Photosynthesis

Sanjay Mahato
IAAS
Introduction to Photosynthesis:
• The synthesis of complex organic material using carbon dioxide, water, inorganic
salts, and light energy (from sunlight) captured by light-absorbing pigments,
such aschlorophyll and other accessory pigments.
• The use of light energy to produce carbohydrates from carbondioxide and a
reducing agent such as water is known as photosynthesis.
• Photosynthesis consists of light reactions and dark reactions.

• This process can be simplified in this equation:

• It means photosynthesis is a process in which carbon dioxide (CO2), water (H2O)

and light energy are utilized to synthesize an energy-
rich carbohydrate like glucose(C6H12O6) and to produce oxygen (O2) as a by-
product.
• Photosynthesis is a vital process among photoautotrophs, like plants, algae and
some bacteria that are able to create their own food directly from inorganic
compounds using light energy so that they do not have to eat or rely
on nutrients derived from other living organisms.

• Photosynthesis occurs in plastids (e.g.chloroplasts), which are membrane-

bounded organelles containing photosynthetic pigments (e.g. chlorophyll),
within the cells of plants and algae.

• In photosynthetic bacteria (cyanobacteria) that do not have membrane-

bounded organelles, photosynthesis occurs in the thylakoid membranes in
the cytoplasm.
Significance of Photosynthesis:
(1) It is the primary source of organic food and food energy (ATP) for all forms of
life, either directly or indirectly.
(2) Excess sugars produced in photosynthesis are either stored in the form of
carbohydrates or used in the biosynthesis of other organic compounds.
(3) In any ecosystem, green plants represent the most essential biotic components as
they are the primary producers.
(4) Photosynthesis helps to purify air and also maintain balance of oxygen and
carbon dioxide in the ecosystem.
(5) Oxygenic photosynthesis was responsible for converting the totally anaerobic
condition on earth into aerobic atmosphere present now.
(6) The fossil fuels (e.g. natural gas, coal, petroleum (oil), etc.) are all energy-rich
materials of an organic origin. The energy stored in all these fuels is basically
solar energy which was trapped and stored during photosynthesis in the
geological past.
Photosynthetic Pigments:
• The photosynthetic products are energy-rich organic compounds. The potential
chemical energy of these compounds comes from the light energy .
• The light energy to be effective in photosynthesis must be absorbed by a siutable
pigments.
• This vital role is performed by green pigment, Chlorophyll, in plants.
• In plants, algae, and cyanobacteria, pigments are the means by which the energy
of sunlight is captured for photosynthesis.
• There are following types of photosynthetic pigments:
1. Chlorophylls
2. Carotenoids
3. Phycobillins
4. Pteridines
1. Chlorophylls:
• Chlorophylls are greenish pigments which contain a porphyrin ring. This is a
stable ring-shaped molecule around which electrons are free to migrate.
Because the electrons move freely, the ring has the potential to gain or lose
electrons easily, and thus the potential to provide energized electrons to other
molecules. This is the fundamental process by which chlorophyll "captures" the
energy of sunlight.
• Chlorophyll consists of tetrapyrrole skeleton formed into a ring , with an atom
of magnesium in the center of the ring.
• There are several kinds of chlorophyll, the most important being chlorophyll
"a". This is the molecule which makes photosynthesis possible, by passing its
energized electrons on to molecules which will manufacture sugars. All plants,
algae, and cyanobacteria which photosynthesize contain chlorophyll "a".
• A second kind of chlorophyll is chlorophyll "b", which occurs only in "green
algae" and in the plants.
• A third form of chlorophyll which is common called chlorophyll "c", and is
found only in the photosynthetic members of the Chromista as well as
the dinoflagellates.
2. Carotenoids (also called Tetraterpenoids):
• Carotenoids are usually red, orange, or yellow pigments, and include the
familiar compound carotene, which gives carrots their color.
• These compounds are composed of two small six-carbon rings connected by a
"chain" of carbon atoms. As a result, they do not dissolve in water, and must be
attached to membranes within the cell.
• Carotenoids cannot transfer sunlight energy directly to the photosynthetic
pathway, but must pass their absorbed energy to chlorophyll. For this reason,
they are called accessory pigments.
• It protect chlorophyll from photodamage.
• Light absorbed by carotenoids has been found to result in the flourescence of
chlorophyll.
• These are located in chloroplasts and chromoplasts.
• These are of two types:
1. Carotenes
2. Carotenols (Xanthophylls)
1. Carotenes:
• Carotenes are orange- yellow in colors consisting of carbon and hydrogen
having general formula C40H56.
• Carotenes are named after carrot in which they are abundant.
• These are capable of absorbing light in violet and blue green parts of the
spectrum.

2. Carotenols (Xanthophylls)
• Xanthophylls are yellow or brown pigment and contains hydroxyl group. These
have general formula C40H56 O2.
• They are capable of absorbing light and help in converting elemental
oxygen to molecular form.
3. Phycobilins:
• Phycobilins are water-soluble pigments, and are therefore found in the
cytoplasm, or in the stroma of the chloroplast. They occur only in Cyanobacteria
and Rhodophyta.
• These are the phycoerythrin (red pigment) and phycocyanin (blue pigment)
found in the red and the blue green algae.
• They are protein- linked pigments which are destroyed by heat.
• These pigments takes active parts in photosynthesis.
• The phycobilins are especially efficient at absorbing red, orange, yellow, and
green light, wavelengths that are not well absorbed by chlorophyll a.

4. Pteridines:
• They are generally found in animals and thallophytes but rarely in higher plants.
• It is yellow pigment and act as photoreceptor
Role of Light in Photosynthesis:
• It has been observed that full sunlight is inhibitory for photosynthesis. The leaves
are oriented in such a manner that the sunlight intensity is effectively reduced to a
level when photosynthesis becomes more efficient.
• In direct sunlight the leaves lie at an acute angle to the rays. In the shade, on the
other hand, the leaves lie at right angles to the general direction of rays.
• It has been stated earlier that the pigments of photosynthetic process absorb light
only in certain regions of the spectrum and transmit the remaining wavelengths.
• The chlorophyll absorbs both the shorter (blue and violet) as well as longer
waves (orange and red) . On the other hand carotenoids absorb shorter
wavelengths only.
• The phycoerythrin absorbs green and yellow colors and the phycocyanin absorbs
the longer wavelengths (absorbing orange and red light, particularly near 620
nm). Together they absorbs most of the visible light.
• The photosynthetic bacteria absorbs even the infra-red light.
• Part of the radiant energy absorbed by chlorophyll is used in producing a
chemical change is its photochemical effect. Some part of radiant energy is re-
emitted as light, which is called as fluorescence.
• Chlorophyll-a absorbs photon of light & becomes activated.
• At this state chlorophyll-a expels one electron & develops a positive charge.
• The expelled electron contains extra amount of energy which is used during the
formation of ATP.
• During photophosphorylation step ATP is formed. This ATP is utilized for dark
reaction to fix carbon.
Absorption Spectra:
• Light is the visible part of spectrum of magnetic radiation. The radiation travels
in the form of waves which have longer and shorter wavelength .
• Visible light consists of radiation having wavelength from 390nm to 760nm
which can be absorbed by photosynthetic pigments. This spectra of wavelength
which can be absorbed by photosynthetic pigments during photosynthesis is
known as absorption spectra.
• Photosynthetic pigments absorbs light energy only in the visible part of the
spectrum. However, certain photosynthetic Bactria absorbs light energy use
infrared, light of comparatively shorter wavelength.
• The effectiveness of different wavelength of light on the photosynthetic activity
of an autotrophic plant is called action spectrum. The action spectrum is closely
related to the absorption spectrum for the photosynthetic plants.
Quantasome – The Photosynthetic Unit:
• A photosynthetic unit is the smallest group of pigment molecules which
collaborate together to cause a photochemical reaction, i.e., the absorption and
migration of a light quantum to a trapping centre where it brings about the
release of an electron.
• Quantasomes are particles found in the thylakoid membrane of chloroplasts in
which photosynthesis takes place.
• There is no actual morphological structure which could be termed as
photosynthetic unit.
• Photosynthesis is now believed to require the coordination of a series of
structures or sub-units distributed throughout the membrane system of the
chloroplast.
• It is assumed that to make a effective photochemical reaction about 250
chlorophyll , 8 molecules of oxidising agents , 8 molecules of Cytochrome F and
16 molecules of cytochrome b are present.
• Park and Biggins (1964) discovered distinct morphological structures in the
thylakoid membrane and many workers considered them to be the
photosynthetic units. However this hypothesis has been found to be
unconvincing. They are now considered as the system of ATP synthesis only but
the complete process of photosynthesis requires the participation of other
structures too.
 Mechanism Of Photosynthesis
➢ Photosynthesis is a complex process which involved successive oxidation and
reduction process.
➢ This involved oxidation of water and reduction of Carbondioxide.

➢ The reaction of photosynthesis occur in two distinct phases:

1. The light rxn: Uses light energy to synthesize NADPH and ATP.

2. The dark rxn: Uses the NADPH and ATP to synthesize carbohydrates from
CO2 & H2O.
1. The light rxn:
(Light Dependent phase, Photochemical rxn., or Hill’s reaction)
• It is a photochemical reaction which takes place in grana of thylakoids of
chloroplast in the presence of sunlight.
• It explains that water is used as a source of electrons for CO2 fixation & O2 is
evolved as a by product.
• The final products of this reaction are NADPH2 , ATP and Oxygen.

➢ The light reaction can be discussed in the following headings:

1. Red Drop & Emersion Enhancement Effect
2. Photosystem or Two pigment system
3. Photo Excitation of Chlorophyll- a
4. Photolysis of Water
5. Photophosphorylation
1. Red Drop & Emersion Enhancement Effect:
• Quantum yield is defined as the number of oxygen molecules released per light
quanta absorbed.
• Robert Emerson concluded that 8 quanta of light energy would be required for
the reduction of one molecule of carbondioxide to carbohydrate ( or for
produucing one molecule of oxygen). The quantum yield (yield per quantum) is
thus 1/8 or 12% only.
• The transfer of four electrons are required in the reduction of one CO2 molecule.
It was suggested that it takes two light quanta to move each electron.
• Emerson and Lewis (1943) determined the quantum yield of photosynthesis
under different wavelength and found that there was a pronounced decrease in
quantum yield at wavelength greater than 680 nm in the red zone. This discovery
of Emerson was termed as “Red Drop”.
• Emerson and his Co-worker later found that the decrease could be prevented &
photosynthetic rate can be enhanced by using shorter wavelength (red light) with
longer wavelength infrared light. The quantum yield is increased when light of
two different wavelength are given simultaneously. This photosynthetic
enhancement is referred to as the Emerson Effect.
Emerson et al. (1957) found that both the red and the
far-red light gave low rates of photosynthesis when
given individually but gave a rate greater than the
sum of the individual rates when the two were
given simultaneously
2. Photosystem or Two pigment system:
• From the discovery of Red Drop and Emerson Enhancement Effect, it was
concluded that photosynthesis is driven by two photochemical processes.
• These processes are associated with two groups of photosynthetic pigments
called as pigment system I and pigment system II.
• Chlorophyll- a exists in its two different forms, one of them absorbs light with
maximum 683 nm , called pigment system I (PS I) and the other with a
maximum of 672 nm , called pigment system II (PS II).
• For PS I the light is absorbed by Chlorophyll-a , P-700 nm, Chlorophyll-b and
carotenoids.
• For PS II light is absorbed by chlorophyll- b, phycobilins and carotenoids. The
P-680 nm constitute the rxn. Center for PS II.
• Each photosystem contains about 300 chlorophyll molecules that trap light
energy.
• In each photosystem, a molecule of chlorophyll –a is the primary pigment.
• The other chlorophyll molecules are known as accessory pigments, as they
absorbs light energy & transfer to chlorophyll-a
• The accessory pigments include carotenoids, phycobillins & chlorophyll b, c &
d.
3. Photo Excitation of Chlorophyll- a
• Light energy is absorbed by different pigments like chlorophyll-a, b, c, d , e
Carotenoids, phycobillins but this energy is ultimately transferred to chlorophyll
–a, which is the main pigment regulating light reaction.
• Chlorophyll-a absorbs photon of light & becomes activated.
• At this state chlorophyll-a expels one electron & develops a positive charge.
• The expelled electron contains extra amount of energy which is used during the
formation of ATP.
4. Photolysis of Water:
• When pigment molecule is photoactivated water molecules gets dissociated into
H+ & OH- ions .
• The dissociation of water molecules into H+ & OH- ions in the presence of light
and chlorophyll is called photolysis of water.

Light energy
4H2O 4H+ + 4OH-
Chlorophyll

• OH- releases electron from itself and becomes a free but highly reactive H+
released gets associated with NADP to form NADPH2
4OH- 4OH + 4e-
• These electrons fill the vacancy in chlorophyll molecule created by
photoactivation of chlorophyll.
• The OH molecule now releases O2 & forms H2O.
4OH 2H2O + O2
• This oxygen is liberated during photosynthesis . It should be noted that O2
produced during photosynthesis comes by dissociation of water and not from
carbondioxide.
5. Photophosphorylation:
• The process of formation of ATP from ADP and inorganic phosphate (Pi) during
photosynthesis process is known as photophosphorylation.

Light
ADP + Pi ATP
Chlorophyll

➢ It takes place in two different ways:

1. Non-Cyclic Photophosphorylation
2. Cyclic Photophosphorylation
1. Non-Cyclic Photophosphorylation (Z scheme):
• In this process, the electron expelled from chlorophyll molecule is not cycled
back to the same chlorophyll P- 680 (PS II).

• Electron transport in two photosystem (PSI & PS II ) takes place in three steps:
1. From H2O to P-680
2. From P-680 to P-700
3. From P-700 to NADPH

• Electrons are transported from water to P-680 (PS II) through Mn containing
enzymes.
• The ultimate source of electrons within the chloroplast is water.
• One molecule of oxygen is evolved for every four electrons donated to P-680
and one NADPH molecule requires two electrons for its production.
• Two quanta (Photons) are needed for excitement of one electron.
➢ The successive steps involved in transfer of an electron from : H2O to PS II to
PS I to NADP are given below:
1. Electron passes from water to PS II or P-680 to fill up the vacancy created by
excitement of chlorophyll molecule and emission of electron.
2. P-680 emits its electron & the electrons are received by a substance Q.
3. This electron is then passed to electropositive substance called photoquinone
(PQ).
4. The next acceptor of electron is cytochrome which exists in two forms
cytochrome b6 and Cytochrome-f (Cyt b6 & Cyt f)
In this step one molecule of ATP is formed by ADP & inorganic Phosphate (Pi).
5. Cyt f then passes its electron to next compound plastocyanin (PC) .
6. Plastocyanin handover the electrons to P-700 (PS I).
7. After absorption of second photon in PS I, this electron is then reached to
NADP from PS I.
8. The electrons from P-700 are received by Ferredoxin reducing substance (FRS).
9. This FRS finally emits electrons and reduces Ferredoxin (FD).
10. Here NADP is reduce to NADPH in presence of an enzyme Ferredoxin-NADP –
reductase. This reduced NADPH is utilized for the reduction of CO2 to
Carbohydrates.
• H+ ions accumulate inside the thylakoids membrane as a result of proton
gradient & diffuse across the thylakoid membrane & is used to produce
ATP. This process is also known as Chemiosmosis.
2. Cyclic Photophosphorylation:
• In this process, the electron expelled from chlorophyll molecule is cycled back to
the same chlorophyll P- 700.
• The electrons released by P700 of PS-I in the presence of light are taken up by the
primary acceptor and are then passed on to ferredoxin (Fd), plastoquinone (PQ),
cytochrome complex, plastocyanin (PC) and finally back to P700 i.e., electrons
come back to the same molecule after cyclic movement. Due to this cyclic
movement of electrons , this process is known as cyclic photophosphorylation.
• The cyclic photophosphorylation also results in the formation of ATP molecules
just like in non - cyclic photo phosphorylation.
• Two molecules of ATP are produced in this cycle.
• During cyclic photophosphorylation, electrons from photosystem - I are not
passed to NADP from the electron acceptor. Instead the electrons are transferred
back to P700.
• This downhill movement of electrons from an electron acceptor to P700 results in
the formation of ATP and this is termed as cyclic photophosphorylation.
• It is very important to note that oxygen and NADPH2 are not formed during cycle
photophosphorylation.
Fig: Cyclic Photophosphorylation
Difference between cyclic and non-cyclic photophosphorylation:

Cyclic photophosphorylation Non-cyclic photophosphoryaltion

1. Involves only PS I 1. Involves both PS I & PS II

2. The movement of electron is cyclic. 2. The movement of electron is non

cyclic or unidirectional

3. Only ATP is formed 3. Both ATP & NADPH2 are formed

4. Oxygen is not evolved 4. Oxygen is evolved

5. Dominant in photosynthetic bacteria 5. Dominant in green plants

6. Photolysis of water does not occur 6. Photolysis of water occurs

2. The dark rxn: (Blackmann’s Reaction)
• This second reaction of photosynthesis is independent of light, hence called dark
reaction.
• The dark rxn utilizes ATP & NADPH2 molecules formed during light rxn for
carbon fixation.
• It occurs in the stroma region of chloroplast.
• The CO2 absorbed by the plants from the environment combines with certain
compounds to form intermediate compounds and ultimately leads to the
formation of sugar and starch.
• Depending upon the initial product after CO2 fixation, three photosynthetic
pathways were recognised in plants:
1. Calvin Cycle(C3 cycle )
2. Hatch and slack Cycle (C4 cycle)
3. CAM (CAM plants)
➢ In C3 plants the first stable product after CO2 fixation is 3- C compound
(Phosphoglyceric acid) and in C4 plants, 4- C compound (oxaloacetic acid) is
the first stable product . In CAM cycle, the first stable product is malic acid.
➢ On the basis of different pathways for Dark rxn., plants are classified as;
1. C3 plants= follows C3 cycle i.e. Calvin Cycle during photosynthesis.
Examples: Most of dicots , rice , wheat, barely,
2. C4 Plants = follows Hatch & Slack cycle during photosynthesis.
Examples: Most of monocots, maize , sugarcane.
3. CAM Plants = follows CAM pathway during photosynthesis.
Examples: Xerophytes plants.
1. Calvin Cycle or C3 cycle:
➢ There are various steps involved in dark rxn of Calvin cycle. These are
discussed on several headings as follows:

1. Carboxylation:
• CO2 is first accepted by ribulose-1,5 diphosphate (RuDP) & forms an unstable
6-C compound.
• This 6-C compound immediately splits to form two molecules of
phosphoglyceric acid (PGA).
• The rxn takes place in the presence of enzyme RuDP Carboxylase (Rubisco).

RuDP + CO2 + H2O 2PGA

RuDP Carboxylase
2. Reduction:
• In second stage the phosphoglycerate is reduced to glyceraldehyde-3-
phosphate.
• Three molecules of CO2 are fixed to three molecules of ribulose-1,5-
bisphosphate to form six molecules of glyceraldehyde -3-phosphate (18
carbons).
• One molecule of this triose phosphate can either be used for energy production
via glycolysis and the citric acid cycle or condensed to hexose phosphate to be
used in the synthesis of starch or sucrose.

PGA + NADPH2 + ATP Triose-Phosphate Glyceraldehyde-3-phosphate

Dehydrogenase
+
ADP
+
NADP
3. Isomerization:
• The glyceraldehyde -3-phosphate molecule is converted into its isomers,
dihydroxyacetone phosphate (DiHAP) in the presence of enzyme triose
phosphate isomerase.

Triose-Phosphate
Glyceraldehyde-3-phosphate isomerase DiHAP

4. Hexoses Fromation:
• One molecule of Glyceraldehyde-3-phosphate combines with a molecule of
dihydroxyacetone phosphate to form Fructose-1,6 –diphosphate in the presence
of enzyme aldolase.

Glyceraldehyde-3-phosphate + DiHAP Aldolase Fructose-1,6-

diphosphate
➢ Fructose-1,6-diphosphate is converted into Glucose in the following steps:

Fructose-1,6-diphosphate

Phosphatase

Fructose-6-phosphate

Phosphofructo isomerase

Fructose-1-phosphate

Mutase

Glucose-1-Phosphate

Phosphatase

Glucose + iP

➢ Hence, Glucose is the main photosynthetic products, but it is stored in the

form of sucrose and starch
 5. Regeneration of RuDP:
• Ribulose diphosphate is regenerated through a series of reaction in the following
ways:
Fructose-6-P + Phosphoglyceraldheyde

Transketolase

Erythrose-4-P + DiHAP Erythrose-4-P + Xylulose-5-P

Aldolase Sedoheptulose-7 phosphate

+
Sedoheptulose-1,7 diphosphate Phosphoglyceraldehyde

Phosphatase Transketolase

Xylulose-5-P + Ribose-5-P
Sedoheptulose-7 phosphate Phosphopentose
Isomerase
Ribulose-5-P

Ribulose-1,5-diphosphate ATP ADP

2. Hatch and slack Cycle (C4 cycle):
• M.D. Hatch and C.R. Slack (1967) gave a new process of CO2 fixation.
• This cycle takes place mostly in monocots plants (Sugarcane, maize, Cyprus)
and in some dicots (Amaranthus).
• Plant in which CO2 fixation takes place by Calvin cycle only are called C3
plants (because first products of CO2 fixation is a 3 carbon containing
compound, Phosphoglyceric acid). But in Hatch –Slack Pathway , first product
of CO2 fixation is a 4 carbon containing compound, Oxaloacetic acid , hence
these plants are called C4 –Plants and the cycle of photosynthesis is known as
C4 - Cycle.
Anatomical Features of Leaves of C4 –Plants:
• Anatomically, the C4 –Plants are characterized by the presence of bundle sheath
cells around the vascular bundles.
• This type of arrangement is called Kranz Anatomy.
• In the mesophyll of leaves of these plants, pallisade tissue is absent.
• Typically C4 –Plants possess two types of chloroplasts. One type in kranz-
sheath, which is large and lack grana but contains enzymes of calvin cycle(C3 –
Cycle). Another type of chloroplast is found in mesophyll cells, which are
smaller with well developed grana and contains enzymes for C4 – Cycle.
➢ So, due to the distinct anatomical features of leaves of C4 –Plants, the dark
reaction of photosynthesis in C4 –Plants involve both C4 –Cycle occurring first in
Mesophyll cells and then occuring C3 –Cycle in Bundle sheath cells synthesizing
carbohydrates.
A. In Mesophyll cells:
• The C4 –Cycle occurs in the chloroplasts of mesophyll cells and the
phosphoenolpyruvic acid (PEP) is the primary acceptor of CO2.
• The PEP combines with CO2 and forms Oxaloacetic acid as the first products of
photosynthesis. The enzyme involved is PEP Carboxylase.

PEP Carboxylase
1. PEP + CO2 + H2O Oxaloacetic acid + H3PO4
(4 C)
2. Oxaloacetic acid is then reduced to malic acid using NADPH + H+ in presence of
enzyme malic dehydrogenase.
Oxaloacetic acid+ NADPH + H+ Malic Dehydrogenase Malic Acid + NADP+
3. The oxaloacetic acid can be also be converted to aspartic acid in the presence of
enzyme transminase.
Oxaloacetic acid Aspartic Acid
Transaminase
B. In Bundle Sheath Cells:
• Malic acid is then transported to the chloroplast of bundle sheath cells where it
is decarboxylated by NADP specific malic enzyme and forms pyruvic acid,
CO2 and NADPH + H+ .
Malic Acid + NADP+ NADP Specific Pyruvic acid + CO2 + NADPH + H+
malic enzyme

• The released CO2 then enter Calvin Cycle which operates in Bundle
sheath Chloroplasts.
• The CO2 is accepted by RuDP and it follows the Calvin Cycle in further steps
to produce starch and to regenerate Ribulose-1,5-diphosphate .
• The Pyruvic acid returns to Mesophyll cells.
• Due to the high affinity of the enzyme Phosphoenol Pyruvate Carboxylase for
CO2 , the photosynthetic rate is higher in C4 –Plants than C3 –Plants.
3. CAM Cycle (CAM plants):
• Crassulacean acid metabolism derives its name from the fact that it involves a
daily fluctuation in the level of acid within the plant and that it was first
discovered to be common in species within the stonecrop family,
Crassulaceae.
• The CAM plants are successful inhabitants of warm, arid sites and include
species of 23 or more flowering plant families as well as a few ferns.
• In a plant using full CAM, the stomata in the leaves remain shut during the
day to reduce evapotranspiration, but open at night to collect
carbondioxide(CO2).
• The CO2 is stored as the four-carbon acid malate, and then used
during photosynthesis during the day.
• The pre-collected CO2 is concentrated around the enzyme RuBisCO,
increasing photosynthetic efficiency.

➢ The CAM cycle can be divided into two parts:

1. CO2 fixation during night & Synthesis of malate
2. Utilization of Malate during day and release of CO2
1. CO2 fixation during night & Synthesis of malate:
• At night, plants using CAM open their stomata, CO2 molecules diffuse into the
spongy mesophyll's intracellular spaces and then into the cytoplasm.
• Here, CO2 can meet with phosphoenolpyruvate (PEP) or pyruvate, which is
a phosphorylated triose to produce malate directly or forming oxaloacetic acid
that is reduced to malate.
• Malate is then transported via malate shuttles into the vacuole, where it is
converted into the storage form malic acid.
➢ The rxn. Involve during night are as follows:
Malic Enzyme Malate + NADP
1. Pyruvate + CO2 + NADH2

2. PEP + CO2 + ADP PEP-Carboxykinase Oxaloacetate + ATP

PEP + CO2 + H2O PEP-Carboxylase Oxaloacetate +Pi

Oxaloacetate + NADH2 Malate Dehydrogenase

Malate + NAD
2. Utilization of Malate during day and release of CO2 :
• At daylight, plants using CAM close their guard cells and discharge malate that
is subsequently transported into chloroplasts.
• There, depending on plant species, it is cleaved into pyruvate and CO2 either
by malic enzyme or by PEP carboxykinase.
• CO2 is then introduced into the Calvin cycle, a coupled and self-
recovering enzyme system, which is used to build branched carbohydrates.

➢ In some CAM plants, malate is directly decarboxylated in the presence of

NADP+ , and malic enzyme into CO2 & Pyruvate.

Malate + NADP+ Pyruvate + CO2 + NADPH + H+

➢ In some CAM Plants, malate is first oxidised to oxaloacetate by a malate

dehydrogenase. The oxaloacetic acid is then converted into CO2 and PEP with
utilization of ATP by carboxykinase.
Malate + NADP+ Oxaloacetic acid+ NADH + H+

Oxaloacetic acid + ATP PEP + CO2 + ADP

➢ The CO2 thus produced in either of the above two ways is then consumed in
normal photosynthetic reaction to yield carbohydrates.

➢ Pyruvate and PEP are also utilized for carbohydrates synthesis during the day.
Pyruvate is first converted into PEP in the presence of enzyme pyruvate
orthophosphate dikinase.

➢ PEP is then converted into 3PGA by reverse reaction of glycolysis. Thereafter

3PGA is utilized in the calvin cycle.
Photorespiration (C2 Cycle):
• It is a metabolic pathway that occurs in plants in the presence of light.
• The enzyme RuBP carboxylase accepts oxygen in place of carbon dioxide,
resulting in the formation of a two carbon compounds, glycolate.
• Since the reaction is an oxygenation reaction, so the enzyme oxygenase is
utilized here.
• The glycolate subsequently metabolizes into CO2.
• It is not related to normal respiration and there is no production of ATP.
• Photorespiration is defined as a light-dependent uptake of oxygen and output of
CO2 .

RuBP Oxygenase Phosphoglycolate + PGA

O2 + RuBP
• The phosphoglycolate is immediately converted to glycolate.
• The peroxisomes present in the cell, metabolizes the glycolate into glycine.
• The glycine is lastly converted into serine and CO2 in the mitochondria without
the production of ATP or NADPH2 .
Fig: Photorespiration
Salient Features of photorespiration:
• It takes place only in the presence of intense light.
• It is a wasteful process as ATP and NADPH2 are used.
• With increase in temperature and oxygen concentration, the affinity of RuBP
carboxylase for oxygen increase and for CO2 decreases. Hence, RuBP functions
as oxygenase rather than carboxylase.
• It occurs in chloroplasts, peroxisomes and mitochondria.
• Increased in temperature leads to more photorespiration that means more loss of
photosynthetically fixed carbon.
• Photorespiration reduces the potential yield of C-3 plants .
• It is not an essential process.
• It occurs usually in C-3 plants like tomato , wheat, oat etc and absent in C-4
plants like maize, Sugarcane etc.
Factors affecting rate of photosynthesis:
A. External Factors:
1. Light
a. Intensity of light:
• The rate of photosynthesis increases with the increase in light intensity until a
saturation point. A very high intensity of light decreases the rate of
photosynthesis due to photorespiration.
b. Quality of light:
• Rate of photosynthesis varies in different wavelengths of light. It occurs only
in the visible part of spectrum ( i.e. 380-760nm wavelengths).
• Plants show maximum photosynthesis in red light, which is followed by the
blue light .
• Green light is less effective in photosynthesis .
• Photosynthesis usually do not takes place in UV-rays and infrared .
2. Temperature:
• In general, the rate of photosynthesis increases with a rise in temperature, over
a range from 6 degree centigrade to 37 degree centigrade. Temperature below
6 degree centigrade and above 37 degree centigrade show adverse effect on the
rate of photosynthesis.

3. Concentration of CO2 :
• Affects markedly as CO2 is one of the raw material for photosynthesis.
4. Water:
• Water is used as raw material in photosynthesis. Plan utilize about 1% of the
water in photosynthesis.
5. Oxygen:
• Warburg(1920) while working on Chlorella reported that higher concentration
of oxygen in mesophyll cells has inhibiting effect on photosynthesis. This
phenomenon of the inhibition of photosynthesis by oxygen is called Warburg’s
effect.
6. Mineral Elements:
• Some elements such as Mg, Fe , Cu etc are essential for photosynthesis. Mg is
one of the component of chlorophyll while Fe is required for the synthesis of
Cytochrome.
B. Internal Factors:
1. Chlorophyll contents:
• Directly related
2. Anatomy of leaf:
• The rate of photosynthesis is influenced by various anatomical structures of
leaves.
• These include thickness of cuticle, position, number and distribution of stomata,
arrangement of pallisade and spongy parenchyma, presence of intercellular
spaces etc.
• They affect the rate of photosynthesis by influencing the diffusion of CO2 and
absorption of light.
3. Leaf age:
• In young leaves, photosynthesis does not start immediately but the rate of
photosynthesis gradually increases as the leaves mature. The rate of synthesis
declines as the leaves become old.
4. Accumulation of photosynthetic products:
• Retards the photosynthesis
5. Demand for photosynthesis:
• In growing plants they require more foods due to which rate of photosynthesis
increases.