Photosynthesis notes

PHOTOSYNTHESIS

1. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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B.Sc. BOTANY - V SEMESTER STUDY MATERIAL
Module 2: PHOTOSYNTHESIS AND RESPIRATION

PHOTOSYNTHESIS
Photosynthesis literally means ‘synthesis with the help of light’. It is the process that gives life to all living
beings. The plants convert light energy into life energy. It is the only biological process that makes use of sun’s
light energy for driving the life machinery. Hence, photosynthesis is regarded as ‘leader’ of all processes both
biological and abiological. It is the most fundamental of all biochemical reactions by which plants synthesize
organic compounds in the chloroplast from carbondioxide and water with the help of sunlight. It is an oxidation
reduction reaction between water and carbondioxide.

History of photosynthesis

320 BC Ancient Indians believed that plants fed from their feet –
Padapa, refers to a plant which drinks from the feet.
1727 Stephen Hales recognised the importance of light and air in
the nourishment of plants.
1779 Jan Ingen-Housz discovered that the green parts of the plant
purify the polluted air in the presence of light.
1782 Senebier showed that as the concentration of CO2 increases,
the rate of O2 evolution also increases.
1845 Von Mayer recognised that green plants convert solar energy
into chemical energy of organic matter.
1845 Liebig pointed out that the organic matter was derived from
CO2 and water.
1920 Warburg introduced the unicellular green alga Chlorella as
a suitable material to study photosynthesis.
1932 Emerson and Arnold showed that the existence of light and
dark reactions in photosynthesis.
1937 Hill demostrated photolysis of water by isolated chloroplasts
in the presence of suitable electron acceptor.
1941 Ruben and Kamen used 18O2 to show that O2 comes from
water in photosynthesis.
1954 Arnon, Allen and Whatley used 14CO2 to show fixation of
CO2 by isolated chloroplasts.
1954 Calvin traced the path of carbon in photosynthesis and gave
C3 cycle (Calvin cycle) and was awarded Noble prize in 1960.
1965 Hatch and Slack reported the C4 pathway for CO2 fixation
in certain tropical grasses.
Significance of photosynthesis
 Photosynthesis is a source of all our food and fuel.
 It is the only biological process that acts as the driving vital force for the whole animal kingdom and for
the non-photosynthetic organism.
 It drives all other processes of biological and abiological world.
 It is responsible for the growth and sustenance of our biosphere.
 It provides organic substances, which are used in the production of fats, proteins, nucleoproteins,
pigments, enzymes, vitamins, cellulose, organic acids, etc.

2. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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 Some of them become structural parts of the organisms. It makes use of simple raw materials such as
CO2, H2O and inexhaustible light energy for the synthesis of energetic organic compounds.
 It is significant because it provides energy in terms of fossil fuels like coal and petrol obtained from
plants, which lived millions and millions of years ago.
 Plants, from great trees to microscopic algae, are engaged in converting light energy into chemical
energy, while man with all his knowledge in chemistry and physics cannot imitate them.
Photosynthetically active radiation
Light:
 The simplest answer is that light is a form of radiant energy, a narrow band of energy within the
continuous electromagnetic spectrum of radiation emitted by the sun.

 The term ‘‘light’’ describes that portion of the electromagnetic spectrum that causes the physiological
sensation of vision in humans.
 In other words, light is defined by the range of wavelengths—between 400 and approximately 700
nanometers—capable of stimulating the receptors located in the retina of the human eye.
 Strictly speaking, those regions of the spectrum we perceive as red, green, or blue are called light,
whereas the ultraviolet and infrared regions of the spectrum, which our eyes cannot detect (although
they may have significant biological effects), are referred to as ultraviolet or infrared radiation,
respectively.


 The radiant energy given off by the sun is measured on the basis of wavelengths
 The range of wavelengths is placed on the electromagnetic spectrum
 Visible light, which drives photosynthesis, is a small segment of all the radiant energy

 The propagation of light through space is characterized by regular and repetitive changes, or waves, in
its electrical and magnetic properties.
 Electromagnetic radiation actually consists of two waves—one electrical and one magnetic—that
oscillate at 90◦ to each other and to the direction of propagation (Figure).

3. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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 The wave propertiesof light may be characterized by either wavelength or frequency. When light is
emitted from a source or interacts with matter, it behaves as though its energy is divided into discrete
units or particles called photons.
 The energy carried by a photon is called a quantum Fluorescence is the emission of a photon of light
as an electron relaxes from the first singlet excited to ground state.
 Since the rate of relaxation through fluorescence is much slower than the rate of relaxation through
thermal deactivation.
 Photoreceptors are defined as pigment molecules that process the energy and informational content of
light into a form that can be used by the plant.
 A pigment that contains protein as an integral part of the molecule is known as a chromoprotein.
 Thus, photoreceptors typically are chromoproteins. The chromophore (Gr. phoros, bearing) is that
portion of the chromoprotein molecule responsible for absorbing light and, hence, color.
 The protein portion of a chromoprotein molecule is called the apoprotein. The complete molecule, or
holochrome, consists of the chromophore plus the protein

Absorption Spectrum
 The absorption spectrum of photosynthesis shows how much of a particular wavelength of light that are
absorbed by the photosynthetic pigments
 The two most common pigments are chlorophyll a and chlorophyll b
 The action spectrum and absorption spectrum shows strong similarities
 Great absorption in violet-blue range
 Also a high level of absorption in the red range
 The lowest absorption in the yellow green range
 There is a close correlation between the action spectrum and the absorption spectrum

4. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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Action spectrum

 A spectrum is a range of wavelengths of electromagnetic radiation (The radiation emitted from the sun)
 The spectrum of visible light is the range of wavelengths from 400 nm to 700 nm
 Each wavelength is colour of light
o 400-525 Violet-blue
o 525-625 Green –yellow
o 625-700 Orange –red
 The efficiency of photosynthesis is not the same in all wavelength
 The action spectrum shows the efficiency of the different wavelengths of light that are used in
photosynthesis




Emerson Enhancment Effect

 The rate of photosynthesis is mearsured as the number of oxygen molecules produced per the quantom
of light absorbed
 Robert Emerson conducted a experiment on Chlorella during the year 1958
 He studied the rate of photosynthesis is varied in wavelenght of light (390-760 nm)
 Monochromatic light of different wavelenght is used and rate of photosynthesis is mearsured
 Sudden fall in photosynthetic yield in the far red region (Greater than 680 nm) compared to the red
region of the electromagnetic spectrum is called Emerson Red Drop
 The increase in photosynthetic yield by the combained effect of red (680nm) is called Emerson
Enhancement Effect
 This work provide experimental evidence for the presence of two photosystem PS I & PS II

5. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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The Emerson effect is the increase in the rate of photosynthesis after chloroplasts are exposed to light of
wavelength 670 nm (deep red spectrum) and 700 nm (far red spectrum). When simultaneously exposed to light
of both wavelengths, the rate of photosynthesis is far higher than the sum of the red light and far red light
photosynthesis rates.

Quantum Requirement and Quantum yield
• The number of photons required to release one molecule of oxygen in photosynthesis is called as
Quantum requirement
• On the other hand, the number of oxygen molecule released per photon of light in photosynthesis is
called quantum yield
• The quantum yield is always in fraction of 1

Site of photosynthesis
 Chloroplasts are the actual sites for photosynthesis.
 All green parts of a plant are involved in photosynthesis.
 Leaves are the most important organs of photosynthesis.
 In xerophytes like Opuntia, the stem is green and it performs photosynthesis.
 Over half a million chloroplasts are present in one square millimetre of a leaf.
 It measures about 4 to 6 micron.
 A typical chloroplast of higher plants is discoid shaped.
 It is a double membrane bound organelle containing chlorophyll, carotenoid, xanthophyll, cytochrome,
DNA, RNA, manganese, etc.
 Chloroplasts are generally considerably larger than mitochondria.
 The space enclosed by the envelope is filled with matrix called stroma.
 In the stroma, many grana are embedded.
 In each granum, several disc shaped lamellae are found.
 These disc shaped structures are called thylakoids.
 They resemble a stack of coins. This structure is known granum.
 Generally a chloroplast contains 40 to 60 grana.
 The photosynthetic pigments are found in grana.
 The stroma contains circular DNA, RNA and enzymes for starch synthesis.

6. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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Photochemical and biosynthetic phases
 The pigments involved in photosynthesis are called photosynthetic pigments.
 They are chlorophyll ‘a’, chlorophyll ‘b’, carotenoids, xanthophyll and phycobilins. Magnesium is an
essential component for the formation of chlorophyll.
 Chlorophyll ‘a’ is a universal pigment present in the plants in which water is one of the raw materials for
photosynthesis.
 Chlorophylls are highly efficient in absorbing solar energy and they are directly linked to photosynthetic
electron transport.
 Photosynthetic pigments other than chlorophyll ‘a’ are generally called accessory pigments eg.
chlorophyll ‘b’, carotenoids and xanthophyll, whereas chlorophyll ‘a’ is regarded as primary pigment.
 Photosynthetic pigments occur in the granum. They constitute the pigment system called photosystem.
 About 250 to 400 pigment molecules are present in a photosystem.
 Two types of photosystems are found in the granum. Photosystem I (PS I) has less accessory pigments
and more chlorophyll ‘a’, while photosystem II (PS II) has more accessory pigments and less
chlorophyll ‘a’.
 The primary function of photosystems is to trap light energy and converts it to chemical energy.

7. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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 The energy absorbed by accessory pigments is transferred to the chlorophyll ‘a’.
 The granal lamella where the photosynthetic pigments are aggregated to perform photosynthetic
activities is called active centre.
MECHANISM OF PHOTOSYNTHESIS
 Biosynthesis of sugars by the chloroplasts of green plants using water and CO2 in the presence of
sunlight is called photosynthesis
 Photosynthesis is also called carbon assimilation
 It is an anabolic process
 Photosynthesis occurs in the chloroplast
 It takes place in two steps namely light reaction and dark reaction
 Light reaction is the first step in photosynthesis
 It occurs in the presence of light
 Light reaction synthesizes energy in the form of ATP and NADPH2 from sunlight


 The overall reaction of photosynthesis can be written as follows.


 The reactions of photosynthesis can be grouped into two – light reactions and dark reactions.
 The reactions involving pigments, solar energy and water that produce ATP and NADPH2, are called
light reactions.
 The photosynthetic reactions in which CO2 is reduced to carbohydrates making use of ATP and
NADPH2 generated by light reactions are collectively called dark reactions.
 Light reaction
 Light reaction is the first stage in photosynthesis
 Water is broken down into 2H+ ion and ½ O2 with the release of electrons
 These help in synthesis of assimilatory powers such as NADP2 and ATP
 It occurs in grana of chloroplast
 Light reaction is faster than dark reaction
 It occurs only in presence of light
 Light reaction is called primary photochemical reaction , it is driven by light
 This is also called Hill reaction –
 Hill 1937 proved that isolated chlorophylls release O2 from water in the presence of light and a
suitable hydrogen acceptor
 There are some photosynthetic pigment system in the thylakoid membrane of grana to perform
photochemical reaction
 They are called Photosystems
 Chlorophyll in these photosystem differ in their absorbance of light
 Photosystem are of two types
 Photosystem -1 (PS-1, PSII)
o PS-I receives the light with all wavelengths
o PSII can receive only wavelength shorter than 680 nm
o It does not receive light with lower wavelength

8. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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o The PS-II and PS-I are spatially separated by protein complexes such as Plastoquinone,
Cytochrome b6f complex and Plastocyanin, Ferredoxine and Ferredoxin NADP reductase
complex (FNR) are associated with PS-I
ATP Synthase is also found in the photosynthetic membrane of chloroplast
– Light absorption by antenna complexes
– Transfer of light energy from antenna complex to chlorophyll a in the reaction centre
– Excitation of chlorophyll-a in the reaction centre
– Photolysis of water and evolution of O2 (Oxidation of water)
– Synthesis of assimilatory powers via electron transport




The overall process of photosynthesis is illustrated

9. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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Light Dependant Reactions
Chlorophyll absorbs light and the energy from the light raises an electron in the chlorophyll molecule to a
higher energy level.
 An excited electron
 The chlorophyll is photoactivated
The chlorophyll is located in the thylakoid membrane and it is arranged in groups of hundreds of molecules
called photosystems
There are two types of photosystems
Photosystem I and Photosystem II


Electron transport system
 The light driven reactions of photosynthesis are referred to as electron transport chain. When PS II
absorbs photons of light, it is excited and the electrons are transported through electron transport chain
of plastoquinone, cytochrome b6, cytochrome f and plastocyanin.
 The electrons released from PS II phosphorylate ADP to ATP. This process of ATP formation from
ADP in the presence of light in chloroplast is called photophosphorylation.
 Now, the PS II is in oxidised state. It creates a potential to split water molecules to protons, electrons
and oxygen.

10. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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 This light dependent splitting of water molecules is called photolysis of water. Manganese, calcium and
chloride ions play prominent roles in the photolysis of water. The electrons thus released are used in the
reduction of PS II.
 Similar to PS II, PS I is excited by absorbing photons of light and gets oxidised. This oxidised state of
the PS I draws electrons from PS II and gets reduced.
 The electrons released to PS I are transported through electron transport chain of ferredoxin reducing
substrate, ferredoxin and ferredoxin NADP reductase to reduce NADP+ to NADPH2.
Cyclic and noncyclic photophosphorylation
 In chloroplasts, phosphorylation occurs in two ways – noncyclic photophosphorylation and cyclic
photophosphorylation.
Noncyclic photophosphorylation
 When the molecules in the PS I are excited the electrons are released. So, an electron deficiency or a
hole is made in the PS I.
 This electron is now transferred to ferredoxin to reduce NADP+. When the molecules in the PS II get
excited, electrons are released.
 They are transferred to fill the hole in PS I through plastoquinone, cytochrome b6, cytochrome f and
plastocyanin.
 When the electron is transported between plastoquinone and cytochrome f, ADP is phosphorylated to
ATP.
 The ‘hole’ in the PS I has been filled by the electron from PS II. Then the electrons are transferred from
PS I to NADP+for reduction.
 Therefore, this electron transport is called noncyclic electron transport and the accompanying
phosphorylation as noncyclic photophosphorylation.
 The noncyclic electron transport takes place in the form of ‘Z’. Hence, it is also called Z-scheme

Non-cyclic photophosphorylation

Photolysis of water
– Photolysis is the splitting of water by light into H+ ion, O2 and electrons e-
– It is also called photodissociation or photodecomposition

11. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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– It is an oxidation reaction occurring during photosynthesis
– It is the first phase of light reaction
– It occurs in the photosystem –II (PS-II) of chloroplast
– Sunlight excites PS-II release energy in the form of electrons
– The electrons are transferred to the reaction centre of photosystem-II
• This energy is received by the oxygen evolving complex (OEC) of PS-II
• The OEC consist of 4Mn++ ions and Ca++ ions
• The OEC combines with 2 molecules of water and splits them into 4 electrons, O2 and H+ ions
• The electrons from the OEC are accepted by phaeophytin and is transported through non- cyclic electron
transport to NADP
• This electron transport synthezes ATP and NADP2 It supplies electrons for non cyclic electron
transport
• It helps in the synthesis of ATP and NADPH2
• It supplies O2 to the earth
• It is the first phase of Photosynthesis

Cyclic photophosphorylation
 Under the conditions of (i) PS I only remains active (ii) photolysis of water does not take place (iii)
requirement of ATP is more and (iv) nonavailability of NADP+ the cyclic photophosphorylation
takesplace.
 When the molecule in the PS I is excited, the electrons are released. The electrons are captured by
ferredoxin through ferredoxin reducing substrate (FRS).
 Due to non-availability of NADP+, electrons from ferredoxin fall back to the molecules of PS I through
the electron carriers - cytochrome b6, cytochrome f and plastocyanin.
 These electron carriers facilitate the down hill transport of electrons from FRS to PS I.
 During this transport of electrons, two phosphorylations take place - one between ferredoxin and
cytochrome b6 and the other between cytochrome b6 and cytochrome f. Thus, two ATP molecules are
produced in this cycle.

12. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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Cyclic photophosphorylation

13. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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LIGHT INDEPENDENT REACTION
Dark reactions (Calvin Cycle)
 The reactions that catalyze the reduction of CO2 to carbohydrates with the help of the ATP and
NADPH2 generated by the light reactions are called the dark reactions.
 The enzymatic reduction of CO2 by these reactions is also known as carbon fixation. These reactions
that result in CO2 fixation take place in a cyclic way and were discovered by Melvin Calvin. Hence, the
cycle is called Calvin cycle. Fixation of carbondioxide in plants during photosynthesis occurs in three
stages – fixation, reduction and regeneration of RuBP.
Fixation
 The acceptor molecule of CO2 is a 5C compound called ribulose-1,5- bisphosphate (RuBP).
 Fixation of a molecule of CO2 to RuBP is catalyzed by the enzyme RuBP carboxylase.
 The resulting 6C compound is highly unstable and gets cleaved to form two molecules of 3C compounds
called phosphoglyceric acid (PGA).


Reduction
 The two molecules of PGA are further reduced to glyceraldehyde-3- phosphates in two steps. First, two
PGA molecules are converted to 1,3 - bisphosphoglyceric acids by the enzyme PGA kinase.
 This reaction consumes two molecules of ATP in the ratio of one ATP for each molecule of 1,3-
bisphosphoglyceric acid formed.
 In the second step, the two molecules of 1,3-bisphosphoglyceric acid are reduced to glyceraldehyde-3-
phosphates by the enzyme glyceraldehyde- 3-phosphate dehydrogenase with the help of the light
generated reducing power NADPH2. So, two molecules of NADPH2 will be consumed during this
reaction.
 To reduce one molecule of CO2 upto reduction two ATP and two NADPH2 are consumed.

Regeneration of RuBP
 The glyceraldehyde 3-phosphate molecules are converted to RuBP through a series of reactions, which
generate 4C, 6C and 7C phosphorylated compounds as intermediates. For better and easy understanding
of these reactions, a simplified scheme of Calvin cycle considering three CO2 molecules fixation
reactions is shown below.
 The reactions of regeneration of RuBP are as follows.

14. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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 1. Some of the Glyceraldehyde 3-phosphate molecules are converted to dihydroxy acetone phosphates.
 2. Glyceraldehyde 3-phosphate combines with dihydroxy acetone phosphate to form fructose1,6-
bisphosphate.
 3. Fructose 1,6-bisphosphate undergoes dephosphorylation to form fructose 6-phosphate.
 4. Fructose 6-phosphate combines with glyceraldehyde 3-phosphate obtained from the fixation of
second molecule of CO2 to form Ribose
 5-phosphate (R5P) and Erythrose 4-phosphate (Er4P).
 5. Erythrose 4-phosphate combines with DHAP obtained from the second CO2 fixation, to form
sedoheptulose 1,7-bisphosphate.
 6. Sedoheptulose 1,7-bisphosphate undergoes dephosphorylation to form sedoheptulose 7-phosphate.
 7. Sedoheptulose 7-phosphate combines with glyceraldehyde 3-phosphate obtained by the third CO2
fixation, to form two molecules of 5C compounds – ribose 5-phosphate and xylulose 5-phosphate
(Xy5P).
 8. Ribose 5-phosphate and xylulose 5-phosphate molecules aretransformed to ribulose 5-phosphate
(Ru5P).
 9. Ru5P molecules are then phosphorylated by ATP to form RuBP molecules, which again enter into the
cycle of CO2 fixation.

15. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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In the above illustration, three CO2 molecules are fixed and the net gain is a 3C called DHAP. These triose
phosphate molecules combine to form hexose phosphates, which are used to form sucrose. For every carbon
fixation 3ATP and 2 NADPH2 are consumed.

Rubisco
 Ribulose bisphosphate carboxykase oxygenase
 It fixes carbondioxide and oxygen
 It is the most import enzyme involved in Calvin cycle during the process of carboxylation
 It is the most important abundant protein present in the earth
 It is a relatively slow enzyme
 Which can fix only 3-10 carbondioxide molecules per second


C3 and C4 pathways
It was once thought that all green plants fix CO2 through Calvin cycle only. Now, we know that certain
plants fix CO2 in a different photosynthetic mechanism called C4 pathway.
Hatch and Slack observed that 4C compounds such as oxaloaceticacid, malate and aspartate were the first
formed compounds, when the leaves of sugarcane were exposed to 14CO2 for one second. So, sugarcane is an
example for C4 plant. When the leaves of rice plant are exposed to 14CO2, 3C compound called phosphoglyceric
acid is formed. So, rice plant is an example for C3 plant.
In C3 plants, photosynthesis occurs only in mesophyll cells. We already learnt that photosynthesis has
two types of reactions – light reactions and dark reactions (Calvin cycle). In light reactions ATP and NADPH2
are produced and oxygen is released as a byproduct. CO2 is reduced to carbohydrates by dark reactions. In C3
plants both light reactions and dark reactions occur in mesophyll cells, whereas in C4 plants, the mechanism of
photosynthesis requires two types of photosynthetic cells – mesophyll cells and bundle sheath cells. The C4
plants contain dimorphic chloroplasts i.e. chloroplasts in mesophyll cells are granal (with grana) whereas in
bundle sheath chloroplasts are agranal (without grana). The presence of two types of cells leads to segregation
of photosynthetic work i.e. light reactions and dark reactions separately.

Hatch-Slack pathway of CO2 fixation (PEP = Phosphoenol pyruvic acid, OAA = Oxalo acetic
acid)

16. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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Hatch-Slack pathway involves two carboxylation reactions. One takes place in chloroplasts of mesophyll cells
arnd another in chloroplasts of bundle sheath cells.
C4 pathway or Hatch & Slack pathway
• Occurs in plants like maize, sugarcane, pearl millet etc.
These plants show presence of two types of photosynthetic cells- mesophyll cells and bundle sheath cells
C 4 Plants
 Produce acids with 4C atoms eg. Oxalo acetic acid
 Which can keep stomata closed on hot days but photorespiration doesnot occur
 Which are mostly tropical and sub tropical
 Eg. Maize, corn, sugar cane
 All most 10 % of all plants are belongs to C4 plans
Characteristics of C4 plants
• They are found in tropical regions.
• Main feature is Kranz anatomy in leaves.
• Bundle sheath has large green barrel shaped cells with one or two concentric arrangement of mesophyll
cells.
• Mesophyll cells lack intercellular spaces.
• Chloroplast in bundle sheath cells are large and donot have well-defined grana.
KRANZ ANATOMY
• This type of anatomy is found in C4 plants.
• In leaves of such plants, palisade tissues is absent.
• There is bundle sheath around vascular bundles.
• Chloroplast in the bundle sheath cells are large without grana.
• Chloroplast in mesophyll cells are small but with well developed grana

17. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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1. The first step involves the carboxylation of phosphoenol pyruvic acid in the chloroplasts of mesophyll
cells to form a 4C compound, oxaloacetic acid. This reaction is catalysed by the enzyme phosphoenol
pyruvate carboxylase.

2. Oxaloacetic acid is converted into aspartic acid by the enzyme transaminase or it may be reduced to malic
acid by NADP+ specific malate dehydrogenase.



3. Malic acid or aspartic acid formed in chloroplast of mesophyll cells is transferred to the chloroplasts of
bundle sheath where it is decarboxylated to form CO2 and pyruvic acid in the presence of NADP+specific
malic enzyme.



4. Now, second carboxylation occurs in chloroplasts of bundle sheath cells. Ribulose bisphosphate accepts CO2
produced in step (3) in the presence of RuBP carboxylase and yields 3-phosphoglyceric acid. Some of the 3-
phosphoglyceric acid molecules are utilised to produce sucrose and starch, while remaining PGA molecules are
used for the regeneration of RuBP




5. The pyruvic acid produced in step (3) is transferred to the chloroplasts of mesophyll cells where it is
phosphorylated to regenerate phosphoenolpyruvic acid . This reaction is catalysed by pyruvate kinase in the
presence of Mg 2+.

The AMP is phosphorylated by ATP in the presence of adenylate kinase to form 2 molecules of ADP. C4 plants
are photosynthetically more efficient than C3 plants, because the net requirement of ATP and NADPH2 for the
fixation of one molecule of CO2 is considerably lower in C4 plants than in C3 plants.

18. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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Difference between C3 Cycle and C 4 Plants
No C3 Cycle C 4 Cycle
1 The primary CO2 acceptor is a 5C
compound –Ribulose bisphosphate
(RuBP)
The primary CO2 is a 3c compound
phoshpoenol puruvic acid (PEP)
2 The first stable compound formed is
phospholyceric acid (PGA) which
contain 3C atoms
The first stable compound formed is
a 4c oxaloacetic acid (OAA)
3 C3 cycle is completed in only one type
of chloroplast present in mesophyll
cell
C4 Cycle is completed in two types
of chloroplast, one occurring in
mesophyll cells and other in bundle
sheath cells
4 It takes place at comparatively low
temperature
It takes place at high temperature and
more light intensities
5 Photorespiration occurs in C3 plants Photorespiration not occurs in c4
plants
6 The rate of photosynthesis is
comparatively lower
The rate of photosynthesis is
comparatively higher
7 It occurs in C3 plants which show
normal anatomy
It occurs in C4 plants which shows
krants anatomy


Photorespiration or C2 cycle

19. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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In animals and bacteria, only one kind of respiration known as dark respiration occurs. This is not
affected by the presence or absence of light. But in certain green plants, there are two distinct types of
respiration – photorespiration and dark respiration. Respiration that occurs in photosynthetic tissues in the
presence of light and results in increased rate of carbondioxide evolution is called photorespiration or light
respiration. Photorespiration involves three organelles – chloroplasts, peroxisomes and mitochondria. Oxidation
of RuBP in the presence of high oxygen is the first reaction of photorespiration. This reaction is catalysed by
Rubisco enzyme called carboxylase. It leads to the formation of 2C compound – phosphoglycolic acid and 3C
compound PGA. When PGA is used up in the Calvin cycle, the phosphoglycolic acid is dephosphorylated to
form glycolic acid in the chloroplasts. From the chloroplast, the glycolic acid diffuses into the peroxisome
where it is oxidised to glyoxalic acid and hydrogen peroxide. In peroxisome from glyoxalic acid, glycine is
formed.
PHOTORESPIRATION (C2 cycle, glycolate cycle)
• In C3 plants, RuBPcarboxylase (RUBISCO) functions as oxygenase at high temp and O2 conc.
• RuBP oxygenase oxidizes RuBP to phosphoglycerate (3C) and phosphoglycolate (2C).
• Phosphoglycolate is converted into glycolate and transported to peroxisomes.
• In the peroxisomes glycolate is oxidized to glyoxylate.
• This glyoxylate is then converted to an amino acid-glycine.
• Glycine enters the mitochondria and here, two glycine gives rise to serine ,CO2.
• Serine again enters the peroxisome and is converted to glycerate
• Glycerate enters the chloroplast where it is phosphorylated to form PGA.
• PGA now enters the Calvin cycle.
• Since there is loss of photosynthetically fixed carbon and no energy- rich compound is produced,
photorespiration is considered as a wasteful process.
• It reduces dry matter production and yield of the plant.
• However, its uses can be- production of glycine and serine that are important for metabolites like
proteins and chlorophyll.

20. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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Glycine molecules enter into mitochondria where two molecules of glycine combine to give a molecule of
serine, NH3 and CO2. During this process, NAD+ is reduced to NADH2. The aminoacid serine is taken to
peroxisome where, it is converted into hydroxy pyruvic acid. Hydroxy pyruvic acid is reduced by NADH2 to
form glyceric acid. The glyceric acid leaves peroxisome and enters chloroplast, where it is phosphorylated to
PGA, which enters into Calvin cycle. During the photorespiratory pathway, one CO2 molecule released in
mitochondria is to be re-fixed. Photorespiration is also known as photosynthetic carbon oxidation cycle or C2
cycle. Under the conditions of high light and limited CO2 supply, photorespiration protects the plants from
photooxidative damage. This means that, if enough CO2 is not available to utilize light energy, excess energy
causes damage to plant. However, photorespiration utilizes part of the light energy and saves the plant from
photooxidative damage. Increased O2 level increases photorespiration whereas increased CO2 level decreases
photorespiration and increases photosynthesis.

CAM PATHWAY (CAM Cycle)
 Crassulacean Acid Metabolism is present in the 5 % of the plants in the earth
 CAM is a variant of the C4 pathway of photosynthesis found in many succulent plants that live in hot
and arid deserts

21. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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 Eg. Pine apple, cactus, succulents etc.
 Crassulacean acid metabolism is also called CAM Photosynthesis
 It is a carbon fixation pathway that evolved in some plants as adaptation to arid plants (Drought)
 The stomata in the leaves close during the day to reduce evapotranspiration and produce sugar but open
at night and collect CO2
 The carbon dioxide stored as four carbon acid ie maliec acid or malate and react with RuBisco produce
sugar through photosynthesis at day

Significance of CAM Pathway
 The CAM mechanism enables plants to maximize their water use efficiency due to the special type
of opening mechanism of stomata
 Typically CAM plants looses 50 to 100 g of water for every gram of CO2 gained compared with the
values of 250-300 nm and 400-5e00 g of C4 and C3 plants respectively
 Thus CAM Plants are specially adapted to arid environment
 Like in C4 plants, the elevated internal concentration of CO2 effectively suppresses the
photorespiratory oxygenation of ribulose bis phosphate and favors carboxylation



Photosynthetic efficiency
The three types of photosynthesis are C3, C4, and CAM
C3 photosynthesis: C3 photosynthesis is the typical photosynthesis that most plants use C4 and CAM
photosynthesis are both adaptations to arid conditions because they result in better water use efficiency.
• In addition, CAM plants can "idle," saving precious energy and water during harsh times, and C4 plants
can photosynthesize faster under the desert's high heat and light conditions than C3 plants because they
use an extra biochemical pathway and special anatomy to reduce photorespiration.
• C3 Photosynthesis C3 plants.
Called C3 because the CO2 is first incorporated into a 3 -carbon compound.
Stomata are open during the day.
RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2.
Photosynthesis takes place throughout the leaf.

22. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under
normal light because requires less machinery (fewer enzymes and no specialized anatomy)..
Most plants are C3.
C4 Photosynthesis : C4 plants.
• Called C4 because the CO2 is first incorporated into a 4 -carbon compound.
Stomata are open during the day.
Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme allows CO2 to be
taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for
photsynthesis.
Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)
Adaptive Value: Photosynthesizes faster than C3 plants under high light intensity and high
temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and
undergo photorespiratio n.
Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to
keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for
photosynthesis.
C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush
pictured here, corn, and many of our summer annual plants.
CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism
Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO2 is
stored in the form of an acid before use in photosynthesis.
•
Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day.
The CO2 is converted to an acid and stored during the night.
• During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis
•
Adaptive Value:
Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when
transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).
•
May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed
night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in
respiration is used for photosynthesis.
• This is a little like a perpetual energy machine, but there are costs associated with running the machinery
for respiration and photosynthesis so the plant cannot CAM-idle forever.
• But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very
quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant
during dry spells).
CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads

Factors affecting photosynthesis
 The important factors affecting photosynthesis are
 External factors and internal factors
 External factors are Light intensity, Carbondioxide concentration, temperature and Water

23. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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 Internal factors are Chlorophyll, Protoplasm, Photosynthesis product (Photosynthate)
 Photosynthesis is influenced by both environmental and genetic factors.
 The environmental factors include light, availability of CO2, temperature, soil, water and nutrient supply
apart from age of leaf, leaf angle and leaf orientation.
 Photosynthesis is not affected by all environmental factors at a given time.
 According to Blackmann who postulated Law of Limiting factor in 1905, photosynthesis is limited by
slowest step of the most limiting factor in the pathway.
 This means that at a given time, only the factor that is most limiting among all will determine the rate of
photosynthesis.
 For example, if CO2 is available in plenty but light is limiting due to cloudy weather, the rate of
photosynthesis under such situation is controlled by the light.
 Further, if both CO2 and light are limiting, then the factor which is the most limiting of the two will
control the rate of photosynthesis. Both quality and intensity of light influence photosynthesis.
 Light between the wavelength of 400nm to 700nm is most effective for photosynthesis and this light is
called photosynthetically active radiation.
 As the intensity of light increases the rate of photosynthesis increases. However, if the light intensifies,
the rate of photosynthesis decreases.
 This is because of higher intensity of light destruction of chlorophyll occurs. Photochemical reactions
and dark reactions of photosynthesis respond differently to temperature. Photochemical reactions in the
thylakoid remain unharmed by temperature, whereas the enzymatic dark reactions get influenced
adversely.
 At higher temperature, the enzymes become inactive. Low temperature also inactivates the enzymes.
The current level of CO2 is about 0.036 per cent or 360 ppm (parts per million), which is very low as
compared to the concentration of other gases in the atmosphere such as O2 and N2.
 The rate of photosynthesis in all plants increases with increase in the concentration of CO2 upto 500
ppm, when other factors are not limiting. Availability of water in soil has a prominent effect on
photosynthesis.
 If the soil water becomes limiting factor, the rate of photosynthesis declines. Among various nutrients,
nitrogen has a direct relationship with photosynthesis.
 Since, nitrogen is a basic constituent of chlorophyll and all enzymes involved in dark reactions, any
reduction in nitrogen supply to plants has an adverse effect on photosynthesis.
 In general all essential elements affect the rate of photosynthesis. Among leaf factors, such as leaf age,
leaf angle and leaf orientation, leaf age has the most prominent effect on photosynthesis.
 If leaf undergoes, senescence, loss of chlorophyll occurs. The photosynthetic enzymes also get
inactivated resulting in reduced rate of photosynthesis.

Limiting Factors
 In a metabolic pathway, if one factor is low, its reaction will be slowed. This has a knock on effect on other
reactions in the pathway. The factor which is closet to its minimum is known as the limiting factor
 Changes to one of these factors can affect the rate of photosynthesis. The factor that is nearest to its minimum at
any one time is called the Limiting factor and this in the one that affect the rate of photosynthesis
 The law of limiting factors states that when a chemical process depends on more than one essential
condition being favourable, the rate of reaction will be limited by the factor that is nearest its minimum
value
 Photosynthesis is dependent on a number of favourable conditions, including:
 Temperature
 Light intensity

24. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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 Carbon dioxide concentration
 Temperature: Low temperature slow all enzyme catalysed reactions in calvin cycle. At high
temperature, RuBisco is denatured so carbon is not fixed. More commonly limiting at low temperature
than high
 Photosynthesis is controlled by enzymes, which are sensitive to temperature fluctuation
 As temperature increases reaction rate will increase, as reactants have greater kinetic energy and more
collisions result
 Above a certain temperature the rate of photosynthesis will decrease as essential enzymes begin to
denature

The Effect of Temperature on Photosynthetic Rate


 Light intensity: If it is low, ATP and NADPH productio will slow rarely the limiting factor
 Light is absorbed by chlorophyll, which convert the radiant energy into chemical energy (ATP)
 As light intensity increases reaction rate will increase, as more chlorophyll are being photo-activated
 At a certain light intensity photosynthetic rate will plateau, as all available chlorophyll are saturated with
light
 Different wavelengths of light will have different effects on the rate of photosynthesis (e.g. green light is
reflected)

The Effect of Light Intensity on Photosynthetic Rate

25. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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 CO2 Concentration: Carobon fixing is limited which leads to less glycerate 3-phosphate RuBP and
NADPH build up. Atomospheric CO2 is low , so this is oftern the limiting factor
 Carbon dioxide is involved in the fixation of carbon atoms to form organic molecules
 As carbon dioxide concentration increases reaction rate will increase, as more organic molecules are
being produced
 At a certain concentration of CO2 photosynthetic rate will plateau, as the enzymes responsible for carbon
fixation are saturated

Effect of Carbon Dioxide Concentration on Photosynthetic Rate

26. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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l


Translocation and Distribution of Photoassimilates
 Transport of dissolved organic substances from the site of synthesis to other parts of the plants is called
translocation of organic solutes
 The organic substances are translocated in soluble form
 This translocation occurs through the phloem
 The fluid transported through the phloem is called phloem sap
 In plants carbohydrates a fore synthesized in leaves and other green parts of the plant tissues. These food
materials are translocatted to the non green parts such as roots for utilization and storage
 The region of synthesis is called supply end or source and the region of utilization I organism called
consumption end or sink
 The organic food materials are always transported from the source to sink
 Translocation of organic solute always takes place from the region of higher concentration to region of
lower concentration
Importance of solute translocation
 Food materials synthesized in green tissues are distributed to other tissues via solute translocation
 By translocation food is served to the areas of utilization such as buds, growing leaves etc.
 The excess food materials are transported to the storage organs such as fruits, seeds, underground stems,
bulbils etc.

27. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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 During germination of seeds, stored food materials are dissolved and transported to the growing points
of the seedlings
Phloem Sap
 The fluid transported through the phloem is called phloem sap
 Water is the most abundant substance in the phloem sap
 The solutes includes; sugars, aminoacids, organic acids, proteins, potassium, chlorides, phosphates,
magnesium, Auxin, gibberlins, Cytokinins, Abscissic acid etc.
 These are found dissolved in water
 Sucrose is the most commonly transported in sieve elements
 Phloem sap may be collected from exudates coming from wounds of plants or from stylets of aphides
after cutting it with a laser beam
 The composition of various substances in the sap has been determined by biochemical tests
Direction and path of translocation
 In plants organic solutes move down ward or upward or radially

Down ward translocation
 In plants, leaves occur at the top and stem and roots occur below the leaves
 Therefore food materials synthesized in the leaves are transported to the stem and rots down wards
through the phloem they may be consumption or storage
Upward Translocation
 This is the translocation of food materials to seeds or to shoot tips from underground storage parts
 During the seed germination, food materials stored in cotyledons are transported to growing the
embryonal axis
Radial translocation
 The translocation of food materials from the phloem to cortical cells in the lateral ways is called
radial translocation
 During the seed or fruit formation, the food materials are transported to them from the leaves in the
radial direction

Mechanism of Phloem Transport
The transport of organic solutes from the sites of synthesis to other parts of the plant through phloem is called
translocation of solutes. In plants sucrose is the main solute for translocation. It is synthesized in the leaves by
photosynthesis. It is transported from the leaves to the stem and root for consumption and storage. Translocation
of solutes mainly occurs through sieve tubes of phloem in higher plants. Usually solutes flow from the site of
their higher concentration (Leaves) to the site of lower concentration (Stem and root). This occurs by
hydrostatic pressure and osmotic pressure. There are many theories to explain the mechanism of translocation
oef solutes in plants.
They are:
1. Munch’s Mass flow hypothesis or Pressure flow hypothesis
2. Diffusion hypothesis

28. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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3. Protoplasmic streaming hypothesis
4. Electro-osmotic theory
Munch’s Mass flow hypothesis or Pressure flow hypothesis
• The mass flow hypothesis for translocation of organic solvents in plants was given by Munch.
• Accordingly, the transport of food takes place through phloem along a concentration gradient.
• Organic food is manufactured in the mesophyll cells of the leaves.
• This increases the osmotic pressure of these cells.
• Due to the increase in OP, water from the xylem elements and neighbouring cells enters these cells, thus raising
the TP.
• This forces some of the dissolved food from these cells into the sieve tubes.
• The cells of the root and storage organs after utilizing the food, will have low TP.
• This creates a turgor pressure gradient between the leaf and the other cells.
• Because of this, movement of water containing dissolved organic food takes place from the upper end to the lower
end of the plant through phloem.
• The leaf is described as the source and the root as sink.
• Major criticism against this is that this theory does not explain the side ways translocation that takes place from
phloem to the periphery of the stem.



Diffusion Hypothesis

Diffusion is a simple process by which a substance moves from its region of higher concentration to the region
of lower concentration. According to this hypothesis, translocation occurs by simple diffusion. It takes place
along the concentration gradient between the supply end and consumption end. The rate of translocation will be
greater when the concentration gradient of the solutes between the sucrose and sink is greater.

Protoplasmic streaming hypothesis
According to this theory, moving protoplasm carries the solutes within the sieve elements and the protoplasmic
fluid move from cell to cell through large protoplasmic connection across the sieve plates. This theory was
proposed first by De Vries in 1885 later it was elaborated by Curtis in 1935.
Circular movements of cytoplasm is called cyclosis has been observed in many different plant cells. The
cyclosis of protoplasm of sieve tubes provides a driving force for the solutes to move from one end to other end.

29. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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From the end of the sieve tube, solutes diffuse through the sieve pores to get into another cell. Therefore,
movement of solutes is due to combined action of cytoplasmic streaming and diffusion.
This theory explains the bidirectional flow of organic solutes through a column of sieve tubes. The evidence in
support of this theory are following
 Streaming of granular substance is observed in all living cells
 Streaming allows a more rapid transport than the diffusion
 Factors that reduce the rate of cytoplasmic streaming ar reducing the translocation of organic solutes
through the sieve tubes.
 Protoplasmic connections are observed between adjacent sieve tubes through the pores present in the
sieve plates
 Organic solutes are transported through the transcellular cytoplasmic strands (Plasmodesmata) existing
between the sieve tubes of phloem
Electro-osmotic theory
This theory was proposed by Fenson and Spanner. According to this theory, the translocation of organeic
solutes through the sieve tubes takes place due to the electrochemical potential developed across the sieve
plates. The electrochemical potential is developed by accumulation and circulation of K
+
ions between the
companion cells and the adjacent sieve tubes. Accumulation of K
+
ions at on end of sieve tube is due to an
active process by the companion cells. Organic molecules then diffuse through the sieve pores to neutralize the
net charge in the sieve tubes.

Phloem Loading
As a result of photosynthesis, the sugars such as sucrose produced in mesophyll cells move to the sieve tubes of
smallest veins of the leaf either directly or through only 2-3 cells depending upon the leaf anatomy.
Consequently, the concentration of sugars increases in sieve tubes in comparison to the surrounding mesophyll
cells. Transfer of photosynthetic products from the mesophyll cells to sieve tubes at the supply end is called
phloem loading. This is the preceding step of phloem translocation. Phloem loading occurs in the sieve tubes of
leaves. Sucrose and other organic solutes are transferred from the site of synthesis to the sieve tubes. It is an
active process that needs energy stored in ATP. The phloem loading at source end and phloem unloading at sink
generate a driving force for pushing the phloem sap to move through the sieve tubes.
Steps involved in Phloem Loading
 Triose phosphate synthesized by photosynthesis is transported from the chloroplast to the cytosol. It is
converted to sucrose during the night, stored starch is converted into sugars which are then converted
into sucrose
 Sucrose is transferred from mesophyll cells to vicinity of the sieve elements at the supply end. It occurs
for a distance of 2-3 cells. Hence it is called short distance transport
 Sucrose enters into the sieve tube and companion cells at the supply end. This process is called sieve
element loading. Both sieve tubes and companion cells take part in the phloem loading
 Sucrose transported through sieve elements in veinlets of leaves is passed to sieve elements of large
veins and others before getting transported to roots. This is called export
 Sucrose is then transported to roots through sieve elements. This is called long distance transport

30. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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Ways of phloem loading
 Sucrose and other sugars synthesized in mesophyll cells are transferred to sieve tubes of phloem either
by symplast pathway or apoplast pathway. In symplast pathway, sugar moves exclusively through the
plasmodesmata between the cells and they enter the apoplast at some points en route to the phloem. On
the other hand in apoplast pathway, sugars are actively loaded from the cytoplasm to apoplast and then
transported through cell walls and intercellular spaces. Finally they delivered in to the sieve elements
and companion cells. It is the energy driven pro.cess. Apoplastic pathway need metabolic energy
Phloem unloading
The transfer of sucrose and other solutes from the sieve tube to receiver cell in the sink organ is called phloem
unloading. Paranchyma cells of stem or root act as receiver cells at the consumption end. Phloem unlading of
sucrose involves the following steps
 Sucrose comes out from the sieve tube in the consumption end of the sing organ. The sucrose then
diffuses through the parenchyma cells for a short distance. This is often called post –sieve element
transport. Finally sucrose is converted into insoluble carbohydrates and stored in the cells of the sink
organ. In the sink organ, phloem unlading occurs via apoplastic and smplastic pathway. Sucrose may
move entirely through symplast or may enter the apoplast at some points. Phloem unloading is
exclusively through symplast is reported inyoung leaves of sugar beet and tobacco and meristems.

Assimilates Partitioning or Photosynthate partitioning
• The products of carbon assimilation or photosynthesis such as hexoses, sucrose, starch etc. i.e. fixed
carbon are called as photosynthates or photaassimilates or simply as assimilates. These assimilates are
produced in green leaves of higher plants which constitute the sources. Within various compartments
of photosynthesizing cells (sources), these assimilates are (i) metabolically utilized, (ii) stored or (iii)
converted inot transport sugars mainly sucrose for export to various sinks (thorugh phloem) such as
young leaves, roots, tubers, stems, fruits and seeds. At the sinks, Assimimilates are metabolicaly

31. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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utilized and/or stored in receiver cells of sinks. Depending upon the nature and specific requirement of
the sinks, the photoassimialtes are differently distributed in different sinks This differential distribution
of photoassimilats in different sinks of plant is called as Assimilates Partitioning
Factors affecting assimilate partitioning
• Competition Among Sink tissues for available Translocated Assimilates
• Photosynthesis and Sink Demand
• Long Distance Signals Between Sources and Sinks
• Turgor Pressure
• Phytohormones
Source Sink Concept
The diversification of photosynthate to various sink organ at different rate is called assimilate allocation. This
product of photosynthesis is called photosynthetic assimilate or assimilates or photosynthate. This assimilate is
synthesized in leaves and transported to other organs via sieve elements of phloem. Here leaves serve as source
end and the receiving organ serve as sinks. The photosynthate move from the region of higher concentration
(Source) to region of lower concentration (Sink).
Transport of assimilates from the leaves to different sink organs is determined by six important factors they are
1. Storage and utilization of photosynthates by leaves
The availability of photosynthates in a leaf for transportation is determined by the rate of storage of the
photosynthates in the leaf cells and utilization of the phtosynthate for the metabolic activities. In leaves, starch
is synthesized from the photosynthages and stored within chloroplast. Large proportion of photosynthate is
utilized by the leaf cells for their metabolic activities.
2. Competition among the sink tissues
In a plant, source is connected with many sinks via inter-connecting vascular strands. Therefore there is a
competition among the sink organs for the photosynthetic assimilates. Leaves, stem, branches, flowers, roots,
fruits and seeds compete for assimilates
3. Photosynthetic rate and sink demand
There is direct relationship between the photosynthetic rate in leaves and sink demand in plants. The higher is
the sink demands the greater would be the photosynthetic rate in the source (Leaf). This condition is observed in
plants in which photosynthetic products are converted into starch instead of sucrose during the day night.
4. Turgor pressure
Turgor pressure of sieve elements is very important for the conduction of solutes between the source and sink.
Rapid phloem loading causes the accumulation of more soluble compounds in the sieve elements and hence
their turgor pressure increases considerably in the source end. Continuous phloem unloading from the sieve
elements decreases the turgor pressure at the consumption end. Therefore, solute move along the sieve elements
from the region of high turgor pressure to the region of lower trugor pressure

32. Dr. Abdussalam, A.K., Dept. of Botany, Sir Syed College, Taliparamba, Kannur 9847654285
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5. Plant Hormones
Plant hormones provide chemical signals for the translocation of solutes. Eg. Auxins, gibberllins, cytokinins
and ABA. They increase the rate of consumption of solutes at the sink side. As a result, the turgor pressure
decreases considerably to import more solutes from the source end.
6. Callose plugs
Higher level of calcium in the cytoplasm leads to the formation of callose plugs in the sieve tubes. In cases
where passage of sieve tube is blocked by callose plugs, the rate of translocation of organic solutes would be
greatly reduced.