General biology of human module 1011 power point

Biology

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General Biology
Unit 3
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The cellular basis of life
Contents
3.1. The cell theory
3.2. Cell organelles
3.3. Structure and functions of organelles
3.4. Cellular diversity
3.5. Transport across the cell membrane
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by Admasu Tassew
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Objectives of the unit
At the end of this unit you will be expected to:
statethe cell theory
enumerateorganelles of typical cells
describethe structure and functions of organelles
discusscell diversity
explainthe mechanisms of transport of materials
across the cell membrane
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by Admasu Tassew
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3.1 The cell theory
Robert Hook (
1665) was the firstto observe plant cells with a
crude microscope. In 1674 A.V.
Leeuwenhoek
observed the
first living cell
Then,
Mathias Schleiden
(1838 - plants) and
Theodore
Schwann
(1839 - animals) proposed that allliving things are
composed ofcells
Rudolf Virchow
(1855) extended this idea by contending that
cells arise only from other cells
The cell theory can be summarized by the following
three
statements:
All organisms are made up of one or more cells Cells are the most basic (structural and functional) unit of life Cells arise from pre-existing cell
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Besides the above statements, modern cell theory includes:
The activity of an organism depends on both the
individual
and the
collective
activities of its cells
Energy flow(metabolism and biochemistry) occurs inside
cells
Cells contain DNAwhich is specifically found in
chromosome and RNAis found in the cell nucleus and the
cytoplasm
All cells are basicallythe same in chemical composition in
organisms of similar species According to the principle of complementarity of structure and
function, the biochemical activities of cells are dictated by
–their shapes or forms, which means their diversity and
–by the relative number of their specific sub-cellul ar structures,
which refers to their complexity
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A typical eukaryotic cell has 3 major parts:
The
plasma membrane
:
»the outer boundary of the cell
The
cytoplasm
:
»the intracellular fluid packed with organelles
The
nucleus
:
»an organelle that controls cellular activities
»Typically the nucleus resides near the cells center
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Fig: 3.1. Generalized animal cell
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3.2. Cell organelles
An organelle is a specialized subunit within a cell that has a
specific function. Organelles are found in the cytoplasm
Eukaryotes possess membrane bound organelles, whereas
prokaryotes
lack
membrane bound organelles
two
kinds of cell organelles based on membrane covering,
Membranous organelles:
–ER (rough and smooth), Golgi bodies, mitochondria,
chloroplasts, nucleus, lysosomes, peroxisomes and
vacuoles
Non-membranous organelles:
–ribosomes (70s and 80s), centrosomes, cilia and flagella,
microtubules, basal bodies and microfilaments
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3.3. Structure and function of organelles
1. The nucleus: •is
oval
shaped largest central structure surrounded by a double-
layered membrane
•In the nucleus, DNA directs protein synthesis by giving codes,
or instruction for directing synthesis of specific structure and
enzymes’ proteins within the cell
•the nucleus indirectly governs most cellular activities and
serves as the
cell’s master
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Three types of RNA are involved in protein synthesis .
•mRNA:
copies instructions in the DNA and carries these to the ribosome
•tRNA:
reads mRNA sequence and transfers each amino acid to ribosome where the protein product is synthesized
•rRNA:
composes the ribosome and binds the corresponding amino acid to a growing peptide chain
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The nucleus can take diverse shapes:
Rounded e.g. in hepatocytes Indented(segmented)e.g. in neutrophils Binucleated e.g. in parietal cells, cardiac muscle cells Multinucleated e.g. in osteoclasts, skeletal muscle cells very large (many DNA)e.g. in megakaryocytes Absent e.g. in mature erythrocytes, blood platelets
The nucleus is surrounded by a nuclear envelope and contains
chromatinand one or more nucleoli
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The Nuclear envelope
surrounds nuclear material
consists of
outer
and
inner
membrane
perforated at intervals by nuclear
pores
Through this pores most
ions
and
water soluble molecules
to
transfer between nucleus and cytoplasm
Chromatin: term chromatin means "
colored material
"
Refers it is easily stained for viewing with micros cope, and it is composed mainly of coils DNA bound to basic protein called
histones
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Nucleoli: the nuclei of most cells contain one or more lightly
stained structures called nucleoli
actively engage in synthesizing of ribosomes does not have a limiting membrane it contains large amounts of RNA and protein nucleolus enlarged when a cell is actively synthesizing proteins The genes of five separate chromosome pairs synthes ize rRNA
and then storeit in the nucleolus
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2. Cytoplasm
It is complex
jelly
like marrow called
cytosol
The cytoplasm contains other organelles -
ER, GC, lysosomes,
peroxisomes, mitochondria and vacuoles
These organelles are similar in all cells with some varia tions
on the cell specialization
Each organelle is a separate compartment with different
function
These organelles occupy about
half
of the total cell volume
The remaining part of the cytoplasm is
cytosol
The cytosol is cell’s interior
not
occupied by nucleus
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3. Endoplasmic reticulum (ER)
Rough ER - Ribosomes attached - Works on protein synthesis - mRNA carries the genetic
message from the nucleus to
the ribosomes “workshop”
Smooth ER - Does not have ribosomes
- it looks smooth
- It does not produce proteins,
but lipid biosynthesis
- smooth ER bud off/pinch off,
giving rise to transport vesicles
- So, it used to make membranes,
detoxify harmful substances,
stores Calcium ion
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4. Golgi complex:
also called
Golgi apparatus or Golgi bodies
It is associated with the ER
and contains sets of flattened,
curved, membrane- enclosed
sacs, or
cisternae
, stacked in
layers Number of stacks vary in
cells, cells for protein
secretion have
hundreds
of
stacks, whereas some have
only
one

Function -
finishes, sorts,
labels and ships proteins
Generally, it performs:
1. Processing the raw material
into
finished
products
2. Sorting
and
directing
finished
products to their final
destination
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5. Lysosomes:
Lysosomes are intracellular “
digestive system
”
They are membrane-enclosed sacs contain powerful hydrolytic enzymes capable of digesting and
removing
–unwanted cellular debrisand foreign materialslike bacteria
vary in size and shape, and about 300μm in a cell Extrinsic material to be attacked by lysosomal enzym es is brought
into the interior by the process of
endocytosis
If the
fluid
is internalized by endocytosis, the process is call ed
pinocytosis Endocytosis is also accomplished by
phagocytosis
. This is achieved
by specialized cells-
white blood cells
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Lysosomes take up old organelles such as mitochondria and
break down into their component molecules
–those molecules can be
reabsorbed
into the cytosol, and the rest
are
dumped out
of the cell
The process by which worn-out organelles are digested is called autophagy
a human liver cell
recycles
about half its content
every week
–Garbage
disposal
and
recycling
In the inherited condition known as lysosomal storage disease
(Tay-Sachs disease) lysosomes are noteffective because they
lack specific enzymes
–As a result, harmful waste products
accumulate
disrupting the
normal function of cells, often with
fatal
results
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6. Peroxisome
: is membrane-enclosed sacs (part of microbodies
along with glyoxysomes
= plant peroxisomes)
containing
oxidative enzymes
and
catalase
–detoxify various wastes such as
ethanol
(liver and kidneys)
major product generated is a powerful oxidant
H
2
O
2
catalase and antioxidant enzyme decomposing
H
2
O
2
into
harmless
H
2
O
and
O
2
This reaction is an important safety reaction that destroys
deadly
H
2
O
2
,
at the site of production
–thereby preventing possible devastating escape into the cytosol
Peroximal disorders disrupt the normal processing of lipids and disrupt the normal function of the nervous system
–by
altering
the structure of the
nerve cell membrane
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7. Mitochondria:
are the “
power houses
” of a cell;
they extract energy from nutrients and transform into usable
form
vary in number based on the energy needs of each cell types
–A single cell may have few hundreds or thousands
they are
rod or oval shaped
about the size of a bacterium
Each is enclosed by a
double membrane
-
–a
smooth outer
that surrounds the mitochondria, and
–an
inner membrane
that forms a series of enfolding called
cristae
, inner cavity filled with a jelly-like matrix
•cristaecontain proteins (the electron transport protein)
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•The enfolding increase the surface area for keeping these proteins
•The matrix contains a mixture of hundreds of differ ent dissolved
enzymes (Citric acid cycle enzymes )

Function

make ATP energy from cellular respiration

sugar + O
2
ATP

fuels the work of life
both animal and plant cells
•Found in both animal and plant cells Mitochondria are unusual organelles in two ways:
In the matrix they have their
own unique DNA
called
mitochondrial DNA
Have the ability to
replicate themselves
even when the cell to
which they belong is
not
undergoing cell division
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Fig. 3.3Mitochondrial structure
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8. Chloroplasts
are useful organelles among plastids participate in the process of
photosynthesis
by converting light energy into chemical energy
are located in outer surface of the cell to receive enough
light
are green colored due to
chlorophyll pigments

Plants make their energy in two ways:

Mitochondria
:
make energy fromsugar + O
2

cellular respiration
:
sugar + O
2
ATP

Chloroplasts
:
make energy + sugar fromsunlight

Photosynthesis
:
sunlight + CO
2
ATP + sugar

ATP =
active energy

sugar =
stored energy
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Fig 3.4:Structure of plant cell
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9. Vesicles:
are membrane bound sacs
are used to
store
or
transport
substances around the cell
Lysosomes are actually Vesicles
10. Vacuoles:
are essentially
larger
vesicles
formed by
joining
many vesicles together
are membrane bound organelles have
no specific shape
contain water with a number of different compounds within it Their function varies depending on the type cell
e.g, In plant cells used to maintain
Turgor Pressure
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11. Cytoskeleton:
is a complex protein network act as ‘
bone and muscle
’ of the
cell This network has at least four distinct elements:
Microtubules,
Microfilaments, Intermediate filaments
and
Microtubular
lattice
Generally, cytoskeletons determine/ provide:
distinct shape, size to the cell structural support organizing its contents substances movement through cell (cilia, flagella a nd
intracytoplasmic vesicles), and
Contribute to movements of the cell as a whole
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12. Plasma/cell membrane
is extremely thin layer of
lipids
and
proteins
forming outermost
boundary of living cell and enclosing the intracellular fluid
(ICF)
It serves as a
mechanical barrier
that
traps
needed molecules
within the cell; plays an active role by
selective permeability
of substances to
pass between the cell and its ECF environment It is a fluid
lipid bilayer
embedded with proteins
It appears as ‘
trilaminar
’ layer structure having
–two dark layers
separated by a
light middle layer
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Fig 3.5: Structure of the cell membrane
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All plasma membrane are made up of
–lipids
and
proteins
plus small amount of
carbohydrates
.
Phospholipids are
–most abundant with a
lesser
amount of
cholesterol
–have a
polar hydrophilic
(water loving)
head
having a
negatively charged phosphate group and
–two
non-polar
(electrically neutral)
hydrophobic
(water fearing)
fatty acid
tails
Such two-sided molecule self-assemble into a lipid bilayer whe n
in contact with water
The hydrophobic tails
bury
themselves in the center away from
the water, while the hydrophilic heads
line up
on both sides in
contact with water
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The water surface of the layer is exposed to
ECF
the inner layer is in contact with the
ICF
Cholesterol
provides to
–the
fluidity
as well as the
stability
;
–lies in between the phosphate molecules,
–preventing the fatty acid chain from
packing
together and
crystallizing
that could
decrease fluidity
of the membrane
–exerts a
regulatory role
on some of the membrane proteins
For fluidity of the membrane,
–it gives
flexibility
to the cell to change its shape;
–transport processes
are also dependent on the fluidity
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Lipid bilayer
forms the basic structure of the membrane,
–is a passage of water-soluble substances between th e ICF and
ECF;
–is responsible for the fluidity of the membrane
–forms the primary barrier to diffusion

Membrane proteins
are variety of different proteins within the
plasma membrane; have the following special functions:
some form water-filled passage ways or channels, ac ross the lipid
bilayer
;
Others serve as carrier molecule that transport speci fic molecule
that cannot cross on their own
Many proteins on the outer surface serve as ‘recept or sites’
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Another group act as membrane-bound enzymes
to
control
specific chemical reactions
Some proteins are arranged as filaments network/ me shwork on
the inner side
Other proteins function as cell adhesion molecules ( CAMs)
Some proteins in conjunction with carbohydrate are used in the
cell’s ability to
recognize
‘self’ and in cell-to-cell interactions
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Fig 3.6: Membrane proteins

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Membrane Carbohydrate: •Short-chain on the outer membrane surface
•serves as
self-identity marker
and
interact
with each other in
the following ways:
Recognition of ‘self’ and
cell-to-cell
interactions
surface markers are important in
growth
Cells do not overgrow their own territory
Some CAMs (cell adhesion molecules) have carbohydra te, on
the outermost tip where they participate in
cell adhesion activity
Functions of biological membranes
Channel protein and carrier protein: facilitate transport Enzymes: membrane proteins sometimes act as enzymes Receptor molecules
Antigens: these act as cell identity markers or ''name tag' '
Glycolipids- involved in cell-cell recognition
Energy transfer in photosynthesis and respiration: proteins in
the membranes of chloroplast and mitochondria take part
respectively
Cholesterol: acts like a plug, reducing even further the escape or
entry of polar molecules through the membrane
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3.4. Cellular diversity
Cells are found in different organisms are very diverse in their size, shape and their int ernal structure this also applies to cells found in the same organi sm diversity is influenced by their rolesand functionwithin body
Cell Shape
Cells have different shapes due to appropriate func tion Body cells have flat, protecting and covering body surface Nerve cells have long extensions Skin cells have a shape which is flat Egg cells have sphere, and some bacteria are rod in shape Some plant cells are rectangular
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Cell Size
•Some cell can be seen with naked eye without using
magnification instruments
•Example,
egg
of birds/reptiles and a
neuron cell
of
giraffe, which is 2 meters in length
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Summary of organelles
•Organelles that involve in
daily metabolism -
feeding,
digestion, oxygen intake, energy generationandwaste
removal: cell membrane, vacuole, lysosome, mitochondria,
chloroplast
•Organelles that involve in
protein synthesis and processing
-
read genesand produce proteins: nucleus, ribosomes,
ER, Golgi complex
•Organelles that involve in
cell division -
copying genetic
materialsandproduce “daughter” cells: nucleus,
microtubules, centrioles
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Review questions
1. State the three statements of cell theory and mention type s of cells
2. What is an organelle? Mentions at least two organelles of t he cell
with their respective functions
3. What is homogenization in cell study? Why is it important?
4. Smooth ER is meant to detoxify harmful substances and stor e
calcium ion. In which cells do they play these roles?
5. What is the function of plant peroxisomes (glyoxysomes)?
6. Distinguish the functions and ER and Golgi complex
7. What dictates cell diversity?
8. Mention diversity with respect to cell shapes and elabora te on how
cell shape suits with cell functions
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3.5. Transport across the cell membranes
•Cell/plasma membrane is selectively
permeable
•Lipid-soluble
and
small ions
passively diffuse down their
electro-chemical gradients
•Uncharged/non-polar molecules O, CO2 and fatty acids are
highly lipid-soluble and readily permeate the membrane
•Charged particle Na/K ions
and polar molecules such as
glucose
and
proteins
have low lipid solubility, but are very soluble in
water
•For water-soluble ions of less than
0.8 nm diameters
, protein
channels serve as an alternate route for passage
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•For the cell to survive some materials need to
enter
and
leave
the cell. There are
4
basic mechanisms:
1. Diffusion (simple and facilitated diffusion)
2. Osmosis
3. Active transport
4. Bulk transport
Two forces are involved in facilitating movement across the
plasma membrane:
1. Forces that
do not
require to expend energy for
movement 
passive force
2. Forces requiring energy (ATP) to be expended to
transport across the membrane 
active force
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1. Diffusion
is the net movement of molecules (or ions)
from a region of
high concentration to lower concentration
The molecules move
down
a concentration gradient
Molecules have
kinetic energy
, which makes them move about
randomly All molecules in
liquid
and
gases
are in continuous random
motion in any direction As a result of this
random movement
, the molecules frequently
collide bouncing off each other in different directions
The
greater the concentration
, the
greater
the likelihood of
collision
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Additional factors that influence the rate of net diffusion are:
1
.
permeability
of the membrane
2
.
surface area
of the membrane
3. molecular weight
of substance (lighter diffuses rapidly)
4
.
distance
through which diffusion must take place
Temperature, pressure etc can also affect diffusion
N.B:-Increasing all the factors increases rate of net diffusion,
except
distance
- thickness, that if increased, decreases the rate of
diffusion; and
molecular weight
if increased, decreases rate of
diffusion 8/19/202242
Fig 3.7: Diffusion of molecules
Movement along electrical gradient
Movement of charged particles is also affected by their
electrical
gradient If a relative difference in charges exists between two ad jacent
areas,
–the
cations
tend to move towards more
negatively
charged area, whereas
the
anions
tend to move toward the more
positively
charged areas
The simultaneous existence of an electrical and concentration
(chemical) gradient for a particular ion is referred to as a n
electro-
chemical gradient
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Fig: 3.8: Concentration gradient (A) and diffusion (B)
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Fig 3.9:Diffusion of lipid molecules

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Carrier-mediated Transport
•All carrier proteins span the thickness of the plasma membra ne
and are able to undergo reversible changes in shape
•This transport displays three characteristics:
1. Specificity
:
each cell possesses
protein
specified to
transport a specific substance
2. Saturation
:
in a given time only a limited amount of a
substance can be transported via a carrier; this limit is
known as
transport maximum
(Tm). When the Tm is
reached, the carrier is
saturated
3. Competition
:
closely related compounds may compete for
ride across the plasma membrane on the same carrier
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Facilitated Diffusion
•Facilitated diffusion uses a
carrier protein
to facilitate the
transfer of a particular substance across the membrane
''
downhill
'' from higher to lower concentration
•This process is
passive
and does
not require energy
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2. Osmosis
•is the net diffusionof water
down
its own concentration gradient
•Water can readily permeate the plasma membrane
•The driving force for diffusion of water is its concentration
gradient from area of higher water concentration (
low solute
) to
the area of lower water (
high solute
) concentration
•This net diffusion of water is known as
osmosis
•Special mechanisms are used to transport selected molecules
unable to cross the plasma membrane on their own
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3. Active transport
•Active transport, requires the carrier to expend energy to
transfer its passenger ''
uphill
'' against a concentration gradient
from an area of lower concentration to an area of higher
concentration
•ions across a membrane against its natural tendency to diffuse in the
opposite direction. Or
transporting against concentration gradient
•The movement of molecules is in one direction only; unlike diffusion
that is reversible the energy is supplied by the br oke down of ATP
The major ions within the cells and their surrounding are Na
+
,
K
+
and Cl
-.
the membrane surface of most cell have
sodium pump
coupled
with a
potassium pump
that actively moves K+ from outside to
inside the cell The combined pump is called the sodium potassium pump (
Na-
K
-
pump
)
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Note that for every
2K+
taken into the cell,
3Na+
are removed
Thus a potential difference is built up across the membrane, with
the inner side of the cell being
negative
This tends to restrict the entry of negatively charged ions (
anions
)
such as
chloride
and favoring diffusion of
cations
into the cell
This explains why chlorideconcentration inside cellsis
less
than
the outsidedespite the fact that chloride ions can diffuse in and out by facilitated diffusion
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Fig 3.11: Sodium-potassium pump
Na+-K+-pump plays three important roles
•It helps regulate cell volume by controlling the concentration of
solutes inside cell and thus minimizing osmotic effects that would
induce
swelling
or
shrinking
of animal cell (
osmoregulation
)
–If the pump is inhibited, the cell swells and burst s because of the building-up of
Na+, which results in excess water entering into th e cell by osmosis
•It establishes Na and K concentration gradients across the
plasma membrane of all cells;
–these gradients are important in the
nerve and muscle
to generate
electrical signals
–high concentrations of K are needed inside cells fo r protein
synthesis, glycolysis, photosynthesis and other vit al processes
•The energy used to run the pump also indirectly serves as the
energy source for the co-transport of glucose and amino acids
across the membrane (intestine and kidney cells)
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Table 3.1 How molecules cross cell membrane
4. Bulk transport (Exocytocis and Endocytisis)
Vesicular Transport
•cell membrane selectively transport ions and small polar
molecules
•But large polar molecules and multimolecular material may lea ve
or enter the cell, such as hormone secretion or ingestion of
invading microbe by
leukocytes
•These materials
cannot
cross the plasma membrane but are to be
transferred between the ICFand ECF
not
by usual crossing
•This process of transport into or out of the cell in a membrane-
enclosed vesicle is -
vesicular transport
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•Transport into the cell is termed
endocytosis
, whereas transport
out of the cell is called
exocytosis
.
•Both are
active processes
involving the bulk transport of
materials through membranes
•In endocytosis, the transported material is wrappedin a piece of
the plasma membrane, thus gain entrance to the interior cell
•Endocytosis of fluid is called
pinocytosis
(cell “
drinking”
),
whereas endocytosis of large multimolecular particle is calle d phagocytosis
(
cell “eating”
)
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Fig 3.12: Exocytosis
Exocytosis is the
reverse
process of endocytosis
Wastes such as
solid and undigested
remains from phagocytic vacuoles
may be
removed
from cells or
useful materials may be secreted
Secretion of
enzymes from the pancreas
is achieved in this way

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Review questions 1. Discuss the nature/structure of the cell membrane
2. Mention factors affecting rate of diffusion along with how they
affect
3. Write down roles of different components of the c ell membrane
4. Discuss endocytosis/exocytosis, phagocytosis/pino cytosis
5. Where does vesicles for bulk transport come from and where
does they go after transport of materials is once a ccomplished?