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Photosynthesis in higher plants.

(1)Chloroplast-The site of photosynthesis : The most active photosynthetic tissue in higher plants is the mesophyll of leaves. Mesophyll cells have many chloroplast. Chloroplast are present in all the green parts of plants and leaves. There may be over half a million chloroplasts per square millimetre of leaf surface. In higher plants, the chloroplasts are discoid or lens-shaped. They are usually 4-10μmin diameter and 1-3μm in thickness.

These are double membrane-bound organelles in the cytoplasm of green plant cells. Chloroplast has two unit membranes made up of lipoprotein. Outer membrane of chloroplast is permeable and an inner one impermeable to protons. Inside the membranes is the proteinaceous ground substance called stroma, which contain a variety of particles, osmiophilic droplets, dissolved salts, small double stranded circular DNA molecules and 70S type ribosomes along with various enzymes. Inside the stroma is found a system ofchlorophyll bearing double-membraned sacs thylakoids or lamellae.

Thylakoids are flattened sacs arranged like the stacks of coins.One stack of thylakoids is calledgranum. Different grana are connected with the help of tubular connections called stroma lamellae or frets.Grana are the sites for light reaction of photosynthesis and consist of photosynthetic unit 'quantasomes' (Found in surface of thylakoids). Photosynthetic unit can be defined as number of pigment molecules required to affect a photochemical act, that is the release of a molecule of oxygen.Park and Biggins (1964) gave the term quantasome for photosynthetic units is equivalent to 230 chlorophyll molecules.

(2)Chloroplast pigments :Pigments are the organic molecules that absorb light of specific wavelengths in the visible region due to presence of conjugated double bonds in their structures. The chloroplast pigments are fat soluble and are located in the lipid part of the thylakoid membranes. There is a wide range ofchloroplastic pigments which constitute more than 5% of the total dry weight of the chloroplast.

They are grouped under two main categories : (i) Chlorophylls and   (ii) Carotenoids

The other photosynthetic pigments present in some algae and cyanobacteria are phycobilins.

(i) Chlorophylls : The chlorophylls, the green pigments in chloroplast are of seven typesi.e., chlorophylla, b, c, d, e, bacteriochlorophyll and bacterioviridin.

Of all, only two typesi.e.,chlorophyllaand chlorophyllbare widely distributed in green algae and higher plants.

Chlorophyll 'a' is found in all the oxygen evolving photosynthetic plants except photosynthetic bacteria. Reaction centre of photosynthesis is formed of chlorophylla. It occurs in several spectrally distinct forms which perform distinct roles in photosynthesis (e.g.,Chla680 orP680, Chla700 orP700, etc.). It directly takes part in photochemical reaction. Hence, it is termed as primary photosynthetic pigment. Other photosynthetic pigments including chlorophyllb, c, dand e; carotenoids and phycobilins are called accessory pigments because they do not directly take part in photochemical act. They absorb specific wavelengths of light and transfer energy finally to chlorophyllathrough electron spin resonance.

Chlorophyllais blue black while chlorophyllbis green black. Both aresoluble in organic solvents like alcohol, acetone etc. chlorophyllaappears red in reflected light and bright green in transmitted light as compared to chlorophyllbwhich looks brownish red in reflected light and yellow green in transmitted light.Chlorophyll is a green pigment because it does not absorb green light (but reflect green light) Chlorophyllapossesses —Cp(methyl group), which is replaced by —CHO (an aldehyde) group in chlorophyllb. Chlorophyll molecule is made up of a squarish tetrapyrrolic ring known ashead and a phytol alcohol calledtail. The magnesium atom is present in the central position of tetrapyrrolic ring. The four pyrrole rings of porphyrin head is linked together by methine(CH=) groups forming a ring system. Each pyrrole ring is made up of four carbon and one nitrogen. The porphyrin head bears many characteristic side groups at many points. Different side groups are indicative of various types of chlorophylls.

Phytol tail is made up of 20 carbon alcohol attached to carbon 7 position of pyrrole ring IV with a propionic acid ester bond.The basic structure of all chlorophyll comprises of porphyrin system.

When centralMg is replaced byFe, the chlorophyll becomes a green pigment called 'cytochrome' which is used in photosynthesis (Photophosphorylation) and respiration both.

Chlorophyll synthesis is a reduction process occurring in light. In gymnosperm seedlings, chlorophyll synthesis takes place in darkness in presence of enzyme called 'chlorophyllase'.The precursor of chlorophyll is chlorophyllide.

(ii) Carotenoids :The carotenoids are unsaturated polyhydrocarbons being composed of eight isoprene (C5H8) units. They are made up of two six-membered rings having a hydrocarbon chain in between. They are sometimes called lipochromes due to their fat soluble nature. They are lipids and found in non-green parts of plants. Light is not necessary for their biosynthesis. Carotenoids absorb light energy and transfer it to Chl.a and thus act as accessory pigments.They protect the chlorophyll molecules from photo-oxidation by picking up nascent oxygen and converting it into harmless molecular stage. Carotenoids can be classified into two groups namely carotenes and xanthophyll.

(a) Carotenes :They are orange red in colour and havegeneral formulaC40H56. They are isolated from carrot.

They are found in all groups of plantsi.e., from algae to angiosperms. Some of the common carotenes areα, β, γandδ carotene; phytotene, lycopene, neurosporeneetc. The lycopene is a red pigment found in ripe tomato and red pepper fruits. Theβ-carotene on hydrolysis gives vitamin A, hence the carotenes are also called provitamin A. β-carotene is black yellow pigment of carrot roots.

(b) Xanthophylls : They are yellow coloured carotenoid also called xantholsor carotenols. They contains oxygen also along with carbon and hydrogen and havegeneral formulaC40H56O2.

Lutein a widely distributed xanthophyll which is responsible for yellow colour in autumn foliage.Fucoxanthin is another important xanthophyll present in Phaeophyceae (Brown algae).

(iii) Phycobilins : These pigments are mainly found in blue-green algae (Cyanobacteria) and red algae. These pigments have open tetrapyrrolic in structure and do not bear magnesium and phytol chain.

Blue-green algae have more quantity of phycocyanin and red algae have more phycoerythrin. Phycocyaninand phycoerythrin together form phycobilins. These water soluble pigments are thought to be associated with small granules attached with lamellae.Like carotenoids, phycobilins are accessory pigmentsi.e. they absorb light and transfer it to chlorophylla.

(3)Nature of light : Sunlight is a type of energy called radiant energy or electromagnetic energy. This energy, according to electromagnetic wave theory (Proposed by James Clark Maxwell, 1960), travels in space as waves. The distance between the crest of two adjacent waves is called a wavelength(λ). Shorter the wavelength greater the energy.

The unit quantity of light energy in the quantum theory is called quantum (hν), whereas the same of the electromagnetic field is called photon. Solar radiation can be divided on the basis of wavelengths. Radiation of shortest wavelength belongs to cosmic rays whereas that of longest wavelength belong to radio waves. Light represents only one part of electromagnetic radiation. Other parts include cosmic rays, X-rays, UV rays, infra-red radiation and radio waves. A visible light has seven separated groups of more or less complete absorption. In a spectrum of sunlight, bands of blending colours are seeni.e.,dark red at one end running through red, orange, yellow, green, blue, indigo, violet and ending in darkest violet. Wavelengths in the violet portion of spectrum are about 400 millimicrons (mμ) in length and at other end of spectrum — the red portion — are much longer about 730mμ. In other words, visible light lies between wavelengths of ultra-violet and infra-red.The visible spectrum of solar radiations are primarily absorbed by carotenoids of the higher plants are violet and blue. However, art of blue and red wavelengths, blue light carry more energy.

Visible light :390nm (3900Å) to 760nm (7600Å). Violet (390–430nm), blue(430–470nm), blue-green (470–500nm), green (500–580nm), yellow (580–600nm), orange (600–650nm), orange-red (650–660nm) and red (660–760nm) Far-red (700–760nm). Infra-red 760nm – 100μm. Ultraviolet 100–390nm. Solar Radiations 300nm (ultraviolet) to 2600nm (infra-red).Photosynthetically active radiation (PAR) is 400–700nm. Leaves appear green because chlorophylls do not absorb green light. The same is reflected and transmitted through leaves.

Absorption and action spectra : The curve representing the light absorbed at each wavelength by pigment is called absorption spectrum. Curve showing rate of photosynthesis at different wavelengths of light is called action spectrum.

Absorption spectrum is studied with the help of spectrophotometer.The absorption spectrum of chlorophylla and chlorophyllb indicate that these pigments mainly absorbblueandredlights. Action spectrum shows that maximum photosynthesis takes place in blue and red regions of spectrum. The first action spectrum of photosynthesis was studied byT.W. Engelmann (1882) using green alga Spirogyra and oxygen seeking bacteria.

In this case actual rate of photosynthesis in terms of oxygen evolution or carbon dioxide utilisation is measured as a function of wavelength.

B. Mechanism of photosynthesis.

Before 1930 it was considered by physiologists that one molecule each ofCO2 andH2O form a molecule of formaldehyde (HCHO), of which 6mols are polymerized to one molecule of glucose (a hexose sugar).

However formaldehyde is a toxic substance which may kill the plants. Hence, formaldehyde hypothesis could not be accepted.

On the basis of discovery of Nicolas de Saussure that "The amount ofO2 released from plants is equal to the amount ofCO2 absorbed by plants", it was considered thatO2 released in photosynthesis comes fromCO2, but Rubenproved that this concept is wrong.

In 1930,C.B. Van Niel proved that, sulphur bacteria useH2S (in place of water) andCO2 to synthesize carbohydrates as follows :

This led Van Niel to the postulation that in green plants, water (H2O) is utilized in place ofH2S andO2 is evolved in place of sulphur (S). He indicated that water is electron donar in photosynthesis.

This was confirmed by Rubenand Kamen in 1941 usingChlorellaa green alga.

They used isotopes of oxygen in water,i.e., H218O instead ofH2O (normal) and noticed that liberated oxygen contains18O of water and not ofCO2. The overall reaction can be given as under :

The fate of different molecules can be summarised as follows :

C. concept of photosynthesis.

Photosynthesis is an oxidation reduction process in which water is oxidised to releaseO2 and CO2 is reduced to form starch and sugars.

Scientist have shown that photosynthesis is completed in two phases.

❒ Light phase or Photochemical reactions or Light dependent reactions or Hill's reactions : During this stage energy from sunlight is absorbed and converted to chemical energy which is stored in ATP and NADPH +H+.

Dark phase or Chemical dark reactions or Light independent reactions or Blackman reaction or Biosynthetic phase : During this stage carbohydrates are synthesized from carbon dioxide using the energy stored in the ATP andNADPH formed in the light dependent reactions.

Evidence for light and dark reactions in photosynthesis : Evidences in favour of light and dark phases in photosynthesis are :

Physical separation of chloroplast into grana and stroma fractions :It is now possible to separate grana and stroma fractions of chloroplast. If light is given to grana fraction in presence of suitable H-acceptor and in complete absence ofCO2, then ATP and NADPH2 are produced (i.e.,assimilatory powers). If these assimilatory powers (ATP andN ADPH2) are given to stroma fraction in presence of CO2 and absence of light, then carbohydrates are formed.

Experiments with intermittent light or Discontinuous light : Rate of photosynthesis is faster in intermittent light (Alternate light and dark periods) than in continuous light. It is because light reaction is much faster than dark reaction, so in continuous light, there is accumulation of ATP and NADPH2 and hence reduction in rate of photosynthesis but in discontinuous light, ATP and NADPH2 formed in light are fully consumed during dark in reduction of CO2 to carbohydrates. Accumulation of NADPH2 and ATP is prevented because they are not produced during dark periods.

Temperature coefficient studies :The temperature coefficient (Q10) is defined as the ratio of the velocity of a reaction at a particular temperature to that at a temperature 10°C lower. For a physical process the value of Q10 is slightly greater than one. In photochemical reaction the energy source is light and any increase in temperature is not sufficient to cause an increase in the rate. Thus here also the value ofQ10 is one. However, in case of chemical reactions the value ofQ10 is two or morei.e., with the rise of 10°C temperature, the rate of chemical reaction is doubled. If the process of photosynthesis includes a hidden chemical reaction in addition to usual photochemical reaction, its value ofQ10 should be two or more.

Blackman found thatQ10 was greater than 2 in experiment when photosynthesis was rapid and that Q10dropped from 2 often reaching unity,i.e., 1 when the rate of photosynthesis was low. These results show that in photosynthesis there is a dark reaction(Q10 more than 2) and a photochemicalor light reaction(with Q10 being unity).

(1)Light phase (Photochemical reactions) :Light reaction occurs in grana fraction of chloroplast and in this reaction are included those activities, which are dependent on light.Assimilatory powers (ATP and NADPH2) are mainly produced in this light reaction.

Robin Hill (1939) first of all showed that if chloroplasts extracted from leaves of Stellaria media andLamium albumare suspended in a test tube containing suitable electron acceptors,e.g.,Potassium ferroxalate (Some plants require only this chemical) and potassium ferricyanide, oxygen is released due to photochemical splitting of water. Under these conditions, no CO2 was consumed and no carbohydrate was produced, but light-driven reduction of the electron acceptors was accompained, byO2 evolution.

The splitting of water during photosynthesis is calledphotolysis. This reaction on the name of its discoverer is known as Hill reaction.

Hill reaction proves that

(i) In photosynthesisoxygen is released from water.

(ii) Electrons for the reduction ofCO2 are obtained from water [i.e.,a reduced substance (hydrogen donor) is produced which later reducesCO2].

Dichlorophenol indophenol is the dye used by Hill for his famous Hill reaction.

According to Arnon(1961), in this process light energy is converted to chemical energy. This energy is stored in ATP (this process of ATP formation in chloroplasts is known asphotophosphorylation) and from electron acceptor NADP+, a substance which found in all living beings NADP*H is formed as hydrogen donor. Formation of hydrogen donor NADPH from electron acceptor NADP+ is known as photoreductionor production of reducing powerNADPH.

Light phase can be explained under the following headings :

(i) Transfer of energy   (ii) Quantum yield   (iii) Emerson effect   (iv) Two pigment systems

(v) Z-scheme   (vi) Cyclic and non-cyclic photophosphorylation

(i) Transfer of energy :When photon of light energy falls on chlorophyll molecule, one of the electrons pair from ground or singlet state passes into higher energy level called excited singlet state. It comes back to hole of chlorophyll molecule within 10–9 seconds.

This light energy absorbed by chlorophyll molecule before coming back to ground state appears as radiation energy, while that coming back from excited singlet state is called fluorescence and is temperature independent. Sometimes the electron at excited singlet state gets its spin reversed because two electrons at the same energy level cannot stay; for some time it fails to return to its partner electron. As a result it gets trapped at a high energy level. Due to little loss of energy, it stays at comparatively lower energy level (Triplet state) from excited singlet state. Now at this moment, it can change its spin and from this triplet state, it comes back to ground state again losing excess of energy in the form of radiation. This type of loss of energy is called as phosphorescence.

When electron is raised to higher energy level, it is called at second singlet state. It can lose its energy in the form of heat also. Migration of electron from excited singlet state to ground state along with the release of excess energy into radiation energy is of no importance to this process. Somehow when this excess energy is converted to chemical energy, it plays a definite constructive role in the process.

(ii) Quantum yield

❒  Rate or yield of photosynthesis is measured in terms of quantum yield orO2 evolution, which may be defined as, "Number ofO2 mols evolved per quantum of light absorbed in photosynthesis."

On the other hand quantum requirementis defined as, "Number of quanta of light required for evolution of one mol ofO2 in photosynthesis."

Quantum requirementin photosynthesis = 8,i.e., 8 quanta of light are required to evolve one mol. ofO2.

Hence quantum yield= 1 / 8 = 0.125 (i.e.,a fraction of 1) as 12%.

(iii) Emerson effect and Red drop :R. Emerson and C.M. Lewis (1943) observed thatthe quantum yield of photosynthesis decreased towards the far red end of the spectrum (680nm or longer). Quantum yield is the number of oxygen molecules evolved per light quantum absorbed. Since thisdecrease in quantum yield is observed at the far region or beyond red region of spectrum is called red drop.

Emersonet al. (1957) further observed that photosynthetic efficiency of light of 680nmor longer is increased if light of shorter wavelengths (Less than 680nm) is supplied simultaneously. When both short and long wavelengths were given together the quantum-yield of photosynthesis was greater than the total effect when both the wavelengths were given separately. This increase in photosynthetic efficiency (or quantum yield) is known as Emerson effect orEmerson enhancement effect.

(iv) Two pigment systems : The discovery ofEmerson effect has clearly shown the existence of two distinct photochemical processes, which are believed to be associated with two different specific group of pigments. One group of pigments absorbs light of both shorter and longer wavelengths (More than 680nm) and another group of pigments absorbs light of only shorter wavelengths (Less than 680nm). These two groups of pigments are known as pigment systems or photosystems.

Pigment system I or Photosystem I : The importantpigments of this system are chlorophylla670, chlorophylla683, chlorophylla695,P700. Some physiologist also includecarotenes and chlorophyllbin pigment systemI.P700 acts as the reaction centre. Thus, this system absorbs both wavelengths shorter and longer than 680nm.

Pigment system II or photosystem II :The main pigments of this system are chlorophylla673,P680, chlorophyllband phycobilins. This pigment system absorbs wavelengths shorter than 680nm only.P680 acts as the reaction centre.

Pigment systems I and II are involved in non-cyclic electron transport, while pigment system I is involved only in cyclic electron transport.Photosystem I generates strong reductant NADPH. Photosystem II produces a strong oxidant that forms oxygen from water.

                                                  Comparison of photosystem I and photosystem II


Photosystem I

Photosystem II


PS I lies on the outer surface of the thylakoids

PS II lies on the inner surface of the thylakoid.


In this system molecular oxygen is not evolved.

As the result of photolysis of water molecular oxygen is evolved.


Its reaction center is P700.

Its reaction center is P680.


NADPH is formed in this reaction.

NADPH is not formed in this reaction.


It participate both in cyclic and noncyclic photophosphorylation.

It participate only in noncyclic photophosphorylation.


It receives electrons from photosystem II.

It receives electrons from photolytic dissociation of water.


It is not related with photolysis of water.

It is related with photolysis of water.

(v) Z-Scheme of light reactions : When sunlight strikes the thylakoid membrane, the energy is absorbed simultaneously by the antenna pigments of both PS I and PS II and passed on to the reaction centers of both photosystems. Electrons of both reaction center pigments are boosted to an outer orbital and each photoexcited electron is transferred to a primary electron acceptor. The transfer of electrons out of the photosystems leaves the two reaction center pigments missing an electron and thus, positively charged. After losing their electrons, the reaction centers of PS I and PS II can be denoted as P700+ and P680+ respectively. Positively charged reaction centers act as attractants for electrons, which sets the stage for the flow of electrons between carriers.

In oxygenic photosynthesis, in which two photosystems act in series, electron flow occurs along three legs-between water and PS II, between PS II and PS I and between PS I and NADP+ an arrangement which is described as the Z scheme.The Z scheme as originally proposed by Hill and Bendall, 1960.

(vi) Photophosphorylation : Light phase includes the interaction of two pigment systems. PS I and PS II constitute various type of pigments. Arnonshowed that during light reaction not only reduced NADP is formed and oxygen is evolved but ATP is also formed. This formation of high energy phosphates (ATP) is dependent on light hence called photophosphorylation.


(Where ADP = Adenosine diphosphate, Pi = Inorganic phosphate and ATP = Adenosine triphosphate).

When the light quantum is absorbed by various types of pigments (Like chlorophylls, phycobilins, carotenoids etc.), it is transferred to reaction centrei.e. P700 in PS I andP680 in PS II. Electrons excite from reaction centres due to funneling of energy.P700 gets photo excited and comes under first excited singlet state. As a result electron is lost, which is accepted by an electron, acceptor in the way. After absorbing light, excited electron liberated from reaction centre interacts with water.



Another important aspect of light reactions is the formation of ATP and NADPH2 (Assimilatory power). H+ from water and electron from chlorophyll are made available to NADP to form NADPH2. The electrons are accepted by NADP after passing through electron carriers. The carriers in the way undergo oxidation and reduction and are arranged in accordance with their redox potential value.

Photophosphorylation is of two types

(a) Cyclic photophosphorylation :It involves only PS I. Flow of electron is cyclic. When NADP is not available then this process will occurs. When the photons activate PS I, a pair of electrons are raised to a higher energy level.They are captured by primary acceptor which passes them on to ferredoxin, plastoquinone, cytochrome complex, plastocyanin and finally back to reaction centre of PS Ii.e.P700. At each step of electron transfer, the electrons lose potential energy. Their trip down hill is caused by the transport chain to pumpH+ across the thylakoid membrane. The proton gradient, thus established is responsible for forming (2 molecules) ATP. No reduction of NADP to NADPH+ H+. ATP is synthesized at two steps.

(b) Non cyclic photophosphorylation : It involves both PS-I and PS-II. Flow of electron is unidirectional. Here electrons are not cycled back and are used in thereduction of NADP to NADPH2. HereH2O is utilized andO2evolution occurs. In this chain high energyelectrons released from 'P-680' do not return to 'P-680' but pass through pheophytin, plastoquinone, cytochromeb6-fcomplex, plastocyanin and then enter P-700. In this transfer of electrons from plastoquinone (PQ) to cytochromeb6-fcomplex, ATP is synthesized. Because in this process high energy electrons released from 'P-680' do not return to 'P-680' and ATP (1 molecules) is formed, this is called Noncyclic photophosphorylation.ATP is synthesized at only one step.

(c) Pseudocyclic photophosphorylation : Arnon and his coworker (1954) demonstrated yet another kind of photophosphorylation. They observed that even in absence of CO2 and NADP, if chlorophyll molecules are illuminated, it can produce ATP from ADP and Pi (Inorganic phosphate) in presence of FMN or vit. K and oxygen. The process is thus very simple and requires no net chemical change but for the formation of ATP and water. Arnon called thisoxygen dependent FMNcatalysed photophosphorylationor pseudocyclic photophosphorylation which involves the reduction of FMN with the production of oxygen. FMN is an auto-oxidisable hydrogen acceptor with the effect that the reduced FMN is reoxidised by oxygen. Thus the process can continue repeatedly to produce ATP.

Since this process can be continuously self repeated, it appears that a single molecule of water should be sufficient to operate pseudocyclic photophosphorylation to meet the requirement of ATP.

D. Dark phase

The pathway by which all photosynthetic eukaryotic organisms ultimately incorporate CO2 into carbohydrate is known as carbon fixationor photosynthetic carbon reduction(PCR) cycleor dark reactions. The dark reactions are sensitive to temperature changes, but areindependent of light hence it is called dark reaction, however it depends upon the products of light reaction of photosynthesis, i.e. NADP .2H and ATP. The carbon dioxide fixation takes place in the stroma of chloroplasts because it has enzymes essential for fixation ofCO2 and synthesis of sugar. The techniques used for studying different steps wereRadioactive tracer technique using14C (Half life – 5720 years),Chromatography and Autoradiography and thematerial used wasChlorella (Cloacal alga) andScenedesmus(these are microscopic, unicellular algae and can be easily maintained in laboratory).

The assimilation and reduction ofCO2 takes place in this reaction by which carbohydrate is synthesized through following three pathways :

(i) Calvin cycle (C3)        (ii) Hatch and Slack cycle (C4)        (iii) Crassulacean acid metabolism (CAM plants)

(i) Calvin cycle : Calvin and Benson discovered the path of carbon in this process. This is known asC3 cycle becauseCO2 reduction is cyclic process andfirst stable product in this cycle is a 3-C compound (i.e., 3-Phosphoglyceric acid or 3-PGA).

Calvin cycle is divided into three distinct phases : Carboxylation, Glycolytic reversal, Regeneration of RuBP.

❒ Carboxylation : CO2 reduction starts with a 5-carbon sugar, ribulose-5-phosphate. 6 molecules of this sugar react with 6 molecules of ATP (Produced in light reactions) forming 6 molecules of ribulose-1, 5-biphosphate (RuBP) and 6 molecules of ADP. (equation i).

+ 6ATP+6ADP                .…. (i)

The reaction is catalysed by the enzyme ribulose biphosphate carboxylase (RUBISCO). Ribulose-1,5-biphosphate (RuBP) (=Ribulose diphosphate) acts asCO2 acceptor and6 mols of RuBP react with 6 mols of CO2 and 6 mols of water giving rise to 12 mols of 3-phosphoglyceric acid (a 3 carbon compound) (equation ii).

         ..… (ii)

❒  Glycolytic reversal :Carboxylation is followed by reactions that involve reversal of glycolysis part of respiration.

12 mols of 3-phosphoglyceric acid react with 12 mols of ATP (Produced in light reactions) giving rise to 12 mols each of 1, 3-diphosphoglyceric acid + ADP (equation iii).

     …. (iii)

12 mols of NADP.2H formed in light reactions are used to reduce 12 mols of 1,3-diposphoglyceric acid leading to the formation of 12 mols of 3-phosphoglyceraldehyde, 12 moles of NADP and 12 moles of phosphoric acid (equation iv).

…. (iv)

In this way bythe reduction of CO2, 12 molecules of 3-phosphoglyceraldehyde are formed. Out of these 12 molecules, 2 molecules go to synthesize sugar, starch and other carbohydrates and remaining 10 molecules are recycled to regenerate 6 molecules of ribulose 5 phosphate.

Out of two mols of 3-phosphoglyceraldehyde one mol is converted to its isomer 3-dihydroxyacetone phosphate (equation v).

                ….. (v)

One mol of 3-dihydroxyacetone phosphate react with 1 mol of 3-phosphoglyceraldehyde to form one molecule of fructose-1,6-biphosphate (equation vi).

     …. (vi)

One mol of fructose-6-phosphate and one mol of phosphoric acid is released from one mol of fructose-1,6-biphosphate with the help of the enzyme phosphatase with utilizations of one mol of H2O (equation vii).

                    .... (vii)

Fructose-6-phosphate can be converted to other sugars (viz., glucose, sucrose, starch, etc.). In this way, the atmospheric CO2 is used in the synthesis of carbohydrates.

Regeneration of RuBP : Both triose phosphates,i.e., 3-phosphoglyceraldehyde and dihydroxy acetone phosphate, actively participate in the regeneration ofCO2-acceptor ribulose-1,5-diphosphate. The sequence of reactions are as follows :









(ix) Xylulose-5-phosphate

2 + 4 = 6 molecules of Ribulose 5 phosphate are formed during the changes from equation (viii) and (ix) these molecule changed in Ribulose 1, 5 diphosphate (RuDP) by the consumption of 6 ATP. These RuDP again ready for reduction of new molecules ofCO2. Hence in this way regeneration of RuDP is going on. They are used in next calvin cycle. In the overall reactions18 ATP molecules and 12 NADPH2 molecules consumed and one molecule of glucose (Hexose) is obtained (1 NADPH2 = 3ATP ∴Total ATP consumed = 54 ATP). The whole photosynthesis can be summarized in terms of equation which is as follows :

Light reaction :

Dark reaction :

Final equation :

E. C4 cycle

Kortschak and Hart suppliedCO2 to the leaves of sugarcane, they found that thefirst stable product is a four carbon (C4) compound oxalo acetic acid instead of 3-carbon atom compound. The detailed study of this cycle has introduced byM.D. Hatch and C.R. Slack (1966).So it is called as "Hatch and Slack cycle". The stable product inC4 plant is dicarboxylic group. Hence it is called dicarboxylic acid cycle or DCA-cycle.C4 plants are true xerophytic plants.

The plants thatperformC4 cycle are found in tropical (Dry and hot regions) and sub-tropical regions. There are more than 900 known species in whichC4 cycle occurs. Among them, more than 300 speciesbelong to dicots and the rest belong to monocots. The important among them aresugarcane, maize, Sorghum,Cyperus rotundus, Digitaria brownii, Amaranthus, etc. These plants have "Kranz" (German term meaning halo or wreath)type of leaf anatomy. The vascular bundles, inC4 leaves are surrounded by a layer of bundle sheath cells that contain large number of chloroplasts. The chloroplasts inC4 leaves are dimorphic(Two morphologically distinct types).The chloroplasts of bundle sheath cells are larger in size and arranged centripetally. They contain starch grains but lack grana. The mesophyll cells, on the other hand, contain normal types of chloroplasts. The mesophyll cells performC4 cycle and the cells of bundle sheath performC3 cycle.

CO2 taken from the atmosphere is accepted by phosphoenolpyruvic acid (PEP) present in the chloroplasts of mesophyll cells of these leaves, leading to the formation of a 4-C compound, oxaloacetic acid (OAA). This acid is converted to another 4-C acid, the malic acid which enters into the chloroplasts of bundle sheath cells and there undergoes oxidative decarboxylation yielding pyruvic acid (a 3-C compound) andCO2.CO2 released in bundle sheath cells reacts with Ribulose-1,5-biphosphate (RuBP) already present in the chloroplasts of bundle sheath cells and thus Calvin cycle starts from here. Pyruvic acid re-enters mesophyll cells and regenerates phosphoenol pyruvic acid.CO2 after reacting with RuBP gives rise to sugars and other carbohydrates. Mesophyll cells have PEP carboxylase and pyruvate orthophosphate dikinase enzyme while the bundle sheath cells have decarboxylase and complete enzymes of Calvin cycle. InC4 plants, there are 2 carboxylation reactions, first in mesophyll chloroplast and second in bundle sheath chloroplast.

C4 plants are better photosynthesizers. There is no photorespiration in these plants. In C4 plants, for formation of one mole of hexose (glucose) 30 ATP and 12 NADPH2 are required. There is difference in differentC4 plants in mechenism of C4 mode of photosynthesis. The main difference is in the way the 4C dicarboxylic acid is decarboxylated in the bundle sheath cells. The three categories of C4 pathways inC4 plants are recognised such as :

(a) SomeC4 plantse.g.,Zea mays, Saccharum officinarum, Sorghum utilise NADP+ specific malic enzyme for decarboxylation. This mechanism of C4 pathway in these C4 plants is said to be of NADP+ –Me Type.

(b) SomeC4 plantse.g.,Atriplex, Portulaca, Amaranthusutilise NAD+ specific malic enzyme for decarboxylation. This mechanism of C4 pathways in theseC4 plants is said to be of NAD+ –Me Type.

(c) SomeC4 plantse.g., Panicum, Chlorisutilise PEP-carboxykinase enzyme. The mechanism of C4 pathway in these plants is called as PCK-me-Type.

Characteristics of C4 cycle

(1)C4 species have greater rate of CO2 assimilation than C3 species. This is on account of the fact that

(a) PEP carboxylase has great affinity forCO2.

(b) C4 plants show little photorespiration as compared to C3 plants, resulting in higher production of dry matter.

(2) C4 plants are more adapted to environmental stresses than C3 plants.

(3) CO2 fixation by C4 plants require more ATP than that by C3 plants. This additional ATP is needed for conversion of pyruvic acid to phosphoenol pyruvic acid and its transport.

(4) CO2 acceptor molecule in C4 plants is PEP. Further, PEP-carboxylase (PEPCO) is the key enzyme (RuBP-carboxylase enzyme is negligible or absent in mesophyll chloroplast, but is present in bundle sheath chloroplast).

(iii) Crassulacean acid metabolism plants (CAM plants) : This dark CO2fixation pathway proposed by Ting (1971).It operates in succulent or fleshy plantse.g.Cactus, Sedum, Kalanchose, Opuntia, Agave,orchid, pine apple andBryophyllumhelping them to continue photosynthesis under extremely dry condition.

The stomata of succulent plants remain closed during day and open during night to avoid water loss (Scotactive stomata).They store CO2 during night in the form of malic acid in presence of enzyme PEP carboxylase. TheCO2 stored during night is used in Calvin cycle during day time. Succulents refix CO2 released during respiration and use it during photosynthesis.

This diurnal change in acidity was first discovered in crassulacean plantse.g. Bryophyllum. So it is called as crassulacean acid metabolism. The metabolic pathways are –

❒ Acidification : In dark, stored carbohydrates are converted to phosphoenol pyruvic acid (PEP) by the process of glycolysis.The opening of stomata in CAM plants in dark causes entry of CO2 in leaf. So, phosphoenol pyruvic acid in presence of PEP carboxylase is converted to oxaloacetic acid (OAA). OAA is then reduced to malic acid in presence of enzyme malic dehydrogenase with the help of NADH2. This malic acid (Produced by acidification) is stored in vacuole.

❒ Deacidification : In light the malic acid is decarboxylated to produce pyruvic acid and evolveCO2. This process is calleddeacidification.

The malate may be decarboxylated in two ways –

(a) In some CAM plants the malate is directly decarboxylated in the presence of NADP+ malic enzyme intoCO2 and pyruvate (ME-CAM plants).

(b) In other CAM plants, the malate is first oxidised to oxaloacetic acid by enzyme malate dehydrogenase which is then converted intoCO2 and phosphoenol pyruvate with the utilization of ATP by enzyme PEP carboxykinase (PEPCK-CAM plants).

TheCO2 produced by any above process is then consumed in normal photosynthetic process to produce carbohydrate.

F. Characteristics of CAM pathway

(1)CO2assimilation and malic acid assimilation take place during the night.

(2) There is decrease inpHduring the night and increase inpH during the day.

(3) Malic acid is stored in the vacuoles during the night which is decarboxylated to release CO2 during the day.

(4) CAM plants have enzymes of both C3 and C4 cycle in mesophyll cells. This metabolism enable CAM plants to survive under xeric habitats. These plants have also the capability of fixing the CO2 lost in respiration.

G.  Photorespiration

Decker and Tio (1959) reported that light induces oxidation of photosynthetic intermediates with the help of oxygen in tobacco. It is called as photorespiration. The photorespiration is defined by Krotkov (1963) as an extra input ofO2 and extra release of CO2 by green plants is light.

Photorespiration is the uptake of O2 and release of CO2 in light and results from thebiosynthesis of glycolate in chloroplasts and subsequent metabolism of glycolate acid in the same leaf cell. Biochemical mechanism for photorespiration is also called glycolate metabolism.Loss of energy occurs during this process. The process of photorespiration involves the involvement of chloroplasts, peroxisomes and mitochondria. RuBP carboxylase also catalyses another reaction which interferes with the successful functioning of Calvin cycle.

Biochemical mechanism

(1) Ribulose bisphosphate carboxylase (RUBISCO), the main enzyme of Calvin cycle that fixesCO2, acts as ribulose bisphosphate oxygenase under low atmospheric concentration of CO(i.e.,below 1%) and increased concentration of O2. In presence of high concentration of O2 the enzyme RuBP oxygenase splits a molecule of Ribulose-1, 5-bisphosphate into one molecule each of 3-phosphoglyceric acid and 2-phosphoglycolic acid.

Ribulose-1, 5- bisphosphate 2 Phosphoglycolic acid +3 Phosphoglyceric acid

(2) The 2-phosphoglycolic acid loses its phosphate group in presence of enzyme phosphatase and converted into glycolic acid –

2 Phosphoglycolic acid +H2O Glycolic acid + Phosphoric acid.

(3) The glycolic acid, synthesized in chloroplast as an early product of photosynthesis, is then transported to the peroxisome. The glycolic acid reacts with O2 and oxidizes to glyoxylic acid and hydrogen peroxide with the help of enzyme glycolic acid oxidase.

Glycolic acid +O2

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