amount of water taken
up by the plant. In fact, the potometer measures water uptake by the leafy shoot.
➢ As the shoot transpires, the water vapor it has lost is replaced by liquid water drawn in from
the potometer via the xylem of the stem.

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Fig: A simple potometer
To calculate the rate of water uptake we proceed as follows:

Rate of water uptake= Distance moved by the bubble (cm)
Time taken in min
8.3 Adaptations of plants to different environmental conditions
➢ Plants can survive in dry areas as well as in or near water or in saline habitats or again in areas
that are neither too dry nor too we.
➢ For this, they have characteristics that enable them to survive in those specified habitat.
Xerophytes, hydrophytes, mesophytes and halophytes are structurally modified depending on
the environment they live in.
8.3.1 Xerophytes

These are plants that grow in dry areas such as the arid and semi-arid areas with little water and
high temperatures.
Adaptations of xerophytes
1. They have a high ability to absorb water from the soil, with roots that are usually more
developed and grow deep into the soil and extend over a wide area.
2. They have water storage tissues. Many xerophytes have swollen stems or leaves which
contain special water storage tissues. Such plants are called succulent plants. Examples
are Bryophyllum, sisal, cactus, Euphorbia, Aloe vera and baobab which has thick stem for
water storage.
3. Xerophytes reduce water loss through transpiration in a number of ways.
a) Some have a thick waxy cuticle which prevents excessive water loss through the leaf
by evaporation.
b) For others, their stomata are found sunken in pits to reduce the rate of transpiration.
c) Others have small leaves, particularly leaves shaped like needles. This reduces the
total surface area of the leaves. The total leaf surface area is also reduced, so that less
Vaseline is applied around the rubber bungs
to ensure an airtight seal, thus the only water
loss from the apparatus is via transpiration.
The function of the reservoir with tap is to
allow the air bubble to travel back to the
start of the measuring scale on repeating the
experiment. As water moves up through the
plant the air bubble moves along the scale
giving a measure of water absorbed by the
plant over time and hence the transpiration
rate.

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water is lost by transpiration.
d) For many others, they have very few stomata which are located on the lower
epidermis away from the direct heat of the sun. Others have reversed stomata
rhythm. Their stomata opens during the night and close during the day unlike
ordinary plants.

4. Some xerophytes have life cycles that enable them to evade dry seasons, for example,
some shed their leaves during the dry season, others survive dry periods as seeds or
spores, others as bulbs or tubers.

Fig.: Some xerophytic plants

8.6.2 Mesophytes
Mesophytes are plants that grow under average conditions of water supply and temperature.
They grow in areas that are neither too dry nor too wet.
✓ Adaptations of mesophytes
1. They have a well-developed root system with long tap or fibrous roots.
2. Presence of stomata on the upper and lower leaf epidermis to allow for efficient gaseous
exchange and also for transpiration.
3. They have thin leaves, which ensure rapid diffusion of gasses from the stomata to the
photosynthetic cells. Internal structures of their leaves have air spaces that allow free
circulation of gases.
4. They have broad and flat leaf blades that provide a large surface area for absorption of
light and carbon dioxide. Their leaves have mosaic arrangement to make sure each leaf
receives maximum amount of sunlight for photosynthesis.

8.3.3 Hydrophytes.

These are plants that live in water or in very wet places. Examples are Nymphaea and water
hyacinth. There are three types of water plants:
(i) Emergent plants: they have roots and part of stem under water. While their leaves are
above water. They have a problem of taking in excess water, for example, reeds.
Floating plants: they float on the water surface with roots in water. Water lilies

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is an example.
(ii) Submerged plants: these are found completely under water, for example, spirogyra.

Adaptations of hydrophytes

1. Thin or lacking cuticle. This permits the plant to absorb water, minerals and carbon
dioxide over its whole surface.
2. As their whole surface can absorb water, their roots are not well developed and if
present, they may be used for anchorage or for food absorption.
3. Presence of a special tissue called aerenchyma which makes the plants buoyant for
support and for gaseous exchange.
4. Submerged leaves do not have stomata and floating types have many on their upper
surface.

8.3.4 Halophytes

These are plants that grow in salty places such as rocky shores, seas and sand dunes which occur
along coastal regions.
➢ They have cells that absorb salt. As a result, they create a higher osmotic pressure which
enables the plant to absorb water. Because of taking much salt, they excrete excess salts
using salt glands.
➢ Some halophytes absorb salt from their habitats and remove it by shedding leaves that
have accumulated salt.

8.4 Translocation

➢ Translocation is the movement of organic products of photosynthesis (photosynthates) from
leaves to other parts of the plant.
➢ The movement of these products throughout the plant is made possible by phloem tissues.
➢ These products move from where they are produced (source, such as mature leaves) to where
they are stored or consumed (sink, such as roots, flowers, fruits, stems and developing leaves).
Note: A storage organ such as a tuber or a bulb may be either a source or a sink depending on
the season. When the storage organ is stockpiling carbohydrates during the summer, it is a sugar
sink.
➢ After breaking dormancy in the early spring, however, the storage organ becomes a source as
its starch is broken down to sugar, which is carried away in the phloem to the growing buds of
the shoot system.
➢ Photosynthates are directed primarily to the roots during early development, to shoots and
leaves during vegetative growth, and to seeds and fruits during reproductive development.
➢ The products from the source are usually translocated to the nearest sink through the phloem.

Structures of Xylem and phloem (see previous pages) relate to their function; the first
transporting water and mineral salts from roots to other parts of the plant, the later
transporting photosynthates from sources to sinks.
A simple observation
When the ring of bark is removed, the phloem beneath it is also removed. After several weeks,

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Swollen part of the bark
The swelling is due to the accumulation of food
substances. They were being transported from the
leaves but could not get across the debarked part
of the stem. That is why there is no swelling on
the lower part of the ring.
swelling above the cut ring is noted.

Unit 9: Gaseous exchange in humans and plants

Learning objectives: After studying this topic, to be able to:

❖ Explain the characteristics and adaptive features of gaseous exchange surface.
❖ Demonstrate the process of inspiration and expiration.
❖ Explain the process of gaseous exchange in plants.
❖ Identify common respiratory diseases and suggest their prevention and treatment.

9.1 The concept of respiration and respiratory surfaces

Gas exchange is the transport of oxygen from the outside air to the cells of the body, and the
transport of carbon dioxide in the opposite direction. This is in contrast to the biochemical
definition of cellular respiration, which refers to cellular respiration. Cellular respiration is the
metabolic process by which an organism obtains energy by reacting oxygen with glucose to give
water, carbon dioxide and ATP (energy); the food taken in by organisms is burnt to produce
energy required for their body functions.
Gaseous exchange is necessary because organisms are able to obtain useful gases from their
environment and get rid of waste gases into the environment. In animals it is the exchange of
respiratory gases whereas in plants it is the exchange of photosynthetic and respiratory gases.

Examples of specialised structures, called respiratory surfaces, for gaseous exchange in animals
include the following:

Organisms Respiratory
surface
Unicellular organisms Cell membrane
Insects Tracheal system
Frogs Buccal cavity, skin
Fish Gills
Mammals, birds, reptiles
and amphibians
Lungs

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Gaseous exchange takes place over the respiratory surfaces. A respiratory surface has a number
of characteristics that make it efficient for gaseous exchange. Some of these characteristics
include:

(i) Thin walls for faster diffusion of gases across it.
(ii) It is moist to dissolve gases as they diffuse across it.
(iii) It has a large surface area for maximum gaseous exchange. If for example your
lungs were laid out flat they would cover an area of between 50-100 m
2
, almost the
size of a volley ball court. Your skin has an area of about 2m
2
.
(iv) In animals with a transport system, the respiratory surface has a rich supply of
blood capillaries (highly vascularized) to quickly transport gases to and from the cells.
(v) They often have a ventilation mechanism to ensure a continuous supply of air or
water to the gas exchange surface. Breathing is an example of ventilation.

Function of Ventilation mechanism
i) It is a process that continuously brings water and air containing more oxygen to the
respiratory surface.
ii) It removes water or air containing a lot of carbon dioxide from it
iii) It maintains a high diffusion gradient at the respiratory surface
iv) It ensures a high rate of gaseous exchange.

9.2 The mechanism of breathing in humans
In humans’ beings, breathing is controlled by medulla oblongata, the respiratory center of the
brain which is sensitive to the concentration of carbon dioxide. When this exceeds a certain level
the medulla oblongata stimulates the ribs and diaphragm to contract more rapidly.
STRUCTURE OF HUMAN GASEOUS EXHANGE SYSTEM

➢ The process that brings air into the lungs and removes it again is known as breathing.
➢ Breathing involves two phases called inhalation (breathing in) and exhalation (breathing
out). Lungs are elastic, and during breathing they are forced to expand or contract as a
result of pressure changes around them.
➢ These pressure changes are caused by the movement of the muscular diaphragm, ribs
and intercostal muscles (rib muscle), and by the force of atmospheric pressure.

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Difference between inhalation and exhalation
Inhalation Exhalation
External intercoastal muscles contract.
Internal intercostal muscles relax
External intercoastal muscles relax.
Internal intercostal muscles contract
Rib cage moves upwards and outwards Rib cage moves downwards and inwards
Diaphragm muscles contract and
diaphragm flattens
Diaphragm muscles relax and diaphragm
forms a dome shape
Volume of the thoracic cavity increases Volume of the thoracic cavity decreases
Air pressure in the lungs and thoracic cavity
decreases compared to external atm
pressure
Air pressure in the lungs and thoracic cavity
increases compared to external atm pressure
External air is driven into the lungs due to the
pressure difference between inside and
outside
Air in the lungs is compressed and forced out
Lungs inflate (expand) Lungs deflate
Fig: Breathing in human
The table below presents the composition of gases in inhaled and exhaled air
Component of air Inhaled air (%) Exhaled air (%)
Oxygen 21 15
Carbon dioxide 0.04 4
Nitrogen 79 79
Water vapor Variable Saturated
We breathe in oxygen and not nitrogen which is more abundant in atmospheric air because
oxygen is required in the body for respiration. Nitrogen is obtained through eating proteins.

Gas exchange in alveoli
Alveoli are air sacs in which gaseous exchange takes place. Air passes the nose or mouth as it
moves down the trachea. It is however more suitable to enter air through the nose than through
the mouth as the nasal cavity warms the air as it enters, acts as filtration and purifies the air by
removing any dust, pollen, and other contaminants, before it passed to the inner body.
The alveolus is a suitable point for gaseous exchange because:

It is supplied with blood which carries the gases being exchanged. It has a very thin wall which allows faster
diffusion of gases between it and the blood.

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• It is lined with a thin film of moisture to dissolve the diffusing gases.
• A ventilation process brings in and takes away air containing the gases being exchanged.
• It has a very large number of alveoli to increase their surface area for gaseous exchange.
As gas exchange between the air within the alveoli and the pulmonary capillaries occurs by
diffusion, oxygen in air, in the alveolar space is at a higher concentration than that in the blood
capillaries. This causes O2 molecules to diffuse across the thin walls of the alveoli and capillaries
and into the blood. CO2 moves out of the blood and into the alveoli in a similar way. The greater
the concentration difference, the greater the rate of diffusion.

Structure of an alveolus

The following factors contribute towards maintaining the diffusion gradient.
(a) Lung ventilation: Breathing movements transport respiratory gases to and from the alveolus.
(b) Blood flow: This constantly replaces oxygenated blood with deoxygenated blood. The
pulmonary artery brings blood low in oxygen concentration and high in carbon dioxide
concentration. The pulmonary vein takes away blood high in oxygen concentration and low in
carbon dioxide concentration.
(c) Hemoglobin: It quickly combines with oxygen and prevents its accumulation in the alveolus.

9.3 Respiratory diseases and smoking

Some of these diseases are caused by micro-organisms while others are genetic. The most
common are the following.

Diseases Causal agent Symptoms Prevention/treatment
Influenza (cold)
serious infection of
nasal passages
(upper airways and
lungs).
Influenza virus type
A or B (more than
200 different viruses)
Muscle and joint
pain, tiredness,
stuffy nose
Difficult nasal
breathing
Cough, fever, sore
throat, sneezing
Good hygiene, getting
vaccinated.
Drinking plenty fluids,
taking over-the-
counter medication to
reduce symptoms,
antiviral medications
Asthma: lung Many agents: cold Difficulty breathing, People with asthma

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inflammation
characterised by an
inflow of mucus
which clogs the
narrow passages
even more.
air, pollen or
Allergens (dust,
mites, cats, dogs,
fungi, etc) ;
It can also be caused
by stress and
anxiety.
In some families the
disease is inherited.
Wheezing sounds
when breathing.
Breathing can feel so
difficult or quick that
the patient can faint.
should: -avoid contact
with allergens (sbces
that trigger asthma
attack); - carry
inhalers that contain a
drug which pacifies
the condition.
Quick-relief
medicines relieve
asthma symptoms
that flare up.
long-term control help
reduce airway
inflammation
Pneumonia (lung
inflammation
especially of the
alveoli) Tissue fluids
accumulate in the
alveoli reducing the
surface area exposed
to air. If enough
alveoli are affected,
the patient may
need supplemental
oxygen
Streptococcus
pneumoniae
(bacteria), viruses or
fungi, Chemical
products
Chills and high fever,
chest pain, rapid and
shallow breathing,
wheezing,
Cough with yellow,
greenish colour or
mucus with some
blood.
Avoid overcrowded
places; provide good
ventilation in living
rooms;
Bacterial and fungal
pneumonia are
treated with drugs
while viral pneumonia
clears by itself.
Lung cancer Cigarette Chronic coughing,
hoarse voice, pain in
the chest, fever
Avoid smoking, avoid
inhaling polluted air,
surgery, chemo-
therapy, radiation
therapy, combination
of surgery, chemo-
therapy and radiation
Bronchitis
It is an infection of
the inner walls of the
bronchi. 2 types:
Acute bronchitis
starts quickly and
stops after a few
days. The symptoms
of acute bronchitis
are like those of a
cold.
Chronic bronchitis
starts slowly and
lasts for a long time.
Caused by bacteria
or air pollutants such
as smoke in inhaled
air.
Excess mucus sec-
retion; cells lining
the bronchi and
bronchioles not
functioning properly,
severe coughing,
difficulty in
breathing
Gettig plenty of rest,
drinking lots of fluids
and taking a cough
syrup (acute
bronchitis);
Avoid polluted air
Avoid smoking
Use of antibiotics in
case of cold with fever

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It is a more serious
kind of infection. It is
commonly caused by
smoking and air
pollution.

Emphysema
This condition results
from long untreated
bronchitis where the
bronchioles in the
lungs become
blocked. Damage of
delicate walls of
alveoli leads to
reduced surface area
for gas exchange, the
lungs become
distended and
inelastic and the
heart
pumps ever-larger
volumes of blood to
the lungs in order to
satisfy the body's
needs. The added
strain can lead to
heart failure.
Weakness due to
insufficient oxygen
supply to tissues.
Running and walking
can prove to be hard
when one has this
condition.
Coughing, shortness
of breath
Early treatment of
bronchitis with
antibiotics to prevent
secondary infection
can help to prevent
emphysema.
Bronchodilators
relieve coughing,
shortness of breath
and breathing
problems.
Tuberculosis (TB)
A serious disease
that attacks lungs
but also attacks
bones, lymphatic
glands and other
parts of the body
Bacillus of Koch
(Mycobacterium
tuberculosis)
Infected dry sputum
in particles of dust
may also cause the
disease;
Raw milk may cause
bovine TB
Dry cough followed
by the spitting of
blood, fever and
excessive sweating;
difficulty in
breathing, loss of
weight, Loss
appetite.
Immunization with
BCG in children;
Avoid taking raw milk.
Boil all milk or drink
pasteurised milk.
Isolating patients. TB
patients should cough
while covering their
mouth with cloth.
Treatment for TB
usually involves a long
course of antibiotics
lasting 6-9 months.
Sinusitis
Inflammation of
sinus cavities usually
that develop after
cold or allergic
reactions
The blockage of
opening of the sinus
cavities
Headache or facial
pain, nasal
congestion, green
and yellow nasal
mucus, cough, sore
throat, fatigue
Avoid cigarette smoke
and pollution; treat
allergies and
respiratory infections
promptly; antibiotics;
sinus surgery to drain
sinus cavities.

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Whooping cough
A condition which
starts like a cold with
fever, running nose
and cough.
Caused by a bacteria
called Bordetella
pertussis from one
person to another
through inhalation of
infected droplets.
Cold with fever,
running nose and
cough; coughing
rapidly many times
without taking a
breath. After the
“whoop”, the patient
may vomit.
Immunisation with
vaccines against
whooping cough
combined, in infants,
with those against
diphtheria, tetanus
and poliomyelitis;
taking antibiotics early
before coughing fits
begin.
Common cold
A cold is a milder
illness than
influenza. Colds
involve the sinuses,
ears and bronchial
tubes.
Caused by a virus
infection located in
the nose. Nasal
secretions containing
cold viruses
contaminate the
hands of people with
colds as a result of
nose blowing,
covering sneezes and
touching the nose.
Sneezing ; Runny
nose ; Nasal
obstruction; Sore or
scratchy throat;
Cough; Hoarseness;
headache, fever,
chills
Limit contact with
known cold patients
during the first three
days of illness and
surfaces they may
have contaminated;
Avoid cough and
sneeze from cold
patients on you or in
your direction

Effects of smoking on the respiratory system

Cigarette smoke contains about 4,000 chemical agents, including over 60 carcinogens.

✓ Smoke particles interfere with the uptake of oxygen in the air sacs.
✓ Tobacco smoke increases the production of mucus in the air passages. A cough by a
smoker is an attempt to remove the excess mucus from the respiratory system.
✓ Tobacco smoke paralyses the cilia in the respiratory tract and stops their movement.
✓ Nicotine in tobacco increases the chance of developing blood clots that cause heart
attacks and strokes increases.
✓ Cigarette smoke also pollutes the environment in public places such as buses, shops and
hotels and forces nonsmokers to inhale the smoke and become passive smokers who
have an increased risk of developing the same diseases as those who smoke.
✓ An excess of particles from smoking or from other sources of air pollution breaks down
the walls of the air sacs and causes the formation of inelastic tissue. This reduces the
functional area of the respiratory surface and in severe cases may lead to emphysema. In
some cases, lung cancer also develops.
✓ Women who smoke often produce less estrogen and tend to experience menopause
earlier than nonsmokers, which may lead to increased bone loss.

9.4 Gaseous exchange in plants

Plants do not have a specialised respiratory system like animals. This is because they are
metabolically less active than animals.
The principal gaseous exchange surfaces for plants are the leaves which have stomatal pores on
their surface where gaseous exchange occurs.

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Stomata, because of their structure, allow:
• Entry of carbon dioxide into the leaf for photosynthesis.
• Exit of oxygen.
• Evaporation of water.

The guard cells control the opening and closing of each stoma.
When the stomata open, gases come into the leaf while others come out along their
concentration gradient. Oxygen, one of products of photosynthesis, diffuses out of palisade cells
to the air spaces. It eventually comes out through the stomata to the atmosphere. On the other
hand, carbon dioxide from the atmosphere enters into the leaf through the stomata into the
airspaces. It then diffuses from the air spaces into palisade cells where it is used during
photosynthesis.
Note that when air is entering or leaving the plant through stomata, the process is not gaseous
exchange, but diffusion. Gaseous exchange refers to the movement of gases between the cells
and their surroundings.

Process of gaseous exchange in leaves

During photosynthesis carbon dioxide diffuses into the palisade cell from the airspaces, because
the concentration of carbon dioxide in the palisade cells becomes lower than in the air spaces
outside the cells.
On the other hand, photosynthesis produces oxygen whose concentration becomes higher inside
the palisade cells than in the air spaces surrounding the cells. Oxygen therefore diffuses out of
the palisade cells into the air spaces.

Fig.: Gaseous exchange in the leaf of a terrestrial plant

The gaseous exchange in stems of woody terrestrial trees and shrubs is made possible by
lenticels. These are formed when the epidermis is replaced by the bark. Lenticels appear.
scattered on the surface of the stem as small, raised openings. They allow gaseous exchange
of oxygen and carbon dioxide between the atmosphere and the internal tissues of the stem.

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Unit 10: Excretion in humans

Learning objectives: After studying this topic, to be able to:
❖ Name excretory organs and excretory products
❖ Outline the structure of kidney and describe a nephron.
❖ Describe the process of urine formation.
❖ Describe the role of the liver in excretion.
❖ Define and explain the need for excretion and develop good habits to maintain healthy
urinary system.

10.1 Need for excretion.

Excretion is the process by which organisms remove waste products of metabolism from the
body such as ammonia, urea, uric acid, carbon dioxide and mineral salts. It is different from
egestion which is getting rid of undigested or unabsorbed wastes from an organism.

Function of excretion
❖ Excretion is necessary to remove excess water, salts, and many other substances from the body.
❖ Through excretion, organisms control osmotic pressure. It also enables them to promote
homeostasis; that is, the balance of the organism’s internal body environment.
❖ Many reactions in cells also producing by-products that could be toxic, it is therefore necessary for these
waste products to be eliminated from the cells.
❖ The waste products if not removed can accumulate to toxic levels resulting to ill health and eventually
death.
E.g: If carbon dioxide, a by-product of respiration, accumulates in the cell, it changes the pH of
the cell, thereby interfering with the functioning of certain enzymes.

➢ Animals have difficulty getting rid of waste products and have a more complex excretory
system than plants for several reasons:
• Animals are more active than plants. Therefore their metabolic processes take place at a
higher rate, producing larger quantities of waste products.
• Animals do not put most of their waste products to other uses like it happens in plants.
• Animals take in certain substances in their food in excess of their needs. These extra
substances, for example proteins, are broken down with the formation of toxic
substances such as ammonia.

10.2 The role of the liver in excretion

Although the kidneys are the main organs of excretion of wastes from the blood, several other
organs are also involved in excretion, including the liver, skin, and lungs.

Table: Excretory organs and their waste products
Excretory organ Excretory products
Skin Urea, lactic acid, excess salts and water in form of
sweat
Kidney Excess salts, excess water and nitrogenous wastes
in form of urine

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Catalase
Lungs CO2 and excess water in form of water vapor

Liver
Bile pigments
Nitrogenous compounds such as ammonia, urea,
uric acid and trimethylamine oxide

The liver is the second largest organ in the body after the skin. The liver has many functions
including maintenance of a constant internal environment (homeostasis) and excretion. The
excretory functions of the liver are described below.

Fig.: Location of liver in the body

a) Deamination:
This is the breaking down of surplus amino acids, end products of protein digestion, in the liver.
From each amino acid, the amino group (-NH2) is converted to ammonia (NH3). Ammonia should
not be allowed to accumulate in the body because it is highly toxic. The remainder of the amino
acid molecule is changed to glycogen or fat for storage.
The ammonia produced from the amino group is quickly converted to a less toxic substance
called urea. During this conversion, ammonia is first combined with carbon dioxide through a
series of enzyme-catalysed reactions. The resulting urea is taken to the kidney in blood and is
eliminated from the body in urine.

b) Detoxification

❖ The liver purifies or detoxifies blood. The inactive substances formed in the liver are
returned to the bloodstream and are finally excreted from the body by the kidneys.
❖ The liver removes harmful substances such as drugs and hormones from the blood.
Within the liver, these substances are converted into inactive or less dangerous forms, for
instance, hydrogen peroxide, a highly toxic by-product of certain metabolic process is
rapidly split into water and oxygen by enzyme catalase in the liver.

2H2O2 (aq) 2H2O (l) + O2 (g)
c) Elimination of Haemoglobin

Haemoglobin from old worn out red blood cells is broken down by the liver cells into pigments.
These pigments are further broken down and eliminated in the bile, giving urine its characteristic
yellowish colour.

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d) Elimination of sex hormones and cholesterol

➢ After sex hormones have performed their functions, some are modified chemically by the
liver cells. Others are sent to the kidney for renal excretion while others are expelled in
bile.
➢ Cholesterol is a sterol lipid (member of a class of steroids playing an important role in
hormone chemistry) synthesized by the liver and transported in the bloodstream to the
membranes of all animal cells but also available in some plants and algae.
➢ It plays a central role in many biochemical processes and, as a lipoprotein that coats the
walls of blood vessels, is associated with cardiovascular disease. If there is a considerable
excess amount of cholesterol in the blood, some may be deposited in the walls of blood
arteries obstructing flow.
Excess cholesterol is also excreted in bile.

10.3 Structure of the human urinary system

➢ The main purpose of the urinary system is to remove urine from the body.
➢ The draining of urine from the bladder, through the urethra and out of the body is known as
urination.
➢ The human urinary system is made up of two kidneys, urinary bladder, two ureters and a single
urethra.
➢ Kidneys are involved in filtering blood and separating waste metabolic substances from it.
➢ The urinary system keeps everything in balance by removing waste, such as urea, extra salt,
extra water and other things the body does not need.
➢ Urea is produced when protein, found in meats, is broken down in the body.

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Human urinary system

10.3.1 Function of parts of urinary system

The two kidneys filter the blood and form urine. Urine is the liquid waste product of the body
that is excreted by the urinary system. Their function is to:
❖ Remove waste from the blood in the form of urine.
❖ Keep substances stable in the blood.
❖ Make erythropoietin, a hormone which helps to make red blood cells.
❖ Make vitamin D active.
❖ Regulate blood pressure.

From the kidneys, urine enters the ureters, which carry it to the bladder. Each ureter is a
muscular tube of about 25 centimeters long. Peristaltic movements of the muscles of the ureter
send urine to the bladder in small spurts. The ureters are two in number whereas the urethra is
single.
The bladder is a hollow organ that stores urine. It can stretch to hold up to 500 milliliters. When
the bladder is about half full, the stretching of the bladder sends a nerve impulse to the sphincter
that controls the opening to the urethra. In response to the impulse, the sphincter relaxes and
lets urine flow into the urethra.
The urethra is a muscular tube that carries urine out of the body. The urethra is longer in males
and shorter in females. Urine leaves the body through another sphincter in the process of
urination. This sphincter and the process of urination are normally under conscious control.

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The kidneys
➢ The kidneys are a pair of bean-shaped, reddish-brown organs about the size of a fist. Each
kidney weighs approximately 142.5 g.
➢ They are located just above the waist at the back of the abdominal cavity, on either side of the
spine.
➢ The kidneys are protected by the ribcage.
➢ They are also protected by a covering of tough connective tissues and two layers of fat, which
help cushion them.
➢ The kidney is supplied with blood from the general circulatory system via the renal artery which
branches off from the aorta.
➢ Blood from the kidney goes back to the general circulation through the renal vein which joins
the vena cava.
➢ The ureter connects each kidney to the bladder located in the lower abdomen.

Internal structure of the kidney

A cross-section of the kidney reveals three main regions: the outer part called cortex, inner part
called medulla and the pelvis.

A cross-section of kidney

A. Cortex: It is the outer part which is dark in colour. It contains a dense network of blood
capillaries that form the glomeruli of nephrons. Nephron is the functional unit of the kidney.

B. Medulla: This part is pale red in colour and lies between the cortex and the pelvis. It contains
several cone-like extensions called pyramids.

C. Pelvis: This part is white in colour. It narrows to form the ureter. Pelvis is a collecting space
leading to the ureter, which takes urine to the bladder.

The nephron

Nephrons are the structural and functional units of the kidneys. A single kidney may have around
1.25 million nephrons. One part of the nephron is in the cortex and the other part in the medulla.

Structure and function of nephron

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Fig.: Structure of the nephron

The nephron has three distinct coiled parts:
✓ The proximal convoluted tubule in the cortex
✓ A U-shaped loop of Henle in the medulla
✓ A distal convoluted tubule in the cortex

✓ The glomerulus (plural glomeruli): a cluster of arteries that filters substances out of the
blood.
✓ Bowman’s capsule: a cup-shaped structure around the glomerulus that collects the
filtered substances.
✓ The renal tubule: a long, narrow tube surrounded by capillaries that reabsorbs many of
the filtered substances and secretes other substances.

10.4 Urine formation

Excretion in the nephron is carried out in two stages: ultrafiltration and selective reabsorption.
Blood coming into the kidney from the artery contains both waste substances and useful
substances. Both substances must enter the nephron, where separation takes place by
ultrafiltration. The body must not lose the useful substances. Therefore useful substances must
be taken back into the blood so that they are not lost. This process is known as selective
reabsorption.

A. Ultrafiltration

Ultrafiltration takes place in the glomerulus. Due to the difference in afferent and efferent
arteriole size, a high pressure of blood is created in the glomerulus. This pressure forces water,
mineral ions and small molecules like glucose, amino acids and urea out of the glomerulus into
the Bowman’s capsule. The liquid collected in the Bowman’s capsule is called glomerular filtrate,
composed of water, salts, glucose, amino acids, and urea. Larger structures in the blood -

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including protein molecules, blood cells, and platelets- do not pass into Bowman’s space.
Instead, they return to the main circulation.

B. Selective reabsorption

Filtrate first enters the proximal tubule. This is where that most reabsorption takes place. Tiny
projections called microvilli line the proximal tubule and increase the surface area for
reabsorption. The proximal convoluted tubule cells are adapted for reabsorption as follows:
- Large surface area due to microvilli
- Numerous mitochondria
- Closeness of blood capillaries
From the proximal tubule, the filtrate passes through the loop of Henle.
The loop of Henle carries the filtrate from the cortex down into the medulla and then back up to
the cortex again. Its primary purpose is to reabsorb water and salt from the fluid. The longer is
the loop of Henle, the more concentrated urine that can be produced.

As the glomerular filtrate passes along the nephron, some substances that are useful to the body
are selectively taken back or reabsorbed into the blood capillaries network surrounding the
nephron.
▪ All amino acids and glucose are reabsorbed by active transport in the proximal
convoluted tubule.
▪ Some salts and water are reabsorbed depending on how much of them the body still
needs. Water is absorbed by osmosis and salts by active transport. Salts are absorbed
mainly in the distal convoluted tubule. Most of the water is reabsorbed in the region of
the collecting duct.
▪ No urea is reabsorbed into the blood.

By the time the filtrate from the glomerulus completes its movement down the nephron, it has a
high concentration of urea, some salts and water. The liquid is now called urine.
Several nephrons empty into one collecting duct, and all the collecting ducts of a kidney empty
into the ureter.
The process of urine formation is a continuous one, and the ureter continuously receives small
amounts of urine. The bladder stores the urine until it is full, then one begins to experience an
un-comfortable feeling. The sphincter muscles must then be relaxed in order to empty the
bladder.

The reabsorption of water by the collecting ducts is controlled by a negative feedback
mechanism. The mechanism involves a hormone secreted by the pituitary gland, called
antidiuretic hormone (ADH). ADH makes the collecting ducts more permeable to water, allowing
more water to be reabsorbed from tubular fluid.

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10.5 Factors that affect urine formation

The volume, colour, odour of urine and frequency of urination is affected by a number of factors
as follows:
Amount of fluids taken: Large intake of fluids lowers the osmotic pressure of blood. This leads
to reduced reabsorption of water in the kidney tubules resulting in the production of large
amounts of dilute urine.

(a) Amount of salt taken: Intake of a salty meal raises the osmotic pressure of blood. This
leads to increased reabsorption of water in the kidney tubules resulting in the production
of coloured, little and smelly urine.

(b) Weather: In hot and dry weather conditions, the body tends to lose a lot of water through
sweating thereby raising the osmotic potential of blood. In this case a lot of water is
reabsorbed resulting in coloured, little and smelly urine.
During cold weather the frequency of urination increases because sweating is so minimal.

(c) Physical activity: During an exercise like running, jumping and playing, we sweat a lot. The
kidney reabsorbs more water resulting in little, coloured and smelly urine.

(d) Diseases: Certain diseases that affect the secretion of hormones that control reabsorption
of water in the kidney tubules can either lead to production of large or small amounts of
urine.

10.6 Diseases of urinary system

Kidney stones: There are hard solids deposited in the kidney. Are formed due to precipitation of
calcium salts. The stones block urine tubes, which eventually interferes with the functioning of
the kidney
Incontinence: Involuntary urination. Common in infants and children due to the predominance of
the involuntary behavior- concentration and relaxation of sphincter muscles, over-conditioned
contraction in early age. In old age, it can appear where the sphincter muscles fail to control the
urethra opening. Incontinence can also be observed in women after child delivery.

Polyuria: An individual urinates large quantities. This is associated with dieresis. It is due to
malfunctioning of the pituitary glands releasing less ADH.

Kidney failure: Due to the fall in the blood pressure, affecting the rate of filtration of blood
plasma.
This leaves the blood with a large amount of wastes, since blood filtration is favored by high
blood pressure of blood.

Dysuria: Characterized by pain during urination. It normally occurs where there is a urinary
infection resulting from bacteria and fungi.

Oedema: Results from the retention of watery fluid caused by excess NaCl that is not eliminated
and spreads across the tissues.
Uraemia: Inflammations of kidneys due to the accumulation of urea in the blood

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Gout: It is due to deposit of uric acid in the joints
Renal colic: Very painful, renal colic is caused by calculi- crystals that accumulate in the pelvis
because of a low dilution of uric acid or salts.

10.7 Practices that maintain healthy urinary system

(1) Avoid holding urine for a long time, since this overstretches the blood and weakens the
sphincter muscles.
(2) Drinking a lot of water, at least 10 glasses of water a day to flush out toxins in the body.
This facilitates the process of dissolving wastes in large amounts and early filtration out of
the blood.
(3) Exercising regularly to keep fit and avoid smoking and alcohol intake.
(4) Avoid taking too many drugs especially pain killers. Stick to prescriptive drugs from a
qualified medical officer.
(5) Eat healthy by avoiding junk food. Eat more fresh fruits and green vegetables. Choose
foods low in sodium, sugar and fats but high in fiber content.
(6) Visit a doctor (urologist) regularly to check the health of the urinary system.

Unit 11: Joints and movement

Learning objectives: After studying this topic, to be able to:
❖ Analyse the structure of immovable and movable joints
❖ Outline the function of bones, ligaments, tendons, cartilage, nerves and synovial fluid in
the joint
❖ Differentiate between hinge joint and ball, and socket joint
❖ Demonstrate how antagonistic muscles bring about movement at the hinge joint
❖ Appreciate the link between skeletal muscles and the bones in movement
❖ Develop good habits that maintain safety of the body joints

11.1 Common terms

➢ Bone: Is a hard, tough connective tissue composed of mineral salts such as calcium and
phosphate. It is abundant in all animal skeletons.
➢ Cartilage: It is a skeletal connective tissue which is softer than a bone. It supports the trachea,
nose, oesophagus and pinna of the ear.
➢ Ligament: This is a fibrous tissue which joins one bone to another. Ligaments are elastic to
allow movement at a joint.
➢ Tendon: A tough connective tissue which attaches a muscle to a bone. They are inelastic.
➢ Muscle: A contractile tissue specialised for contraction and relaxation. They cover the skeleton.
Muscles are responsible for locomotion and other type of movement in animals.
A joint (also called an articulation), is a point at which two or more bones make contact. They
are constructed to allow movement and provide mechanical support for the body, except for
pelvic, sacral, skull and sternal bones.

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11.2 Types of joints

Movement is important in determining the type of joint that develops. The functional
classification gives mainly two types of joints: immovable and movable joints.
• Immovable joint or fixed joint: bones are connected by dense connective tissue, which is
usually collagen. Immovable joints, like those connecting the cranial bones, commonly known as
sutures, have edges that tightly interlock, and do not allow movement.
The connective tissue at immovable joints serves to absorb shock that might otherwise break the
bone.

• Movable joints: They allow movement of body parts to take place. Some movable joints allow
only a small degree of movement while others allow a wide range of movement.
(a) Gliding joints: they consist of two opposing flat surfaces that allow slight amount of
gliding motion. They have no fluid between them but instead have a larger cartilage
between them known as intervertebral disc which reduces friction during movement.
They are found in wrist and ankle.
(b) Synovial joints: They are the most movable. The word "synovial" comes from the Latin
word for "egg". Synovial joints contain a space between the bones of the joint (the
articulating bones), which is filled with synovial fluid. The fluid reduces friction between
the articular cartilage and other tissues in joints and lubricates and cushions them during
movement. There are many different types of synovial joints, and many different
examples.
i) Saddle joints: It is a joint which allows one bone to slide over another, such as
between the carpels of the fingers and comprised of two saddle-shaped articulating
surfaces that are oriented at right angles to each other. Example is the joint of the
thumb.

Fig.: Saddle joint

ii) Hinge joints: In the hinge joint, the articular surfaces fit together in such a way as to
permit motion only in one plane, forward and backward, the extent of motion at the
same time being considerable. The joints at the elbow, knee and finger joints show this
back and forth movement as the only type of movement.

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Fig.: Hinge joint of the knee

iii) Pivot joints: is formed by a process that rotates within a ring, the ring being formed
partly of bone, and partly of ligament. An example of a pivot joint is the joint
between the radius and ulna that allows you to turn the palm of your hand up and
down.

iv) Ball and socket joint: It consists of a ball-shaped head end of one bone that fits into
the cavity (socket) of an adjacent bone. This type of joint allows wide range of
movement in almost any direction. The shoulder and hip joints are examples of a ball
and socket joint.

fig.: Ball and socket joint at the hip
The table below summarizes the types of joints

Categories Type and description Examples
Immovable Sutures Between cranial bones
Slightly
movable
Symphysis: disc of fibrous cartilage
between bones
Between vertebrae; between pubic
bones

Freely
movable
Ball-and-socket: movement in all
planes
Scapula and humerus; pelvic bone
and femur
Hinge: movement in one plane Humerus and ulna; femur and tibia
Pivot: rotation Atlas and axis; radius and ulna
Gliding: side-to-side movement Between tarsals and phalanges
Saddle: movement in several
planes
Carpo-metacarpal of thumb

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Structurally, joints are classified as follows:

(a) Cartilaginous joints- the bones are connected by cartilage. They are slightly movable.
(b) Fibrous- the bones are connected by dense fibrous tissue rich in collagen.
(c) Synovial- there is a space between the bones called synovial cavity that is filled with a
fluid known as synovial fluid.

11.3 Action of antagonistic muscles in the movement of a hinge joint

➢ Antagonistic muscles occur in pairs and oppose a specific movement such that when one
muscle contracts, the other relaxes. This means that they never contract or relax at the
same time.
➢ The biceps and triceps are referred to as antagonistic muscles. They are found on the
upper part of the forearm. To lift the arm, the biceps contracts while the triceps relaxes
➢ The movement of the arm is purely brought about by the opposing actions of the biceps
and triceps.
➢ Joints provide a fulcrum point for muscles to act.
➢ The joint action in the upward movement of the arm is flexion resulting to the decreasing
of the joint angle.
➢ Biceps muscles are therefore called flexor muscles.
➢ To straighten the arm, triceps muscles contract while biceps relaxes; the joint action is
extension that results to increase in the joint angle.

11.4 Practices that promote healthy bones and joints

Simple things can be done to reduce your likelihood of developing conditions such as
osteoporosis (=disease, occurring especially in women following menopause, in which the bones
become extremely porous and are subject to fracture) and arthritis (=Inflammation of joints
causing pain and/or disability, swelling and stiffness) or at least prevent them from becoming
worse.

1. Eating healthy food appropriate for bone health, for example:
(a) Incorporating more calcium rich foods into the diet. Foods rich in Calcium are milk, dodo
and cabbages, cereals and bread
(b) Eating foods that contain Vitamin D. Foods that contain vitamin D include: egg yolk and
beef liver. Vitamin D is also synthesized when our skin get exposed to the sun.

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(c) Getting enough of Vitamin C, which is necessary in repairing tissues, including the cartilage
in joints. Foods that are rich in vitamin C include: fruits, such as oranges, lemons,
watermelon, sweet potatoes and tomatoes.
2. Avoiding alcohol consumption. Taking alcohol interferes with the body’s ability to absorb
vitamins and minerals. It also results in an increase in hormones that deplete bone density.
3. Avoiding smoking. Smoking has been shown to cause bone mass depletion besides many other
health problems.
4. Maintaining a healthy body weight and posture. If you are underweight, you are at a higher
risk of bone loss. If you are overweight, you may be causing extra stress on your joints. This may
put you at risk of developing osteoarthritis.
5. Plenty of physical activity and exercises such running, walking, dancing, playing. They help the
body to build more bone mass and maintain the bone density.

Unit 12: Infectious diseases

Learning objectives: After studying this topic, to be able to:
❖ Define pathogen, transmissible disease, host
❖ Explain the symptoms, prevention and control of malaria, Ebola and HIV/AIDS
❖ Explain how the body’s defense mechanism can be enhanced by vaccination
❖ Apply practices of hygiene and sanitation to control the spread of diseases

12.1 Definition of some common terms

i) Good health: state of mental, social, physical and emotional well-being.
ii) Disease: any physical or mental disorder or malfunction with characteristic set of signs and
symptoms. A disease is a particular abnormal condition, a disorder of a structure or function that
affects part or all of an organism.
iii) A condition is a permanent health problem that affects a particular part of the body.
iv) An infection is an illness that is caused by microorganism. It can affect one part of the body
and spread from one person to another.
v) Endemic disease: a disease that is always present in an area. E.g Malaria in East Africa;
HIV/AIDS
vi) Epidemic disease: a sudden appearance of a disease, especially an infectious one that affects
a large number of people within a population at the same time. E.g Meningitis
vii) Pandemic disease: a disease that affects almost everyone in a very large area. E.g. influenza
(flu, common cold)
viii) Chronic disease: A disease that lasts for a long time, usually at least six months. A chronic
disease may be stable (does not get any worse) or it may be progressive (gets worse over time).
ix) Progressive disease: a disease whose typical natural course is the worsening of the disease
until death, serious debility, or organ failure occurs.
x) Pathogen (also known as germ): It is an agent that can cause a disease, e.g: bacteria,
protozoa, and viruses
xi) Transmissible disease: These are diseases capable of spreading from one person to another;
e.g; Ebola, common cold.
xii) Host: An organism that harbours a disease-causing micro-organism, for example, water snails
are hosts to schistosomes that causes bilharzia.

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xiii) Vector: A living organism that can transmit infectious diseases between human beings or
from animals to human beings. Female anopheles mosquito is a vector to the plasmodium
parasite malaria.
xiv) Carrier: A person who has been infected but develops no signs or symptoms. This person is
capable of transmitting the disease to other people.
xv) Signs: Visible expressions brought about by a disease, for example, red spots on the body.
xvi) Symptoms: A physical or mental feature that is regarded as indicating a condition of disease,
particularly such a feature that is felt by the patient, for example, nausea, dizziness and
headache.
xvii) Epidemiology: The study of all factors that contribute to the appearance of a particular
disease.

12.2 Transmission of infectious diseases

Infections can be spread in several different ways. Some of them are listed below
a) By micro-organisms: Some diseases are caused by micro-organisms. The micro-
organisms that cause the diseases are known as pathogens and include varieties of
bacteria, viruses, protozoa and fungi.
b) Through direct contact, through blood or other body fluids;
c) Though indirect contact, hand-to-mouth contact with infectious material on surfaces,
d) by bites of insects, vectors (vector-borne) or other carriers of the disease,
e) From contaminated water (water-borne), air (air-borne), atmosphere or food (food-
borne) often via fecal contamination), etc.
f) Sexual intercourse. (STD/STI) E.g. HIV/AIDS, Syphilis, Gonorrhea,

12.3 Defense against infection

The body has many ways of defending itself from invasion by harmful micro-organisms. In most
cases the body’s defense mechanism prevents this from happening.
Some of these defense mechanisms are made in a way to block pathogens from entering into the
body. Other mechanisms eliminate pathogens that are already in the body. There are also
defense mechanisms that attack micro-organisms when they persist inside the body.

➢ Immunity is the ability of the body to resist disease infection. This means a person can be
exposed to factors that can cause a disease, yet they do not become sick. The body’s
defense mechanism protects an organism from infections.
➢ Resistance is the ability of the body to prevent itself from diseases using its lines of defense.
If pathogens do not encounter resistance from the body’s defense mechanism, almost all
diseases would be fatal. Organisms must find a way of defending themselves against harmful
micro-organisms like viruses, bacteria and fungi.
Body defense mechanisms can either be first line or second line.

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(a) Body’s first line of defense
Is a combination of physical and chemical barriers that prevent all types of foreign agents from
penetrating the outer layer of the body.
This line of defense prevents harmful micro-organisms from entering the body. It is the external
defense system of the body. Some examples are discussed below:
i) The skin. It creates a physical barrier that protects the cells inside the body against the
entrance of pathogens. A healthy skin is rarely penetrated by pathogens. Certain
chemical secretions produced by the skin helps to stop the growth of bacteria and
fungi.

ii) The nose and passages leading to lungs. They are lined with cells that produce sticky
fluid called mucus that traps invading microbes and dust. Tiny hairs called cilia lines
the trachea. They can move back and forth in a wave-like motion. Cilia trap microbes
and dust particles and prevent them from entering the lungs. These particles then
combine with mucus and are either coughed, sneezed out or swallowed and then
passed out of the body in faeces.
iii) The stomach: It produces hydrochloric acid which destroys many of the microbes that
enter the body in food and drinks we take.
iv) Tears: They act as a barrier to pathogens. Tears are a watery secretion produced by tear
glands located in the outer eye. They contain a powerful enzyme that can digest and
breakdown harmful substances. Therefore, harmful micro-organisms cannot enter the
body through the opening of the eye.
v) Clotting of blood: occurs when an open cut or wound exposes blood to air. Such a cut
causes a break in the skin exposing the body to harmful micro-organisms. When a clot
forms, it seals the opening and enables the wound to heal.

The body’s first line of defense mechanisms.

Fig. :

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(b) Body’s second line of defense

This is a group of cells, tissues and organs that work together to protect the body. It is the
immune system.
Once the first line has failed to protect the body, the second line of defense is activated. This
happens through a sequence of steps called immune response. White blood cells, known as
phagocytes, are adapted to fight the disease causing micro-organisms by surrounding invading
microbes and engulfing them, by releasing digestive enzymes which breakdown the trapped
microbes before they can do any harm.

Fig.: Phagocytosis in white blood cells
Immunity

Immunity is the ability of an organism to resist a particular infection or toxin by the action of
specific antibodies or some white blood cells.
An antibody is a protein produced by the body’s immune system when it detects harmful
substances, called antigens such as bacteria, fungi, parasites, viruses, and chemicals. Each type of
antibody is unique and defends the body against one specific type of antigen.
Antibodies detect and neutralises the antigen by binding to it. This signals the production of
other cells of the immune system to get rid of the invading microbes before it spreads or attacks
the body.

Antigen-antibody reaction

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Types of immunity

Types of immunity (Natural and artificial immunity)

i) Natural immunity
a) Active natural immunity: The organism makes its own antibodies as a result of contact with
antigen from disease-causing pathogen. Once the organism recovers from the disease, it can
produce antibodies very quickly should the pathogens invade again.
b) Passive natural immunity: This is the immunity acquired by the foetus from the mother
through the placenta and also through breast milk. This type of immunity is short-lived.

ii) Artificial immunity is obtained by introducing antigens into the body of an organism to
protect the organism from a disease.

a) Active artificial immunity: Is induced by introducing antigens into the body of an organism
through the use of a vaccine. A vaccine contains antigens composed of living, dead or weakened
pathogens. They are used to stimulate the body to recognise certain disease antigens and
respond to them. Vaccines usually do not cause the disease.

b) Passive artificial immunity: Is the transfer of immunity in the form of ready-made antibodies.
This is immunity that comes from using antibodies produced in one organism to protect another
organism from a specific disease. These antibodies are usually extracted from the serum (plasma
without soluble proteins) of an animal that has recovered from the disease.
Such immunity does not last long.

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12.4 Symptoms, prevention, control and treatment of common infectious diseases

Disease Type Cause/transmiss
ion
Symptoms Prevention
Malaria, a
protozoan
disease. Infants,
children under 5
years, pregnant
women and
people with
HIV/AIDS are
more exposed.
Infectious,
vector
borne
disease
The most
common
forms of
human
malaria
are caused
by
Plasmodiu
m
falciparum
, P. vivax,
P.
knowlesi,
and P.
malariae.
Anopheles
mosquito.
Several parasites
cause the
disease. The
most dangerous
are P. falciparum
and P. vivax
Plasmodium
falciparum,
common in sub-
Saharan Africa,
and P knowlesi,
common in
Southeast Asia,
are especially
dangerous.
Sensation of cold,
shivering
Headaches,
vomiting, fever,
pain in joints,
general body
weakness,
anaemia
Sleeping in the
Mosquito nets; clearing
bushes and treatment of
standing water; fish-
eating mosquitoes,
Spraying of households
with insecticides to
control mosquitoes
(Indoor Residual
Spraying), medication
HIV/AIDS. Viral
disease
As the virus
destroys and
impairs the
Infectious,
Std.
opportuni
stic
diseases
Sexual
intercourse;
Mother to child
transmission;
transfusion of
Fever and
diarrhea for more
than an month,
unexplained
weight loss of
Abstinence; faithfulness;
use of sterilized
instruments, Anti-
retroviral drugs for
patients of HIV/AIDS;
function of
immune cells,
infected
individual
become immune-
deficient
are those
which
develop
following
the
immuno-
deficiency
infected blood or
transplant of
infected organs;
Sharing
unsterilized skin
piercing tools
over 10% of body
weight,
tuberculosis,
meningitis,
lymphoma,
Kaposi’s sarcoma
Mothers should give
birth to hospitals and go
for regular checkup;
screening blood before
transfusion.

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Tuberculosis
(TB)
People with low
immune systems,
such as people
living with HIV
and AIDS,
malnutrition,
diabetes, or
people who
smoke, have a
much higher risk
of being infected.
Infectious,
Air-borne
disease
A bacterium,
Mycobacterium
tuberculosis.
One can also be
infected by
drinking raw milk
from a cow
suffering from
bovine TB
Expectoration of
mucus and
sputum;
Fever;
Weight loss;
Chest pain; night
seats
Avoiding overcrowded
places;
Avoiding taking raw
milk. Boil or pasteurize
milk before
consummation;
Covering one’s
mouth/nose when
sneezing; isolate
patients,
BCG vaccine; anti-TB
drugs
Typhoid
Typhoid fever is
highly
contagious. A
small number of
people (carriers)
recover from
typhoid fever but
continue to have
the bacteria.
Infectious, Salmonella typhi.
The bacteria are
spread by eating
food or drinking
water
contaminated
with faeces or
urine of patients
and carriers.
High fever,
headache,
splenomegaly
causing
abdominal
discomfort,
constipation or
diarrhea,
decreased
appetite
Isolating patients,
proper disposal of
faeces in toilets; water
treatment and
purification, washing
hands properly after
visiting toilets,
vaccination; antibiotic
treatment
Cholera.
It affects both
children and
adults and can
cause death
within hours.
Infectious,
water
borne
disease
Vibrio cholera.
Cholera is
endemic in poor
countries and
strikes areas with
poor sanitary
conditions and
where drinking
water is not well
treated.
Diarrhea,
vomiting, rapid
heart rate, loss of
skin elasticity;
wrinkled skin and
sunken eyes
because of
dehydration
Drink and use safe
water, washing hands
with soap and safe
water, use of latrines or
bury feces; not to
defecate in water or on
the ground, cook food
well, keep it covered,
vaccination during
epidemics, use of
antibiotics
Ebola Infectious,
spreads by
contact
with the
skin or
Ebola virus,
people remain
infectious as long
as their blood
contains the
Hemorrhagic
Fever, headache,
weakness,
stomach, lack of
appetite, internal
No cure for Ebola, avoid
contact with infected
ones;
Avoid getting into
contact with wild
bodily
fluids of
an
infected
animal.
virus. and external
bleeding with
blood oozing from
the gums, ears
and eyes, bloody
diarrhea, impaired
kidney and liver
function
animals or eating raw
meat, burial of dead
patients should be done
by health and protected
workers.

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Unit 13: Immunity and vaccination

Learning objectives: After studying this topic, to be able to:

❖ Define active immunity, pathogen and antibody production in the body
❖ Demonstrate the importance of passive immunity for breastfed infants
❖ Observe and differentiate the immune responses
❖ Use a diagram to illustrate antibody-antigen reaction sites
❖ Advocate for vaccination and breastfeeding as a sustainable disease prevention method

The Ability of the body to defend itself against foreign bodies and disease-causing micro-
organisms is known as immunity. The system that is responsible for defending the body against
diseases is known as the immune system.
The immune system consists of specific cells (lymphocytes, macrophages, etc.), lymphatic organs
(thymus, spleen, tonsils, lymph nodes) and diffuse lymphatic tissue-collections of lymphocytes
and other immune cells dispersed in the lining of the digestive and respiratory tracts and in the
skin whose function is to protect the body from numerous pathogens and toxins in our
environment.

13.1 Antibodies and antigens

An antibody also known as an immunoglobulin is a large Y-shaped protein produced mainly by
plasma cells. It is used by the immune system to identify and neutralise pathogens such as
bacteria and viruses. The antibody recognises a unique molecule of the harmful agent, called an
antigen. The antibody then binds to this antigen. Antibodies are specific to their antigens.

Structure of antibody and antigens

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The prototype of an immunoglobulin molecule is made up by heavy chains, light chains,
constant and variable regions, carbohydrates and disulfide linkages.
Antigen is any substance that is capable of stimulating an immune response, specifically
activating the body to produce antibodies. (= antibody generator).
We distinguish foreign antigens from self-antigens.
Foreign antigens come from outside the body. All pathogens (viruses, bacteria, protozoa) and
certain proteins are examples of foreign antigens.
Self-antigens come from within the body. In individuals with autoimmune disorders, normal
body substances provoke an immune response, leading to the generation of auto-antibodies.

Antigen-antibody reaction

➢ All antibodies are Y-shaped. With its two-armed Y-shaped structure, the antibody can attack two
antigens at the same time with each arm.
➢ They however, differ at the antigen binding site. Each antibody has a site complementary to a
certain antigen. This means that each antibody can only bind to a specific antigen.
➢ An antigen and antibody reaction works like a lock and key mechanism.
➢ Antibodies themselves do not destroy antigens but they inactivate and tag antigens for
destruction by phagocytes.
➢ All antibodies form an antigen-antibody (immune) complex when they combine with antigens.
Formation of an antigen-antibody complex

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Defensive mechanisms used by antibodies are neutralization, agglutination, precipitation and
plasma complement system.
❖ Neutralization: Antibodies bind to and block specific sites on antigens, viruses and
bacteria. This prevents antigens from binding to sites called receptors on tissue cells. They
are later destroyed by phagocytes.
If the antigen is a toxin produced by pathogenic bacteria that cause an infection, the binding
process of the antibody will neutalize the antigen's toxin.
❖ Agglutination: Antibodies bind the same determinant on more than one antigen. An
example is when an antibody surrounds a virus, such as one that causes influenza, it
prevents it from entering other body cells.
❖ Precipitation: soluble molecules are cross-linked into large insoluble complex. After which
they fall out of solution and are phagocytized.
❖ Plasma complement system: The antibodies coat infectious bacteria and then white
blood cells will complete the job by engulfing the bacteria, destroying them, and then
removing them from the body.

13.2 Immune response

Immune response refers to the process by which the body recognises and defends itself against
bacteria, viruses and substances that appear foreign and harmful.
The immune system is a collection of cells, tissues and molecules that protect the body from
numerous pathogens and toxins in our environment.
Immunity can be innate (inborn) or acquired (adaptive). The acquired one can be either active
or passive. In active immunity, the body makes its own antibody against a pathogen whereas
in passive immunity the body acquires antibodies produced by another person or an animal.
Either type of immunity can occur naturally or, for treatment and prevention purposes, it can be
induced artificially.

a) Innate immunity
Innate immunity also called non-specific immunity is the body defense system that one is born
with. Innate immunity involves barriers that keep harmful materials from entering your body. It
consists of cells and proteins that are always present and ready to fight microbes at the site of
infection.
It includes the components of the body’s first line of defense; the main components are:
i. Physical epithelial barriers
ii. Phagocytic leukocytes
iii. Dendritic cells
iv. A special type of white blood cells called a natural killer (NK) cell
v. Circulating plasma proteins

b) Adaptive immunity
Adaptive immune system, also known as acquired immunity, is immunity that develops after
exposure to various antigens.
It is called into action against pathogens that are able to evade or overcome innate immune
defenses.

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The immune system includes certain types of white blood cells, for example, lymphocytes. There
are B (formed in the bursa of birds and the bone marrow of other animals) and T (from the
Thymus) type lymphocytes.
✓ B lymphocytes are cells that produce antibodies. Antibodies attach to a specific antigen
and present it to other immune cells for destruction.
✓ T lymphocytes are cells that attack antigens directly and help control the immune
response.
✓ The thymus and bone marrow are known as primary lymphoid tissues. Secondary
lymphoid tissues, namely the lymph nodes, spleen, tonsils and mucosa-associated
lymphoid tissues are important sites for generating adaptive immune responses and
contain the lymphocytes.

Once B cells and T cells are formed, a few of those cells will multiply and provide "memory" for
the immune system. This allows the immune system to respond faster and more efficiently the
next time an individual is exposed to the same antigen.

Active immunity.

This is the immunity that results from the production of antibodies by the immune system in
response to the presence of an antigen. It is either natural or artificial.
• Naturally acquired active immunity occurs when a person is exposed to a live pathogen. The
individual develops the disease and becomes immune as a result of the primary immune
response.
• Artificially acquired active immunity can be induced by a vaccine, a substance that contains
alternated form of the antigen. A vaccine stimulates a primary response against the antigen
without causing symptoms of the disease.

Passive immunity

This is a short-term immunity that results from the introduction of antibodies from another
person or animal. Passive immunity is either artificial or natural.
• Artificially acquired passive immunity is a short-term immunization. It is achieved by the
injection of antibodies that are not produced by the recipient's cells.
• Naturally acquired passive immunity occurs during pregnancy, in which certain antibodies are
passed from the maternal body into the foetal bloodstream.
✓ Infants have passive immunity because they are born with antibodies that are transferred
through the placenta from their mothers. They also get some antibodies from the
mother’s breast milk. These antibodies disappear between ages 6 and 12 months.

✓ Passive immunisation may also be due to injection of antiserum, which contains
antibodies that are formed by another person or animal. It provides immediate
protection against an antigen but does not provide long-lasting protection. E.g: BCG
given to a child at birth to protect it from TB is an example of passive immunisation.

13.3 Immunisation and auto-immunity

Immunization is the process by which an immune response is triggered by the administration of a

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vaccine towards an infectious disease. Small amounts of an antigen, such as dead viruses, are
given to activate immune system "memory". Memory allows the body to react quickly and
efficiently to future exposure. Such small doses of antigens are referred to as vaccines.

Importance of immunization
-Immunization is an important method of disease prevention.
-By receiving a vaccination, usually in the form of an injection, a person can be 'immunised'
against a disease and reduce their likelihood of developing the illness.
-Immunization can also reduce the spread of the disease in the population.

Auto-immunity

Autoimmunity is a condition whereby the immune system attacks and kills own cell. Any disease
that results from such an immune response is termed an autoimmune disease.
An efficient immune response protects against many diseases and disorders. Wrong immune
response causes immune system disorders. An overactive immune response can lead to the
development of autoimmune diseases, in which antibodies form against the body's own tissues.
Complications from altered immune responses include:
i. Allergy or hypersensitivity
ii. Autoimmune disorders
iii. Immunodeficiency disorders
iv. Serum sickness
v. Transplant rejection

Type 1 diabetes and the immune system
This is a chronic condition in which the pancreas produces little or no insulin. Insulin is a
hormone needed to enable sugar (glucose) to enter cells to produce energy.
The pancreas is the organ responsible for controlling the amount of sugar (glucose) in the body.
Specific cells in the pancreas known as the beta cells are responsible for manufacturing insulin.
Insulin released into blood regulates the amount of sugar in the body keeping it at normal levels.

Once the body is attacked by the virus having same antigens like those of the beta cells in the
pancreas, the T-cells mistakenly recognise beta cells as foreign to the body. They attack these
cells and destroy them.
Once the beta cells of the body are destroyed, no insulin is produced. This leads to an increase in
blood sugar level and therefore increases the possibility of having diabetes.
Those viruses having same antigens like those of the beta cells include:
• German measles
• Mumps
• Rotavirus (which generally causes diarrhea)
Note: Type 2 Diabetes mellitus occurs when the body cells become resistant to insulin or when
less insulin is produced by the body.

Unit 14: Sexual behavior and sexual response

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Learning objectives: After studying this topic, to be able to:
❖ Describe male and female responses to sexual stimulation.
❖ Outline responses to sexual attraction and stimulation at puberty by boys and girls
❖ Recognize that sexual relationships require emotional and physical maturity.
❖ Develop self-confidence and control towards sexual thoughts and feelings.

Sexual stimulation is any stimulus (including bodily contact) that leads to, enhances and maintains
sexual arousal, and may lead to orgasm (=a pleasant sensation considered as the climax or peak of
sexual pleasure occurring during sexual activity which may include in males’ ejaculation and vaginal
contractions in females).
The term sexual stimulation often implies stimulation of the genitals, but may also include
stimulation of other areas of the body, stimulation of the senses (such as sight or hearing) and
mental stimulation (i.e. from reading or fantasizing).
Sufficient stimulation of the penis in males and the clitoris in females usually results in an orgasm.

14.1 Male and female sexual responses

The sexual response cycle refers to the sequence of physical and emotional changes that occur as a
person becomes sexually aroused and participates in sexually stimulating activities.
The sexual response cycle as has four distinct phases: excitement, plateau, orgasm, and resolution.
Both males and females may experience these phases during sexual activity, but the duration of the
phase may differ.

14.1.1 Excitement
The excitement phase is the first phase of the sexual response cycle. It varies from person to person
and situation to situation. The characteristics of the excitement phase can last from less than a
minute to several hours and include the following signs:
• Myotonia (muscle tension increases)
• Heart rate and blood pressure increase
• Breathing accelerates
• Nipples become erect
• Vaginal lubrication begins and the labia majora begin to separate.
• Female breasts enlarge
• Male testes swell and the scrotum tightens
14.1.2 Plateau: This is a phase that prepares both man and woman for orgasm. Within this phase, the
general characteristics of the excitement phase continue, but become more intensified. These
characteristics include the following:
• Increases in muscle tension spasms of the feet, hands, and face
• Continued increase of heart rate, blood pressure, and breathing rate

14.1.3 Orgasmic phase
This is the climax of the sexual response cycle. It is the shortest of the phases and generally lasts only
a few seconds. This phase is accompanied with sexual pleasure and satisfaction with release of sexual
tension.
General characteristics of this phase include:
• Blood pressure, heart rate and breathing which are at their highest rates, accompanied by a
rapid intake of oxygen.

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• In a female, the vaginal muscles contract and the uterus undergoes repeated contractions.
• In a male, repeated contractions occur at the base of the penis and semen is ejaculated.

14.1.4 Resolution
In this phase, the body slowly returns to its original, unexcited state. Body parts return to their
normal size and hue. Some of the changes occur rapidly, whereas others take more time. The
resolution phase is often accompanied by a general sense of well-being, intimacy, and fatigue. The
resolution phase begins immediately after orgasm if there is no additional stimulation.

14.2 Puberty in boys and girls

During puberty, growing boys or girls undergo sexual maturation and become capable of
reproduction. Normally, before puberty, there is no big physical difference between males and
females apart from their sexual organs.
Puberty comes as a result of hormones that originate from the gonads: testicles in men and ovaries in
females.
Teenagers normally face challenges during this period of growth and development. The greatest
challenge is increase in sexual desire in both females and males which can result to unwanted
pregnancies and STDs.

14.2.1 Puberty in boys

Puberty in males takes place between the ages of 11 and 13 on average. When a boy reaches puberty
at an early stage, he may have benefits such as popularity with his fellow peers because of being tall
and therefore a leader. Late onset of puberty can lead to low self-esteem in males and such may also
face problems of anxiety and fear. Sexual desire is at its peak. Boys start dating girls at this stage.
This is a critical time where boys need guidance and counseling because most of them face problems
associated with increased body activity and sexual desire. The problems faced at this stage include:
✓ Abuse of drugs and alcohol
✓ Lack of respect for authority
✓ Fighting among peer groups
✓ Parental hatred
✓ Poor hygiene and general body cleanliness.
In order to avoid or control such emotional conditions, adolescents should keep busy by joining
useful groups. Such groups include: creative clubs, sports clubs, religious organisations and
community developments groups.

14.2.2 Puberty in girls

On average, girls begin puberty at an early age of about 10-11years. Puberty in girls occurs fast as
compared to boys. Menstruation in girls is a characteristic that shows the onset of puberty.

Early onset of puberty in girls is detrimental, causing low self-esteem and stress. This is due to breast
enlargement and a lot of fat deposition on the body that causes different body shapes between the
girl and her peers.
Early onset is caused by a number of factors which include diet and environmental exposure.

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During puberty:
Girls tend to become more private and sensitive emotionally. Their social behaviors tend to change.
They become more attracted to males than to females.
➢ Parents should therefore be more careful.
➢ The main challenge that girls face during puberty is unwanted pregnancies. Other challenges
include rejection from peers, clothes to wear and physical appearance.

14.3 People and sexual relationships

People develop different types of relationships ranging from acquaintances to friendships to
romantic relationships.
It is important to note that sexual relationships between close members of a family is unacceptable.
It is considered a taboo in the society and is also punishable by law. This form of sexual relationship is
called incest. If it happens, children born normally suffer from genetic disorders and lack of vigor.

The following are forms of sexual relationships:
a) Exogamy: this is about finding one’s mates outside their family circles since incest is a taboo.
Endogamy is the opposite of exogamy where individuals are allowed to marry their distant
cousins.
b) Monogamy: This is a long-term relationship between a man and a woman in which none has other
sexual partners. In the Rwandan society, monogamy is the legal form of sexual relationship.

c) Polygamy: This is a long term sexual relationship of having more than one wife or husband at the
same time. The Rwandan government is not in support of polygamous sexual relationships since
these relationships are source of sexually transmitted diseases and also contribute to poor family
planning that hinders development in families.
d) Casual relationships: This is an act of having sex with someone you do not have an attachment to.
Casual relationships are dangerous since they can be sources of sexually transmitted diseases
including HIV and AIDS and other forms of sexual and physical abuse.
e) Dating relationships: This is when two people (a boy and a girl) go out together. It leads to
engagement and later marriage. Dating should be done carefully. You should avoid sexual intercourse
until at appropriate time e.g. after marriage. Sexual intercourse can lead to unexpected pregnancies
or STDs such as HIV and AIDS.
Partners in a healthy relationship should have the following characteristics:
• Treat each other with love and respect.
• Be honest to each other.
• Take special interest in activities that each likes.
• Respect each other’s emotional, physical and sexual limits.

14.4 Sexual problems and disappointment

A sexual problem or sexual dysfunction refers to a problem experienced during any phase of the
sexual response cycle preventing the individual or couple from experiencing satisfaction from the
sexual activity.
In males

✓ Premature ejaculation: This is a condition whereby a man cannot delay ejaculation long
enough to satisfy the woman.

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✓ Impotence: This is the inability to produce or maintain an erection. Impotence occurs as a
result of fear, anxiety, disease such as diabetes and drug abuse.
✓ Inflammation: Blockage of the urethra which occurs as a result of an infection or due to
swelling of the prostate gland.

In females

✓ Vaginal discharges: Any abnormal discharge from the vagina. It can be a white discharge with
irritating smell especially after sexual intercourse.
✓ Functional disorders: This is the inability of a woman to experience sexual pleasure and
satisfaction. This results from emotional problems, feeling of guilt and trauma.

14.5 Sexuality, age and culture

Sexual behaviors are based on the morals and values of the particular society as it comes to the
society to dictate the norm, what is considered acceptable or unacceptable.
The following sexual behaviors are unacceptable in the Rwandan culture and constitute a crime:

i) Incest: This is the act of having sex with a close relative, especially immediate family members and
first cousins. Children born from incest normally suffer from genetic disorders and lack of vigor.
ii) Homosexuality: The state of being sexually and romantically attracted primarily or exclusively to
persons of the same sex. People of the same sex are engaged in a sexual relationship. This state is
linked to many other health problems: depression, drug and alcohol abuse, sexual abuse, higher risks
of HIV and other sexually transmitted infections, death of a society (as no child is produced from
homosexuality)
iii) Masturbation: Manual erotic stimulation of the genitals or other erotic regions, often leading to
organism, either by oneself or a partner. It is the act of self-stimulation that leads to sexual
satisfaction. Masturbation leads to other psychological, physiological, mental and social problems,
such as thinning of hair and hair loss, premature and early ejaculations, inability to control the
release of urine, impotence, low sperm count and sperm leakage, vaginal discharge, dryness and
infections in females, ….
iv) Pornography: The explicit literary or visual depiction of sexual subject matter. Among effects
of pornography abnormal sexual behaviors, sexual aggression and abuse, sexual partners
considered as sex objects or instruments for self-pleasure, erectile dysfunction, higher risks STI,
shame, low self- esteem, social isolation, ….
v) Rape: The act of forcing some one into sexual intercourse without consent.
vi) Adultery: Having sex outside wedlock.
vii) Fornication: Engaging in sex before marriage.
viii) Sex slavery: The act of taking advantage of vulnerable individuals in a forced commercial sex.
ix) Prostitution: Engaging in sexual relationships for financial gains.
x) Pedophilia: Sexual attraction to children by adults. This can result in child sexual exploitation
where children are exploited for money, power and/or status. Children or young people are tricked
into sexual relationships by being offered money. They might be invited to parties and given drugs
and alcohol. They may also be recruited online.
xi) Sexual assault: Any forced or coerced sexual contact or behavior that happens without consent.
Sexual assault includes rape and attempted rape, child molestation, and sexual harassment or use of
threats.
During ageing process, there is a decline in hormone secretion resulting in sexual dysfunction
such as erectile dysfunction (for males), vaginal dryness, decreased libido, and decreased ability to

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achieve orgasm (for females).
For many older adults, the concept of sexuality includes practices such as kissing, hugging and
fondling. Such activities may replace actual intercourse and become increasingly important for those
who no longer desire or are in capable of sexual intercourse.

Unit 15: Pregnancy prevention

Learning objectives: After studying this topic, to be able to:
❖ Identify effective ways of preventing unintended pregnancy
❖ Outline forms of contraceptive methods and their working mechanisms
❖ Appreciate the importance of making informed choices about reproduction and family
size
❖ Recognize the importance child spacing
❖ Demonstrate negative health side effects of artificial methods of birth control

15.1 Infertility

15.1.1 Male infertility

Male infertility refers to the inability of a male to achieve a pregnancy in a fertile female.
The most common cause of infertility in males is a low sperm cell count (oligospermia). If the
sperm cell count drops to below 20 million sperm cells per millilitre of semen, the male is usually
infertile.
Fertility is reduced if the sperm cell count is normal but sperm cell structure is abnormal
(teratospermia). Abnormal sperm cell structure can be due to chromosomal abnormalities or
genetic factors.
Reduced sperm cell motility (astenozoospermia) also results in infertility.

Often semen with a decreased sperm concentration may also show significant abnormalities in
sperm morphology and motility ("oligoasthenoteratozoospermia").
In the case of male infertility, except in the complete lack of semen (aspermia) or absence of
sperm cells in semen (azoospermia), fertility can be sometimes be achieved by collecting the
sperm cells, followed by their introduction into the female’s reproductive tract, a process called
artificial insemination.
15.1.2 Female infertility

Causes of infertility in females include:
✓ Malfunction of the uterine tubes,
✓ Reduced hormone secretion from the pituitary or ovary,
✓ Interruption of implantation.

15.2 Contraception

Contraception is the prevention of conception that is preventing the fusion of the male gamete
with the female gamete. Both natural and artificial methods exist.

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Importance of using contraceptives

A woman can become pregnant while breastfeeding, from about 10 days after childbirth, and
even during her menstruation. One can prevent an unwanted pregnancy with: total abstinence
or use of contraceptives.
Effective contraception provides both health and social benefits to mother and her children.
Family planning involves using birth control methods to decide how many children to have and
when to have them.
It allows couples to have their desired number of children and control the spacing and timing of
their births.

15.2.1 Natural contraceptives

Natural family planning methods work by observing and recording the body’s different natural
signs or fertility indicators on each day of the menstrual cycle. This knowledge enables couples to
avoid having sex when the woman is fertile.

Advantages of natural contraception

a) Effective method of birth control.
b) Have no negative health side effects.
c) An alternative for women who cannot or do not want to use hormonal methods.
d) Promotes positive body awareness.
e) Consistent with many religious beliefs and lifestyles.
f) Alerts women to reproductive health and fertility concerns.
g) Fosters communication between partners and encourages male involvement.

Disadvantages of natural contraception
a) Requires time to learn (usually 3 to 6 cycles).
b) Requires discipline and commitment to chart fertility signs and follow the rules to avoid
pregnancy.
c) Times of abstinence from intercourse may be a challenge for some couples.

15.2.1.1 Types of natural contraception
Behavioural methods
i) Abstinence
Abstinence or refraining from sexual intercourse is a sure way to prevent pregnancy when
practiced consistently.
It is very effective for preventing unwanted pregnancy and sexually transmitted diseases and
allows couples to engage in other forms of sexual expression.

Advantages of abstinence
a) Prevents transmission of STIs including HIV and AIDS.

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b) No side effects.
c) No need to visit a health care provider.
d) No cost involved.

ii) Lactation Amenorrhea Method (LAM)
Lactation Amenorrhea Method (LAM) is used by a mother who has just given birth and is
exclusively breastfeeding. This method is highly effective for the first six months after childbirth.
The mother has to breastfeed the baby at least every four hours during the day and every six
hours through the night.

iii) (Basal body) Temperature method
A woman measures her basal body temperature: temperature when fully at rest especially after
waking up. It is believed that ovulation may cause a slight increase in basal body temperature.
Elevated waking temperature for three days in a row is considered confirmation of the post-
ovulatory less fertile phase.
iv) Calendar method
Calendar method is also known as Rhythm method or cycle beads. It is a much less effective
natural birth control method. It predicts a woman’s fertile days using calculations based on the
length of past cycles and not daily observations of fertility signs.

v) The Billings Ovulation Method
The cervix produces different types of mucus in response to changing hormone levels. Once a
woman is familiar with these changes, they are able to identify their patterns of fertility and
infertility in their cycle.
It allows a woman to understand her fertile and infertile days and helps to safeguard the
woman’s reproductive health, but it is not an effective method since the body system can fail.

15.2.2 Artificial contraceptives

Artificial birth control is any product, procedure or practice that uses artificial or unnatural
means to prevent pregnancy.

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Most types of artificial contraceptives work by:
• Preventing an egg from being released every month (hormones).
• Preventing sperms from reaching the egg (barrier and some IUD methods).
• Blocking the reproductive function in men or women (sterilisation).
• Preventing a fertilised egg from implanting in the uterus (hormones).

15.2.2.1 Barrier methods
a) The male condom
Condoms are usually made of latex. It is the most used contraceptive. It collects semen at its tip
so that they are not released into the vagina.

Advantages
(i) It is easy to use.
(ii) Affordable
(iii) It can offer the protection against sexually transmitted infections.
(iv) It can be bought over the counter without prescription from a doctor.
Disadvantages
(i) A new condom is required every time one engages in sex.
(ii) If used improperly it can bust, this could lead to unwanted pregnancy or transmission of
STIs.

b) The female condom (Femidom)

c) Spermicide
This method contains chemicals (creams, film, foams and gels) that stop sperms from fertilizing
the ova. They can be used alone, or can be used with other birth control methods such as
diaphragm, to make them more effective.

It offers 95% effective protection against pregnancy, as well as
some protection against STIs.
Female condoms are generally more expensive than the male
ones but they are less likely to burst.
They can be inserted up to eight hours before sex.

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Spermicide does not always offer the best protection against pregnancy. Also, it does not protect
against STIs.

d) The Diaphragm
This is a rubber cap inserted into the vagina to cover the cervix. It prevents sperms from entering
the uterus and reach the egg.
Since every woman's body is a slightly different shape, diaphragms must be fitted to make sure
they're the right size.
Diaphragm should be used together with spermicides which kill the sperms.
Depending on the material and type of the diaphragm, it can be reused many times.
The diaphragm does not protect against STIs and a doctor is needed to direct on how it should be
used.

15.2.2.2 Hormonal based contraceptives
This is the use of a mixture of artificial hormones resembling Oestrogen and Progesterone. They
are administered in three ways:
• Orally (oral contraceptives)
• Through injections
• Administration of implants

a) The Pills
It is the most popular form of female contraception. It is taken orally in form of a tablet, every
day at almost the same time. Synthetic oestrogen and progesterone in oral contraceptives (birth-
control pills) effectively suppress fertility in females. These substances reduce LH and FSH release
from the anterior pituitary gland, among many other effects.

✓ The contraceptive pill will prevent pregnancy 95%. It comes close to providing 99%
protection if one pill is taken every day as prescribed.
✓ The pill does not provide any protection against STIs and a doctor's prescription is
required to buy it.

b) Contraceptive Injections
This is an injection that contains artificial progesterone (progestin). Progesterone-like chemicals,
such as medroxy progesterone (Depo-Provera), which are injected intramuscularly and slowly
released into the circulatory system, can act as effective contraceptives. Injected progesterone-

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like chemicals can provide protection from pregnancy for up to 3 months, depending on the
amount injected.
Injections are about 99% effective. It fails if the user forgets to renew the contraceptive shot in
time.
Once the shot is given, it cannot be reversed. One becomes effectively infertile for the next three
months. Just like the pill, contraceptive injections do not protect against STIs.

c) The contraceptive implant
The implant offers long term protection. It lasts for about three years on average. The
contraceptive implant contains progestin (progesterone), the same hormone as the
contraceptive pill. A thin silastic tube containing these chemicals, such as the Norplant system
can be implanted beneath the skin, usually in the upper arm, from which they are slowly
released into the circulatory system. The implant can be effective for up to 5 years. Menstruation
does not normally occur in women using these techniques while the progesterone levels are
elevated.
The implant has a much higher effectiveness rate than the pill around 99.99%. The implant does
not protect against STIs.
Advantages of injected and implanted progesterone-like contraceptives over other chemical
methods of birth control are that they don’t require taking pills on a daily basis.

d) Other hormonal devices

The same hormones that make birth control pills effective can be distributed to the body by
other means, such as:
• The birth control patch: The patch is usually placed on the arm, back or thigh. It
distributes hormones through the skin and has to be replaced every few weeks.
• The birth control ring (vaginal ring): The ring is inserted into the vagina once a month. It
releases hormones to prevent pregnancy from occurring.

15.2.2.3 Non-hormonal-based contraception methods

a) The Intrauterine Device (IUD)

Often 'T'- shaped, this device offers long-acting reversible contraception, and it is the most
effective type of reversible birth control. It is kept inside the vagina for up to five or ten years.
One type of IUD works by releasing hormones, and another type is made of copper, which affects
the mobility of sperms and stops them from fertilizing the egg.
The copper IUD is believed not to interfere with hormones or cause hormonal birth control-
related side effects.

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15.2.2.4 Preventing pregnancy after sex

❖ Emergency contraception

a) Mifepristone
Mifepristone (RU486) blocks the action of progesterone. It causes the endometrium of the
uterus to slough off and to be expelled from the uterus as it does at the time of menstruation.
It is used to induce menstruation and reduce the possibility of implantation when sexual
intercourse has occurred near the time of ovulation. It can also be used to terminate
pregnancies.
b) Morning-after pills
Morning-after pills are similar in composition to birth-control pills.
The elevated oestrogen and progesterone levels may:
- Inhibit the preovulatory LH surge in some cases;
- Alter the rate of transport of the fertilized ovum from the uterine tube to the uterus;
- Inhibit implantation.

15.2.2.5 Surgical and Permanent contraception methods
a) Sterilization
Sterilization is a procedure that closes or blocks fallopian tubes in women and sperm duct in
men. This prevents movement of the ova and sperm respectively. This option is available to both
men and women.
i) Vasectomy
This involves tying off and cutting the tubes that carry sperms, the vas deferens. This procedure
prevents sperm cells from passing through the ductus deferens and becoming part of the
ejaculate. The sperm cells are reabsorbed in the epididymis.
This method provides no protection against STIs and the effects are permanent. In very rare
cases (less than 1%), the tubes can grow back, making pregnancy a risk.

ii) Tubal ligation
• Surgical sterilization: This involves cutting and tying the fallopian tubes so that they cannot link
the ovaries with the uterus anymore. The effects are permanent.
• Non-surgical sterilization: This involves placing a coil in each fallopian tube through the vagina
and uterus. Scars appear and eventually block each tube completely. The scars may take up to 3
months to completely block the tubes, so there is a need to use another method of
contraception in the meantime.
In rare cases the blocked tubes can grow back and reconnect. The method offers no protection
against STIs.

b) Abortion:
This is the deliberate process of prematurely terminating a pregnancy usually before the embryo
or foetus is capable of independent life. Surgically, it is done by insertion of an instrument
through the cervix into the uterus. The instrument scrapes the endometrial surface while a
strong suction is applied. This technique is normally used only in pregnancies that have
progressed less than 3 months.

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❖ Side effects of using modern contraceptive methods

A number of side effects have been associated to the use of modern (artificial) contraceptive
methods:
✓ Menstrual irregularities
✓ Inter-menstrual hemorrhage and menstrual pain
✓ Behavioral changes, nausea, weight gain/loss, mood swings, breast swelling or
tenderness, growth of facial hair or hair loss
✓ Heart attacks, high blood pressure, blood clots and strokes
✓ Increased risks for developing urinary tract infections, breast and cervical cancers,…
✓ Depression, anxiety and diminished sexual desire (reduced libido)
✓ Ectopic pregnancies, diabetes, severe acne, migraine
✓ Risk of miscarriage,
✓ Encourages sexual abuse, unfaithfulness, spread of STDs

Unit 16: Reducing risk of STIs and HIV/AIDS

Learning objectives: After studying this topic, to be able to:
❖ Explain how STIs and HIV are transmitted, treated and prevented.
❖ Recognize symptoms and complications of STIs and HIV
❖ Explain how culture and gender affect personal decision making regarding sexual
relationship.
❖ Appreciate behaviors that reduce the risk of STIs and HIV transmission.

16.1 Sexually transmitted diseases

Sexually transmitted diseases (STD), also referred to as sexually transmitted infections (STI) and
venereal diseases (VD) [venereology=branch of medicine that studies these diseases], are
illnesses that have a significant probability of transmission between humans by means of sexual
behavior, including vaginal intercourse, anal sex and oral sex.
Examples of these diseases are:

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✓ Chlamydia, caused by a bacterium Chlamydia trachomatis. The disease is spread by oral,
vaginal or anal sex, and also through touch, for example, touching the eyes with a
contaminated hand, may lead to conjunctivitis (Inflammation of the conjunctiva, a
mucous membrane that lines the inner surface of the eyelid and the exposed surface of
the eyeball or sclera). It also causes inflammation of the cervix in women, urethra and
rectum in both men and women. Other complications like pelvic inflammatory disease
(PID) or infertility may occur from this infection.
✓ Gonorrhea: caused by a bacterium called Neisseria gonorrhoeae. If left untreated, the
infection spreads from vagina or male urethra to the other reproductive parts and may
eventually block the passages resulting to infertility.
✓ Syphilis: caused by a bacteria called Treponema pallidum. In its tertiary stage, lesions
develop and cause extensive tissue damage that may lead to paralysis, insanity, blindness
and eventually death.
✓ Trichomoniasis, sometimes called "trich", is caused by a protozoan called Trichomonas
vaginalis
✓ HIV and AIDS

Some STIs can also be contracted by using drug needles after their use by an infected person, as
well as through any incident involving the contact of a wound with contaminated blood or
through childbirth or breastfeeding.

16.1.1 Symptoms of STDs
Sometimes, there are no symptoms of STDs. If symptoms are present, they may include one or
more of the following:
• Bumps, sores, or warts near the mouth, anus, penis, or vagina.
• Swelling or redness near the penis or vagina.
• Skin rash.
• Painful urination.
• Weight loss, loose stools, night sweats.
• Aches, pains, fever, and chills.
• Yellowing of the skin (jaundice).
• Discharge from the penis or vagina. (Vaginal discharge may have an odor.)
• Bleeding from the vagina other than during a monthly period.
• Painful sex.
• Severe itching near the penis or vagina.
Transmission of HIV (Human immunodeficiency virus) infection
HIV is the virus that causes AIDS. AIDS stands for Acquired Immuno-Deficiency Syndrome.
Acquired: the disease is not hereditary but develops after birth from contact with a disease-
causing agent (in this case, HIV).
Immuno-deficiency: the disease is characterized by a weakening of the immune system.
Syndrome: a group of symptoms that collectively indicate or characterize a disease. In the case
of AIDS this can include the development of certain infections and/or cancers, as well as a
decrease in the number of certain cells in a person’s immune system.
It is mainly transmitted through:
a) Certain body fluids: blood, semen, pre-seminal fluid, rectal fluids, vaginal fluids and breast milk
from a person who has HIV. These fluids must come in contact with a mucous membrane or

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damaged tissue or be directly injected into the bloodstream (from a needle or syringe) for
transmission to occur. Mucous membranes are found inside the rectum, vagina, penis and
mouth.
b) Having anal or vaginal sex with someone who has HIV. Anal sex is the highest-risk sexual
behavior. Vaginal sex is the second highest-risk sexual behavior.
c) Sharing needles or syringes or other equipment used to prepare drugs for injection with
someone who has HIV.

Less commonly, HIV may be spread through:
d) Mother to child during pregnancy, birth or breastfeeding. Although the risk can be high if a
mother is living with HIV and not taking medicine.
There is a 75% chance that the child will be completely unaffected if the HIV-positive mother
does nothing. The odds of having an infected child are less than 8% if the mother is treated with
ZDV (AZT) during the pregnancy, and as low as 2% with HAART (Highly Active Antiretroviral
Therapy).

In some rare cases, HIV has been transmitted through:
a) Receiving blood transfusions, blood products, organ or tissue transplants that are
contaminated with HIV.
b) Eating food that has been pre-chewed by an HIV-infected person. The only known cases are
among infants.
c) Being bitten by a person with HIV. There is no risk of transmission if the skin is not
broken.Contact between broken skin, wounds or mucous membranes and HIV-infected
blood or blood-contaminated body fluids.
d) Deep, open mouth kissing if both partners have sores or bleeding gums.
e) Oral sex.

16.2 Ways of reducing STIs and HIV infection
The following are ways of reducing STIs and HIV infection:
a) Abstinence is the only sure way to prevent STIs.
b) Being faithful to one trusted partner.
c) Avoid sharing towels or underclothing.
d) Get a vaccination for hepatitis B.
e) Get tested for HIV.
f) Avoiding alcohol consumption and abuse of drugs. Individuals who are drunk or on drugs
often fail to have safe sex. The use of so-called 'recreational drugs' such as cocaine and cannabis,
together with sexual activity, may well also' enhance susceptibility to infection because of their
immunosuppressive effects. Similarly, nitrites, which are often used by homosexual men to
promote and prolong erection, have marked immunosuppressive activities.

16.3 Treatment of STIs and HIV infection

❖ Antibiotics like penicillin, erythromycin or tetracycline are used to treat STDs although
some strains can be resistant to certain antibiotics.

Biology and Health Sciences, S2
Page 99

❖ Viral STIs cannot be cured, but the symptoms can be managed with medications. For
instance, there is a vaccine against hepatitis B, but it will not help if you already have
the disease.
❖ HIV cannot be cured but can be managed. A person may choose to visit a Voluntary
Counselling and Testing (VCT) centre to undergo HIV and AIDS counselling. After
conselling he or she can make an informed decision about whether to be tested for HIV or
not.
❖ Antiretroviral therapy (ART) is the use of HIV medicines to treat HIV infection. People on
ART take a combination of HIV and AIDS drugs every day. ART cannot cure HIV and AIDS,
but helps people infected to live longer and healthier lives. ART also reduces the risk of
HIV and AIDS transmission.

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