This slideshow explains the details about Photosynthesis process. It has covered all the aspects such as definition, significance, photosystems, Hill reaction, Calvin cycle, HSK cycle, CAM pathway, Photorespiration, etc. of photosynthesis. This slide show will be useful to College students and the students who are appearing for various competitive examinations. .This slide show is equally beneficial to the students who want to pursue career in the biological sciences.

1. Photosynthesis
Dr. Anil V Dusane
Sir Parashurambhau College
Pune
anildusane@gmail.com
www.careerguru.co.com
1

2. Introduction of Photosynthesis
• This is the most important physico-chemical process.
• The existence of life on the earth is dependent on the photosynthesis.
• Definition: It is physico-chemical process in which green cells fix
(convert) energy of sunlight into chemical form with the help of CO2,
water and chlorophyll pigment.
• Photosynthesis is the physiological process that converts the radiant
energy into chemical energy and the chemical energy is stored in the
form of organic food material.
• 6CO2 + 12H2O----------------------------------------C6H12O6+ 6H2O+ 6CO2
sunlight and chlorophyll pigments
2

3. Importance of photosynthesis
• Source of energy for all the living activities: To carry out all the living activities in the
autotrophic and heterotrophic organisms the solar energy captured by the process of
photosynthesis. About 200 billion tonne of sugar produced and about 1017 KJ energy is
captured by this unique process.
• Balance of CO2 and O2 in air: Every year green plants produce about 500 billion tonne of
O2 by this process. Thus, O2 and CO2 balance in the air is maintained by respiration and
photosynthesis as these processes are exactly opposite to each other.
• Raw material for industrial products: The products such as textile fibres, timber, pulp,
cellulose, alcohol, tanning gum, resins, etc. are ultimately derived from photosynthesis
process.
• Renewable and non-renewal energy sources: Biomass, alcohol. biogas, etc. are the ultimate
products of photosynthetic activity.
3

4. Photosynthetic apparatus
• Photosynthetic apparatus of higher plants is present in chloroplast which is the
site of light and dark reactions of photosynthesis.
• The light absorbing pigments (chlorophylls) of thylakoid membranes are
arranged in functional sets (arrays) called photosystems.
• In spinach chloroplasts, each photosystem contains about 200
molecules of chlorophylls and about 50 molecules of carotenoids.
• Almost all the pigment molecules in photosystems absorb photons.
These are called light harvesting or antenna molecules.
• Light absorbed by hundreds of antenna molecules are then transferred
at extremely high rate to a site at which chemical reaction occurs. This
site is called reaction center.
4

5. Photosynthetic reaction centre
• Only few chlorophyll molecules those at
reaction center mediate the
transformation of light energy into
chemical energy.
• Chlorophyll molecules at the reaction
center are chemically identical with other
chlorophyll molecules in photosynthetic
unit. The only difference is that the
energy level of their excited state is lower
than that of other chlorophylls, which
makes them energy trappers.
5

6. Photosynthetic unit- Quantasomes
• Quantasomes represent photosynthetic unit.
• The photosynthetic unit is the smallest group of photosynthetic
pigments and other associated compound, which are necessary for the
photochemical act.
• The photochemical act involves the tapping of light and excitation of
electrons.
• These are arranged in monolayer on thylakoid.
• Size of quanta: 18X17X10 nm (smaller than ribosomes).
• It is with a molecular weight of about 2 million.
• Internally the quantasome is differentiated into two parts i.e., peripheral
region (enriched with pigment molecules) and the middle region or
energy center.
6

7. Composition of Quantasomes
• 230 chlorophyll molecules (160 chl a, 70 chl b),
• 48 molecules of carotenoids,
• 47 quinones,
• 115 phospholipids,
• 144 diagalactosyldiglyceride,
• 346 monogalactosyldiglyceride,
• 48 sulfolipid,
• Sterols and unidentified lipids.
• In these specialized quanatasome structures, only a single molecule of
pigment can absorb energy at a time and all the other molecules support
the action of light reaction of photosynthesis
7

8. Quantasomes
8

9. Photosynthetic pigments
• Chlorophylls are the complex structures consisting
of four pyrrole rings around a central Mg++ atom
and a long hydrocarbon chain of phytol.
• It is a derivative of porphyrin.
• Chlorophyll is the most efficient light absorbing
pigments as it contains extensive network of
conjugated double bonds (polyenes) that gives
capacity to absorb visible light and transfer
excitations.
• Chlorophylls and carotenoids are important
photosynthetic pigments.
9

10. Types of Photosynthesis pigments
1. Chlorophyll a (C55H72O5Mg) is the principle element of energy trapping.
2. Chlorophyll b (C55H70O6Mg): It occurs in green and higher plants absent in BGA, red
algae, brown algae and diatoms.
3. Chlorophyll C: Abundant in diatoms and brown algae.
4. Chlorophyll d: Abundant in red algae.
5. Carotenoids: are protective against photooxidation of chlorophylls and absorb light
energy of the middle of visible spectrum and transfer it to Chlorophyll a. These are
categorized as carotene and xanthophylls.
6. Carotene (C40H56) is orange in colour. B-carotene is found in higher plants and most of
algal species,
7. Xanthophylls (C40H56O2) are a Oxygen derivative of carotene.
8. Phycobilins are accessory pigments distributed only in Cyanophyta and red algae. There
two types - Phycoerythrin (red) and phycocyanin (blue).
10

11. Red drop and Emmerson effect
• Emmerson and Lewis (1943) worked on the quantum yield of photosynthesis.
They found that an average quantum yield is 12% and this quantum yield
decline sharply at the wavelength greater than 680 nm (in red zone).
• The decline in quantum yield in red zone of spectra, called red drop.
• The work of Haxo and Blinks (1950) on Porphyra nereogitis (red alga) also
confirmed the phenomenon of red drop.
• Emmerson and Chalmers (1951) observed that the red drop can be cancelled
by simultaneously providing shorter wavelength.
• They also noted that the effect of two wavelength (shorter and long
wavelength) simultaneously on the rate of photosynthesis exceeds the sum
effect of both wavelengths uses separately.
• This enhancement of photosynthetic rate under influence of two wavelengths
(long and short) of light is called Emmerson effect. 11

12. Red drop and Emmerson effect
12
Emmerson effect = Rate of release of O2 under both wavelengths –
Rate of release of O2 in short wavelength (red)/ Rate of evolution of
O2 under long wavelength (far red)

13. Significance of Emmerson effect
• This effect had made radical change in the understanding of the
mechanism of light reaction.
• For the first time he claimed that two photosystems viz. Photosystems I
and II occur in chloroplast. Photosystem-I contains the long red
wavelength absorbing form chl a 683 (P700) while photosystem system II
contains short red wavelength absorbing form Chl.a 673 (P680).
• Emmerson also claimed that both the photosystems work harmoniously
and responsible for photolysis of water and reduction of NADP.
• The photosystem-I is under the influence of long wavelength and
photosystem II is under the influence of short wavelength and that is why
the two wavelengths (short and long) enhance the photosynthesis rate.
13

14. Photosystems
• A photosystem is the assemblage of 200-400 pigment
molecules together with a primary electron acceptor and
a series of electron carriers.
• The photosystems are located within thylakoid
membrane.
• The function of photosystems is to capture the light
energy for photosynthesis.
• Based on the type of photochemical reaction center, a
set of antenna molecules and electron carriers
photosystems are categorized into viz. PSI and PSII 14

15. Photosystem-I
• This is called as photosystem-I because it was discovered first.
• The reaction centre of PS-I is designed as P700 (because chl.a absorbs light
wavelengths that are of P700 nm (far red).
• This photosystem has a high ratio of chl a to chl b.
Composition of PSI:
• 200 Antenna chlorophylls
• 50 Carotenoids
• 1 P700 reaction centre
• 1 Bound ferrodoxin
• 1 Soluble ferrodoxin
• 1 Cytochrome b563 cyt b6
• 1 Cytochrome C552 or cytf
• 1 Plastocyanin
• 1 Ferridoxin-NADP-reductase.
15

16. Mechanism of Photosystem-I
• Antenna molecules first capture the light. The absorbed energy is sent to
(moves to) P700 by resonance energy transfer mechanism.
• The excited reaction centre (P700) loses an electron to an electron acceptor and
becomes P700+. This is now a strong oxidizing agent. It quickly acquires
electron from plastocyanin.
• Electron from P700 is transferred to ferrodoxin (bound) and then to soluble
ferredoxin. The reduced ferrodoxin transfers its electron to NADP+ to form
NADPH.
• The conversion of NADP+ to NADPH is catalyzed by ferrodoxin–NADP
reductase.
2 ferrodoxin (reduced) + H+ + NADP+ ------ (ferrodoxin-NADP reductase) + 2
ferrodoxin (oxidized) + NADPH.
• Importance: PSI generates NADPH via reduced ferrodoxin and light
absorption by PSI creates a powerful reducing agent.
16

17. Photosystem II
• It is discovered after photosystem I.
• Composition of PSII.
• 200 Antenna chlorophylls (a and b)
• 50 carotenoids
• 1 P680 reaction centre
• 1 Primary electron donor (Z)
• 1 Primary electron acceptor (Q)
• 4 Plastoquinone
• 6 Manganese atoms
• 2 Cytochrome b6.
17

18. Photosystem II
• It consists of 3 pigments-chlorophyll a, b (c or d according to species)
and carotenoids.
• The reaction centre molecule (Chl a) is called P680 because it absorbs
wavelengths that are 680 nm (red) and less.
• The primary electron acceptor of PSII is substance Q (of unknown
molecular weight.).
• The other carriers in order of decreasing free energy are Plastoquinone
(PQ), Cytf, and PC.
• The electron flow through PSII is as follows P680→Q→PQ→Cyt→PC.
• It is during PSII oxygen is evolved.
• Photosystem II generates a strong oxidant that splits water.
• The redox potential of this reaction centre is about + 0.8 V.
18

19. Mechanism of Photosystem II
• The electron from the photoactivated reaction centre of PSII is transferred to Q
(which is tightly bound molecule of Plastoquinone).
• The electron is then transferred from Q to a pool of mobile plastoquinones and then
to cyt b559 (b6).
• This electron is then transferred to Cytc 552 (formerly called cyt f).
• The final electron carrier from PSII to PSI is plastocyanin (which contains Copper).
Cu atom facilitates electron transfer as Cu alternates between +1 and +2 oxidation
states.
• The electron transfer from reduced plastocyanin to oxidized form of P700 enables
this reaction centre to again serve as an electron donor to form NADPH when
illuminated.
• The overall reaction initiated by light in PSII is
4 P680 + 4H+ + 2 QB + light (4 photons) ----- 4 P680+ + 2 Q2H2.
• The water splitting activity is an integral part of PSII reaction centre. 19

20. Coordination between PSI and PSII
• According to Junge (1982), each granum has 200 units of PSI and PSII.
• The PSI and PSII functions in close co-ordination because they jointly
function to transfer the electrons from water to NADP. Since these systems are
located a little apart that is why certain intermediates (carriers) help to carry
electrons from PSI to PSII.
• These carriers are found to be of two types.
• i) Plastocyanins (copper containing proteins) in its reduced form can carry
an electron form PSII to PSI.
• ii) Plastoquinones (group of quinones) carry two electrons and two H+ from
PSII to PSI. Besides the above additional components in the form of
cytochromes occurs in thylakoids Cyt b6 and Cyt f in between PSI and PSII
and help in electron transport between them. Cyt b3 and Fd (FeS protein) also
participate in the light mediated electron transport.
• Electron flow accompanied by proton pumping across the thylakoid
membrane and the proton motive force thus created drives ATP synthesis.20

21. Light reaction
• It produces ATP and NADPH.
• Hill reaction: Robert Hill (1939) discovered that the isolated chloroplasts evolve oxygen in
the presence of water, light and a suitable electron acceptor (ferricyanide). He recorded that
the reduction of ferricyanide to ferrocyanide takes place as follows:
• 2H2O + 4 Fe+++ --------- (illuminated chloroplast) O2 + 4H+ + 4 Fe ++
• Hill reaction has clearly shown that O2 evolution takes place in the absence of CO2.
• Hill used non-biological hydrogen acceptor, a dye-2,6 dichlorophenolin-dophenol now known
as Hill reagent. Which is in oxidized form (A) is blue and its reduced form (AH2) is
colourless. When leaf extracts supplemented with the dye and illuminated, the blue dye
become colourless and O2 was evolved. This was the first clue, which indicated that how the
absorbed light energy is converted into chemical energy.
• Hill found that CO2 was not required for this reaction.
21

22. Diagrammatic representation of Hill reaction
22

23. Hill reaction
• The hill reaction represents light reaction in photosynthesis.
2H2O + 2A---------------(light, chloroplast) 2AH2 + O2.
A.artificial hydrogen acceptor (oxidized form) and AH2- reduced form.
• After Hill’s experiment, Arnon (1951) showed that NADP+ serves as electron
(hydrogen) donor and plays significant role in the reduction of CO2.
• The Hill reaction may be expressed as
• 2NADP+ + 2H2O---------------------(light, chlorophyll) 2NADPH +2H+ +O2
• NADP+ (Nicotinamide Adenine Dinucleotide Phosphate) is reduced to form
NADH
• For every O2 molecule liberated or for every molecule CO2 reduced, at least 2
molecules of NADPH are produced.
• Besides NADPH, ATP molecules are also formed through phosphorylation.
23

24. Significance of Hill reaction
• Hill reaction is considered as a landmark in elucidation of the mechanism of photosynthesis
due to following reasons:
• It has shown that oxygen release can occur without reduction of CO2. (The artificial electron
acceptors such as ferricyanide can substitute for CO2).
• It confirmed that evolved oxygen comes from water and not from CO2 (as CO2 was not
present). Use of radiotracers 18O later proved this.
• It showed that the isolated chloroplasts could perform a partial photosynthesis.
• It revealed that a primary event in photosynthesis is the light activated transfer of an electron
from one substance to another against chemical potential gradient. The reduction of ferric to
ferrous ion by light gives a clue for the conversion of light energy into chemical energy.
• This reaction shows that the source of oxygen is water, and this water undergoes
splitting (photolysis).
• The splitting of water releases electrons and these are required for reduction of
CO2. The splitting of water, H2 and O requires light and chlorophyll
24

25. Photophosphorylation
• Arnon (1954) showed that isolated chloroplast could produce ATP from ADP and Pi in the
presence of light.
• Arnon termed the formation of ATP molecules as ‘photophosphorylation’ (as mitochondrion
is not involved).
• 2ADP + 2 Pi + 2 NADP + 4H2O ------ (light, chloroplast) 2 ATP + 2NADPH2 + 2 H2O + O2.
• Definition of phosphorylation: Chlorophyll molecules when excited by sunlight, energy rich
electrons are emitted, these then transferred through a series of electron carrier in a stepwise
manner.
• During this sequential transfer of electrons, the energy of electrons is lost bit by bit and the
lost energy is picked up by ADP and Pi molecules to synthesize energy rich molecule (ATP).
• This process of synthesis of ATP molecules due to light energy is known as
photophosphorylation. ATP produced (by chemosynthesis mechanism) in above process is
used up for CO2 assimilation (fixation) in dark reaction of photosynthesis.
• There are two pathways of electron transfer (photophosphorylation) viz. Cyclic
photophosphorylation (cyclic electron transfer) and non-cyclic photophosphorylation (non-
cyclic electron transfer). 25

26. Cyclic photophosphorylation
• The electrons released from chlorophyll (P700) returns (get back) in a continuous chain
again to chlorophyll. In this the electron travels in a cyclic fashion hence named as cyclic
phosphorylation.
• Salient features:
1. Arnon and his associates established the steps of cyclic photophosphorylation.
2. This takes place in presence light and chlorophyll.
3. This mechanism is possible only when PSI (P700) participates.
4. Substrates required are ADP and Pi. The products are ATP and H2O.
5. Water does not participate hence there is no evolution of O2
6. There is no formation of reduced NADPH molecule. Only ATP is produced.
7. In this process no outside electron donor is needed.
8. The flow of electron is PSI—FRS—Fd---Cyt b6---Cyt f ---PC and PSI sequence.
9. Overall reaction is ADP + Pi------------------------(light, chlorophyll) ATP + H2O
26

27. Mechanism of cyclic phosphorylation
27

28. Significance of cyclic phosphorylation
• ATP formed during in any one type of
photophosphorylation (including cyclic) is
insufficient for the fixation of CO2 during
carbohydrate formation. Therefore, cyclic and non-
cyclic must work harmoniously.
28

29. Non-cyclic phosphorylation
• An electron released from chl a does not returns to chl.a hence this process is
called non-cyclic photophosphorylation. This is also called Z scheme electron
transport because the electron travels in a zig zag fashion (not in a cyclic
manner).
Salient features:
1. Flow of electron is unidirectional
2. This is the major pathway of light reaction involving both, PSI and PSII.
3. Oxygen produced or ADP phosphorylated is proportional to the amount of
NADP+ present in the system and this process goes on until all NADP+ is
reduced.
4. During this process quanta causes photolysis of water molecule.
5. It is broken down to two electrons, two protons and one oxygen atom. There
is evolution of oxygen in this process electrons are provided by water
molecules (tracer technique has shown this). H2O--- 2e- + 2H+ + O. 29

30. Non-cyclic phosphorylation
6. Electron flow normally takes place along electrochemical gradient i.e. from more electron
negative components to more electrons positive one. However, in this the electron transport
from pigment II to compound Q takes place against electron gradient.
Chl. (PSII)---Q—PQ--Cyt b6—Cytf---PC----PSI---FRS-----(Fd.UQ)---Fd------NADP+
7. The two electrons from water replace two electrons that have left the reaction centre (P680)
and PSII.
8. In this process two molecules of ATP are formed per two molecules of NADP+ reduced or
one molecule of oxygen evolved, or two molecules of water oxidized.
9. During sequential transfer of electrons an ATP molecule is produced in between Cytf and
plastiquinone.
Over all reaction is 2ADP+2Pi+2NADP++2H2O-----(light) 2 ATP +2 NADPH +H+ +O2.
30

31. Non-cyclic photophosphorylation
31

32. Mechanism of Non-cyclic photophosphorylation
• Light (photon) absorbed by PSI (P700) result in the excitation of one
of electron from chlorophyll molecule.
• This excited electron is picked up by FRS (Ferrodoxin Reducing
Substance) and then finally goes to ferrodoxin.
• The reduced ferrodoxin transfers its electron to NADP+ to form
NADPH (in presence of ferrodoxin–NADP reductase). However, PSI
remains in the excited state, as it has not got back its electron.
• Some electrons are used for reduction of NADP+ whereas others is
passed to Cytf via Cyt b6.
• Thus Chl.a remains in an unstable state since there is deficiency of
electron due to photoreduction.
32

33. Mechanism of Non-cyclic photophosphorylation
• Meanwhile PSII (P680) absorbs light energy and ejects one of its
electrons.
• This excited electron is picked up by plastoquinone. An electron produced
in the photolysis of water (H2O-------2e- + 2H+ +O) replaces the electron
lost by PSII. Plastoquinone accepts electron and then it gets reduced
(plastoquinone + 2H ===reduced plastoquinone).
• The reduced plastoquinone then donates the electron to Cyt b6. The
electron is transferred to cytochrome f then plastocyanin and P700 (PSI).
• P700 can accept an electron only if it has lost one of its own when excited
by the energy in second photon of light.
• In above process excess electron energy is used for the synthesis of ATP
(from ADP + Pi). ATP formation takes place in between Cyt b6 and cyt f.
33

34. Cyclic photophosphorylation Vs Non-cyclic photophosphorylation
Cyclic photophosphorylation Non-cyclic photophosphorylation
1 Expelled (excited) electrons returns to
chlorophyll
Expelled (excited) electrons do not return to
the chlorophyll.
2 Only PSI functions Both PSI and PSII functions
3 In bacteria only cyclic
photophosphorylation occurs
In green plants both cyclic and non-cyclic
occurs.
4 No evolution of O2 as water does not
participate
O2 is evolved as water participates
5 Energy is captured in form of 1 ATP. Energy captured inform of ATP and NADPH
6 Electron flow-chl (PSI)---FRS—Fd—Cyt
b6—Cyt f—PC—Chl (PSI)
PSII—Q--PQ---cytf--PC---PSI—FRS—Fd—
NADP+
7 Not inhibited by DCMU (Dichlorophenyl-
dimethyl urea)
Stopped by DCMU
34

35. Electron carriers
• These maintain an optimal geometry for the energy transfer between chlorophylls.
1. Quinone (Q): When photosystem II is excited that time e- is transferred to primary
electron acceptor
2. Plastoquinone (PQ)-Group of quinones. PQ + 2H ===reduced plastoquinone. The
reduced PQ then donates its e- to cyt b6 (these also carry 2e- and 2H+ from photosystem II
and I.
3. Cyt b6 (Cyt b559): Proteins are associated with Fe i.e. haeme. Its Redox potential is near
to 0. Its stereoscopic properties are similar to that of Cyt b of mitochondria.
4. Cyt f: It is a c type of cytochrome called Cyt c 552/c554. It is insoluble membrane bound
protein.
5. Plastocyanin PC: A copper protein. It is a soluble copper containing e- transfer protein. It
is having 2 Cu atoms, which are the sites of oxidation-reduction. PC is blue in oxidized
state and colourless in reduced state. It is likely that this may be immediate e- donor to
P700.
6. FRS (Ferrodoxin Reducing Substrate): These compounds are also designated as X or Z.
From photosystem-I e- are accepted by FRS. 35

36. Dark reaction
• This can take place either in absence or presence of light.
• The overall photosynthetic rate is regulated by the light reactions.
• Light does not have any direct effect on this process.
• This is also known as carbon assimilation phase the carbohydrate
synthesis takes place. In this reaction products of light reaction viz. ATP,
NADPH is used to reduce CO2 into glucose.
• The use of CO2 isotopes helped to trace the secrets of the dark
reaction.
• CO2 is fixed by C3 pathway (Calvin cycle), C4 pathway (Hatch Slack
cycle) and CAM (Crassulacean Acid Metabolism).
• Blackmann (1905) claimed that light is not necessary for reduction of
CO2. 36

37. Calvin cycle
• Malvin Calvin (1953) for the first described path of carbon in photosynthesis.
• Calvin was awarded with a Noble Prize for this work in 1961.
• Chromatography and radioautography helped Calvin to trace the path of carbon in
photosynthesis.
Salient features:
1. It has been named variously such as Calvin cycle, Bassham and Calvin cycle, Carbon
assimilation, Blackmann reaction, Calvin–Benson cycle, etc. It is also known as C3
pathway because the product of first reaction contains three carbon atoms.
2. It occurs in stroma (as it contains most of the enzymes required for CO2 fixation).
3. In this CO2 combines first with Ribulose diphosphate (RuDP) to form two 3-carbon
molecules of phosphoglycerate.
4. 18 ATP molecules are required for the synthesis of one glucose molecule.
5. The overall efficiency of photosynthesis under standard conditions is at least 30%.
6. Net reaction for Calvin cycle:
6CO2+18 ATP+12 NADPH+12H2O---- C6H12O6+18 ADP+18 Pi+12 NADP++6H+
37

38. Steps in Calvin cycle
1. Carboxylation,
2. Reduction of (PGA),
3. Formation of fructose 6-phosphate and
4. Regeneration of acceptor (RuDP).
Carboxylation:
• It is the first reaction in which CO2 combine with 5-carbon acceptor i.e.,
Rubulose1,5 biphosphate (RuBP). An unstable 6C compound is immediately breaks
into two molecules of a 3-carbon acid i.e., Phosphoglyceric acid (PGA).
• Here the first product of CO2 fixation has 3C atoms that is why Calvin cycle is also
referred as C3 pathway. The reaction is catalyzed by Ribulose biphosphate
carbosylase/oxygenase (RUBISCO)
• Ribulose1-5 disphoshpate (5C)+CO2 (1C)+H2O ==(Rubisco) 3-PGA (2
molecules, 3C + 3C) + 2H+
38

39. Steps involved in Calvin cycle
• Reduction of PGA: Conversion of 3-phosphoglycerate to glyceraldehyde-3-phosphate takes place. This
requires a large input of energy.
• Each PGA is phosphorylated by ATP and reduced by NADPH to form PGAL.
• PGA + NADPH + H+ --------- (ATP) PGAL.
• This reaction takes place in two sub steps.
• i)3 phosphoglycerate kinase is in the stroma, and it catalyzes the transfer of phosphate from ATP to 3-PGA
and yields 1,3 diphosphoglycerate.
PGA + ATP ---------(phosphoglycerokinase) 1,3-diphosphoglyceric acid + ADP.
• ii) 1,3-diphosphoglycerate+NADPH+H+-------(glyceraldehyde3-phosphate dehydrogenase) 3 PGAL+NADP
• NADPH donates electrons and reduction is catalyzed by glyceraldehyde –3-phosphate dehydrogenase
producing glyceraldehyde 3-phsophate.
• The ATP and NADPH required in the above reaction come from the light reaction.
• Five out of every six molecules (5/6 th) of this sugar combine with 3 ATP molecules to form 3 molecules of
RuDP (which is used up to pick up more carbon dioxide) and one out of every six molecules (1/6 th) used to
synthesize more complex end products (sugars).
• It takes six turns to produce two PGAL molecules that from one glucose molecule.
• The 5/6 of total PGA produce a number of 4C, 7C and 5C sugars by series of intermediate reactions one of the
5C intermediates is ribulose-5-phosphate and it reacts with ATP resulting regeneration of ribulose-1-5
diphosphate. 39

40. Steps involved in Calvin cycle
Formation of Fructose-6-phosphate:
3PGA to Fructose 6- phosphate. Glyceraldehyde 3-phosphate + Dihydroxyacetone phosphate
(DHAP) condenses in the presence of aldolase.
Generation of RuDP:
i)Fructose-6-phosphate (C6) + glyceraldehyde-3-phosphate (C3) ---- (transketolase) xylose-5-
phosphate (C5) + erythrose 4-phosphate (C4).
ii)Erythrose 4-phosphate (C4) + DHAP (C3) ------ (aldolase) Sedoheptulose, 1,7 diphosphate.
iii)Sedoheptulose, 1,7 diphosphate (C7) + glyceraldehyde 3-phosphate (C3)-------
(transketolase) ribose-5-phosphate (C5) + xylulose-5-phosphate (C5).
iv)Phosphopentose epimerase converts xylulose 5-phosphate into ribulose-5-phosphate.
v)Xylulose 5-phosphate--------- (phospho pentose epimerase) ribulose 5-phosphate.
40

41. Steps involved in Calvin cycle
vi)Phospho pentose isomerase converts ribose-5-phosphate into ribulose-5-phosphate.
vii)Ribose-5-phosphate ----------(phospho pentose isomerase) ribulose-5-phosphate.
viii)Phospho ribulose kinase catalyzes phosphorylation of ribulose-5-phosphate to regenerate
ribulose1,5 diphosphate (which is CO2 acceptor).
• Ribulose 5-phosphate+ATP----(phospho ribulose kinase) ribulose 1,5 diphosphate + ADP+
H+
• Sedoheptulose 1, 7 diphosphate ----- (phosphatase) Sedoheptulose 7-phosphate.
• If all the six molecules of RuDP combine with 6 molecules of CO2 and form 6 molecules of
glucose, the cycle cease to operate and for the next turn there will be no RuDP to accept CO2
molecules.
• Thus, for entry of every 6 CO2 molecules one goes for hexose formation and rest go for the
regeneration of RuDP.
41

42. Steps involved in Calvin cycle
• Formation of sugars:
• The PGAL molecule (1 out of 2) undergoes isomerization to form a molecule of DHAP
the enzyme triose phospho isomerase catalyzes this reaction.
• 3 PGAL --------- (triose phospho isomerase) DHAP
• 3 PGAL + DHAP (aldolase) ----fructose 1, 6 diphosphate.
• Fructose-1-6 diphosphate gets converted into 6-mono phosphate due to the loss of one
phosphate group under the influence of phosphatase.
• Fructose 1,6 diphosphate------ (phosphatase, H2O) Fructose-6-monophosphate (FMP).
• FMP isomerases into glucose-mophosphate under the influence of isomerase
• FMP -------- (isomerase) Glucose-6-monophosphate (GMP).
• FMP or GMP undergoes dephosphorylation to form either fructose or glucose. These
monosaccharides unite to form sucrose and various other carbohydrates
• For every 3 CO2 molecules fixed, one molecule of triose phosphate (PGAL) is
produced, and 9 ATP and 6 NADPH are consumed.
42

43. Calvin cycle
43

44. Overview of Calvin cycle
44

45. Significance of Calvin cycle
• CO2 is absorbed and fixed to form carbohydrates i.e., starch. ATP and
NADH2 are used and are again converted into ADP and NADP for reuse
in light reaction.
• RuDP is regenerated and again ready for picking CO2 molecules. An
enzyme, RuDP carboxylase catalyzes the joining of CO2 with RuDP or
O2 with RuDP.
• The two molecules are alternative substrates that compete with each other
for the same active site of enzymes. The more abundant of two molecules
gets occupy the active site and subsequently joins with RuDP. When the
concentration of CO2 in stroma is higher as compared to O2 concentration
then CO2 joins with RuDP and Calvin cycle is initiated. When reverse the
case CO2 joins with RuDP and a pathway known as photorespiration is
initiated. 45

46. HSK/C4 pathway/B-carboxylation
• It has been named variously such as Hatch, Slack and Kortschak or
B-carboxylation or Bicarboxyllic acid Cycle or Co-operative
photosynthesis or C4 pathway, etc.
• It is known as C4 pathway because the first product formed contains
4C atoms. For the long time it was thought that C3 pathway is the
only pathway for the fixation in all green plants.
• Kortschak et al (1954) discovered an alternative pathway for CO2
fixation in photosynthesis. Kortschak found that the first product
formed was 4C compound in sugarcane. Later MD Hatch and CR
Slack (1966) confirmed it in several plants.
46

47. HSK/C4 pathway/B-carboxylation
• The pathway occurs in majority of grasses viz. Panicum
maxicum, Artiplex spongosa, Chloris guyana, etc, also in the
plant like Maize.
• Besides Graminae it occurs in plants belonging
Euphorbiaceae, Portulacaceae, Cyperaceae, Amaranthaceae,
Composiatae, Chenopodiaceae, etc.
• However, C4 pathway does not occur in all graminaceous
plants e.g., wheat, rice, etc.
47

48. Structural (anatomical) peculiarities of C4 plants
• Plant with C4 pathway has several unique features known as Kranz anatomy (in
German Kranz = wreath).
• The Kranz anatomy is characterized by the lack of differentiation of mesophyll
into palisade and spongy parenchyma.
• In this, vascular bundles are surrounded by a layer of distinct parenchyma cells,
which are arranged radially. These constitute bundle sheath.
• In transverse section of vascular bundle, the bundle sheath appears like a
wreath hence the name Kranz anatomy.
• In addition to this, there are two distinct kinds of photosynthetic cells i.e.bundle
sheath cells and mesophyll cells.
• The chloroplasts in bundle sheath cell are large but with poorly developed
grana. Mesophyll cells with small chloroplasts and having well developed
grana.
48

49. Kranz anatomy
49

50. Mechanism of HSK
• Phosphoenol pyruvate (PEP) is the initial acceptor of CO2. Phosphoenol
pyruvate carboxylase is found abundantly in mesophyll cells. This catalyzes the
condensation of CO2 with PEP and formation of 4C compound i.e. Oxaloacetic
acid (OAA) takes place. The reaction requires the participation of water.
• PEP+CO2 (1C) + H2O ----(PEP carboxylase) OAA (4C) + H3PO4 (phosphoric
acid)
• The function of PEP carboxylase is similar to Rubisco. OAA formed is
relatively unstable and it readily gets converted into either malic acid or
aspartic acid. OAA is reduced to malic acid by light generated NAPH2. The
reaction is catalyzed by malate dehydrogenase.
• OAA + NADPH + H+ ----------- (malate dehyrogenase) Malic acid + NADP+.
• The malic acid is transferred to the chloroplast of bundle sheath cells.
50

51. Mechanism of HSK
• In bundle sheath cells, malic acid undergoes oxidative decaboxylation and pyruvic acid is
formed. This reaction is catalyzed by malate dehydrogenase. Here the malic acid is
decarboxylated.
• Malic acid+NADP+ ------- (malate dehydrogenase) Pyruvic acid+ NADPH+H+CO2.
• NADPH formed in above reaction travels back to mesophyll cells to regenerate malic acid.
Pyruvic acid also goes in mesophyll cells where it utilized the light generated ATP to
produce PEP again.
• Pyruvic acid +ATP+ H3PO4----------- (pyruvate phosphate dikinase) PEP + Pi.
• The release CO2 (formed due to the oxidative decarboxylation of malic acid) enters the
Calvin cycle in the usual way by condensing with RuBP. The Calvin cycle operates in
bundle sheath cells.
• In this cycle, the carboxylation takes place at two sites therefore this pathway is also known
as ‘decarboxylation pathway’.
51

52. HSK pathway/C4 pathway
52

53. Biological significance of HSK pathway
This pathway is considered one of incredibly significant pathway.
• Adaption to the tropical climate: The real significance of C4 pathway is the
adaption to a tropical climate. In tropical climate, temperature is high, so stomata
are partially opened so less CO2 is available for photosynthesis. This cycle makes
more efficient use of CO2 and increases the uptake of CO2 though low-level CO2
is present. This pathway is efficient even at 10 ppm concentration of CO2.
• High temperature tolerance: CO2 fixing enzyme i.e., PEP carboxylase is
insensitive to high temperature while RuBP carboxylase is sensitive to
temperature. The plants lacking C4 pathway lose 25% -50% of their fixed carbon
by photorespiration. The plants with C4 pathway have little photorespiration
because the oxygenation of RuBP is inhibited by high concentration of CO2 in
their bundle sheath cells. The carboxylation in bundle sheath increases CO2
concentration. This CO2 is used in C3 pathway.
53

54. Biological significance of HSK pathway
• Economical use of water: Less water is lost through C4 plants
because their stomata are partially close. So, the plants release less
water during CO2 fixation as compared to C3 plants and thus make
more economical use of water.
• Through this pathway nature has experimented with variation and has
provided alternative pathway for the fundamental process like
photosynthesis. To have only one pathway for the process like
photosynthesis is catastrophic. Thus, the nature has provided some
variation through this pathway.
54

55. Distinguish between C3 pathway and C4 pathway
Characteristics C3 pathway C4 pathway
CO2 acceptor RuBP PEP
First product PGA ( 3C) OAA (4C)
Chloroplast Normal Dimorphism
Photosystem PSI and PSII. Bundle sheath cell lack PSII.
Enzymes for C3 In mesophyll cells In bundle sheath cells.
CO2 compensation of photosystem 50-150 ppm 0-10 ppm
Photorespiration Present Absent or negligible
Temperature (optimum) 10-25oC 30-450C
Pathways involved Only C3 Both C3 and C4
High rate of O2 Inhibits photosynthesis No effect on photosynthesis
Number of ATP required to synthesize
1 glucose molecule
18 ATP 30 ATP
55

56. Crassulacean Acid Metabolism (CAM)
• This is one of the pathways of CO2 fixation (other than C3 and C4
pathways).
• This is called as CAM because it was first recorded in Crassulaceae
family.
• About 2,000 species show CAM mechanism for CO2 fixation.
• This pathway is quite common in many of the plants belonging to
Crassulaceae, Cactaceae, Orchidaceae, Bromeliaceae,
Asclepediaceae, Euphorbiaceae etc.
• Many orchids and bromeliads that grow as epiphytes also show
CAM pathway.
56

57. Important features of CAM pathway
• CAM plants keep their stomata open mainly at night and close
during day yet achieves net fixation of CO2. During daytime CO2
uptake is negligible.
• Most of the CAM plants grow in acidic habitat and have
exceptionally low rates of transpiration and with succulent
habit.
• CAM plants fix atmospheric CO2 mainly at night when stomata
are open, but they cannot use the Calvin cycle because this
operates only in the light.
• These plants have a diurnal fluctuation of organic acid, some
degree of succulence and large storage vacuoles.
57

58. Important features of CAM pathway
• In CAM plants, initial CO2 fixation and the Calvin cycle operate
at different times but in the same cells in contrast to C4 plants
where they operate at the same time but in different cells.
• There is limit to the amount of malic acid, which can be stored.
This usually, determines the overall photosynthetic capacity.
• An efficiency of CAM is less as compared to C4 plants. But this is
the price paid for the conservation of water.
• Metabolic pathway of CAM is similar to C4 pathway in many
respects. However, it is much more flexible photosynthetic
strategy than C4 pathway
58

59. Mechanism of CAM (Night time)
Stomata are open, CO2 is fixed through the action of PEP carboxylase
to malic acid i.e., this acid is formed at the night by carboxylation of
phosphopyruvic acid in the presence of enzyme PEP carboxylase.
• PEP + CO2 ------------- (PEP carboxylase) OAA
• Later O.A.A. gets converted to malic acid with help of malate
dehydrogenase.
• OAA + NADPH ---------- (malate dehydrogenase) malic acid +
NADP+
• This malic acid gets accumulated and decreases pH.
59

60. Mechanism of CAM (Daytime)
• Malic acid gets converted into starch, glucose, etc. in the presence of light.
Decarboxylation of malic acid during daytime yields CO2 inside the
photosynthetic tissues and this CO2 is fixed by C3 cycle.
• Malic acid ----------------- (malic enzyme) Pyruvic acid + CO2.
• Thus, CO2 fixation takes place without CO2 entry directly from air. During
daytime, the pH is comparatively higher than that of night. It undergoes
decarboxylation to form pyruvic acid and CO2. This CO2 enters in C3 cycle and
pyruvic acid goes for regeneration of PEP.
• In CAM plants however, both C3 and C4 pathways occur in mesophyll cells. C4
and C3 and pathways occur simultaneously in C4 plants while in CAM plants
they occur during night and day respectively. Thus, C3 and C4 pathways are
separated in space in C4 plants while in CAM plants C3 and C4 pathways are
separated in time. 60

61. CAM pathway
61

62. Significance of CAM
• CAM cycle is considered as the environmental adaption because
most of CAM plants grow in xeric conditions.
• It is an important physiological and biochemical adaption of
photosynthetic carbon metabolism to water-stress.
• CAM plants have evolved mechanism for optimum utilization of
CO2 with minimum loss of water.
• In CAM plants CO2 is stored in the form of malic acid and the
decarboxylation of malic acid results in the release of CO2
during daytime.
62

63. Significance of CAM
• During day light, amount of titratable acidity decreases
significantly (with increase of cell sap pH) known as light
deacidification.
• During dark, increased amount of titratable acidity (with
decrease of cell sap pH) known as dark acidification.
• Portulaca oleracea (C4 plant) may show CAM activity under
certain environmental conditions.
• In Kalachoe blossfeldiana short days may induce CAM in young
leaves than due to factor of ageing.
63

64. Photorespiration
• Decker and Tio (1959) coined the term photorespiration.
• It was generally believed up to mid-1950 that the rate of respiration of a green
leaf is the same in light as well as in dark. But it was found that the rate of
respiration in chlorophyllous tissues of higher plants (except many monocots)
increases in the light than in dark.
• The phenomenon of increased rate of respiration induced by light is called
photorespiration. It is nothing but the release of CO2 in respiration in the presence
of light.
• The respiratory substrate in normal respiration is glucose but for photorespiration
it is glycolic acid (2C).
• It is found that under conditions of high O2 concentration (high O2/low CO2).
• Rubisco does not fix CO2 but undergoes oxygenase activity.
• Site of photorespiration:
Three organelles viz. chloroplast, peroxisome and mitochondria are involved in the
process of photorespiration.
64

65. Evidence for occurrence of photorespiration
1. Experimental evidences reported by RGS Bidwell and et al (1969) shown that the light
stimulated release of CO2 (called photorespiration) contains a high proportion of carbon
fixed. He feed tobacco leaves with 14C radiotracers and revealed that first carbon atom of
glycolic acid is liberated as CO2.The liberation of CO2 is related to rise in temperature from
25oC to 35oC during photorespiration. The carboxylic group of glycolate is thought to be
donor of CO2 to photorespiratory release. Zelitch (1966) also proved the presence of
photorespiration by using glycolate oxidase inhibitor.
2. The work of Tregunna (1966) and Hew (1968) shown that the effects of light on CO2
production are eliminated by loss of chlorophylls from leaves (due to mutation or nutrient
deficiency). In non-chlorophyllous cells, the photorespiration process is absent.
3. Illuminated corn (maize) leaves which do not release CO2 in the CO2 free air begin to
release when treated with photosynthetic inhibitors like DCMU (Dichlorophenyl-1-
Dimethyl Urea). This inhibitor does not check the dark respiration.
65

66. Normal respiration and photorespiration
66
Feature Normal respiration Photorespiration
1 Respiratory
substrate
Carbohydrate or fat or protein Glycolic acid (2C)
2 Occurrence In all living cells In photosynthetic cells
3 Biosynthesis of
substrate
Substrate may be recently formed
or stored
Substrate always recently formed
4 Site Cytoplasm and mitochondria Chloroplasts
5 H2O2 Not formed Formed
6 ATP Several are formed No ATP formed.
7 NAD and NADH NAD reduced to NADH NADH oxidized to NAD
8 Transamination Does not occur It occurs
9 O2 concentration Not dependent totally on O2 conc. Depend totally on O2 conc.

67. Steps of Photorespiration
1. Oxygenase activity: RuDP ------ (Rubisco O2) Phosphoglylic acid (2C)+ 3 PGA(3C).
2. Phosphoglycolic acid undergoes dephosphorylation to form glycolic acid.
Phosphoglycolic acid + H2O -----(phosphatase) Glycolic acid + Phosphoric acid.
3. In peroxisomes glycolic acid is converted into glyoxylic acid in presence of glycolate
oxidase. Glycolic acid ---------(glycolate oxidase) glycolic acid.
4. Formation of H2O2:
Glyoxylic acid+O2----(glycolate oxidase, light) glyoxylic acid + H2O2.
5. H2O2 is converted into O2 and water in presence of catalase.
2H2O2 ----------(catalase) 2 H2O+O2.
6.Glyoxylic acid+glutamic acid ----(aminotransferase) glycine + -ketoglutarate.
•Glyoxylic acid is converted into glycine in presence of glutamate glyoxylate
aminotransferases. Glycine is then transported to mitochondria via cytosol.
67

68. Steps of Photorespiration
7. In mitochondria two glycine molecules reach to produce serine, CO2 and NH3.
2 Glycine + H2O + NAD+ ----------- (multienzyme) Serine + CO2 + NH3 + NADH.
At this place light induced CO2 is liberated. Ammonia released by mitochondria is
assimilated by chloroplast within the
same cell.
8. Serine is then transported out of mitochondria into peroxisome where it is converted into
hydroxy pyruvic acid and glyceric acid.
Serine ------------ hydroxy pyruvic acid + glyceric acid.
9. Finally glyceric acid is transported to chloroplast where it undergoes phosphorylation by
ATP to form 3-PGA which is ultimately used in photosynthetic carbon reduction cycle.
Thus, overall process becomes cyclic series of reactions and may be called photorespiratory
carbon oxidation
68

69. Photorespiration
69

70. Significance of Photorespiration
• The photorespiration occurs in the temperate plants while tropical plants do not show
this type of mechanism.
• Presence of photorespiration process decreases the photosynthetic efficiency of
plants.
• The presence of photorespiration process decreases the photosynthetic potential
(under certain circumstances as much as 50%). Up till now what is the actual role of
photorespiration is not known.
• According to one hypothesis photorespiration provides the protective mechanism
against the light destruction of chloroplasts in C3 plants.
• This mechanism reduces the oxygen injury to chloroplast. By consuming oxygen, the
process helps in maintaining low oxidative state in chloroplast of C3 plants.
• According to some research workers photorespiration utilizes the excess of ATP and
reducing power (NADPH) produced at higher levels of light. 70

71. Significance of Photorespiration
• At least it appears that photorespiration has the following functions in plants.
i) Amino acids like glycine and serine are synthesized during photorespiratory
metabolism.
ii) These are the precursors for many important metabolites such as proteins, chlorophyll,
nucleotides, etc.
iii) Conversion of glycolic acid to glyoxylate consumes NADH (which is generated in the
light reaction of photosynthesis).
Thus, photorespiration involved in dissipating excess reducing power.
• Glycolic acid is formed during this process may be involved the protection against the
destruction action of photo-oxidation.
• Many scientists have questioned the significance of these functions.
• According to some scientists, RuBP carboxylase apparently emerged early in the
evolution when atmosphere was rich in CO2 and almost devoid of O2.
• However, as the CO2 content of air increased it started functioning as oxygenase as
well. Thus, it is inevitable process as it consequence of CO2/ O2 concentration.
71

72. Chemiosmotic theory
• The word chemiosmosis refers to conversion of chemical energy
e.g., in the oxidation of NADH by oxygen) to osmotic energy (i.e.
difference in the concentration of proton on two sides of
mitochondrial membrane) the membrane.
• Energy released by proton flow is used to form ATP from ADP and
Pi.
• This hypothesis provides general mechanism for coupling the energy
in electron transport to phosphorylation.
• This hypothesis was proposed by Peter Mitchell in (1961).
• This was proposed to explain the process of ATP formation both in
respiration and photosynthesis (photophosphorylation).
72

73. Chemiosmotic theory
This hypothesis having five main postulates.
1. The reactions occur on thylakoid membrane which forms closed vesicles
and which is almost impermeable to the passive flow of protons.
2. Electron donors and acceptors are arranged vectorially in the membrane.
3. Electron transfer is obligatory to the pumping of hydrogen ions from the
stroma into the osmotic space of the intrathylakoid membrane, leading to
build-up of a transmembrane proton concentration (PH) and the
transmembrane field ()
4. The combination of (PH) and () for as store of energy, tending to expel
protons from the intrathylakoid space.
5. There is a back flow of protons via the enzyme ATP synthetase (also called
Coupling factor) located across
• Support to this theory:
Data from several types of experiments support this hypothesis.
73

74. ATP synthesis through chemiosmosis in non-cyclic photophosphorylation
74

75. Significance of Chemiosmotic theory
• It provides understanding about biological energy transductions including process of
oxidative phosphorylation in mitochondria and phosphorylation in chloroplasts.
• The mechanism of energy coupling is similar in both cases.
• The conservation of free energy involves the passage of electrons through a chain to
membrane bound oxidation-reduction (redox) carriers and concomitant pumping of protons
across the membrane producing electrochemical gradient the proton motive force. The force
drives the synthesis of ATP by membrane bound enzyme complexes through which protons
flow back across the membrane, down their electrochemical gradient proton motive force
also drives other energy requiring processes of cells.
• The chemiosmotic theory also explains the formation of ATP in thylakoid membrane of
chloroplast. In these cases, the transfer of protons across the thylakoid membrane occurs
through alternate reduction (oxidation) of plastoquinone. This plastiquinone plays similar
role as that of ubiquinone plays in ETS chain of mitochondria.
• In photosynthesis the energy required for the flow of electrons from water to NADP+ and for
transport of protons across the membrane is supplied by absorption of light. 75

76. Questions on Photosynthesis
1. What are C4 plants? Describe the biochemical reactions involved in HSK
pathway.
2. What is photorespiration? Give schematic representation of glycolate cycle.
3. Describe Crassulacean Acid Metabolism and give its significance.
4. What is photophosphorylation? Give an account of chemiosmotic mechanism
of photophosphorylation.
5. What is oxidative phosphorylation? Explain ATP synthesis by chemiosmotic
theory.
6. Write about Red drop and Emmerson effect.
7. Describe the mechanism of cyclic and non-cyclic photophosphorylation.
8. What are C3 plants? Describe the various steps involved in C3 cycle.
9. Describe the role of pigments in photosynthesis.
10.Describe HSK pathway. 76

77. Questions on Photosynthesis
11. Explain cyclic photophosphorylation.
12. ‘Photosynthesis to certain extent is reverse of respiration’ Justify
and amplify the statement.
13. What are photosystems? Explain the working of both the
photosystems in photosynthesis.
14. Compare C3 and C4 plants.
15. Give the composition and function of two photosystems.
16. Write a note on the Hill reaction.
17. Write an account on CAM pathway.
18. Write short note on i) Quantasomes ii) Emmerson effect
77