Photosynthesis in Higher Plants Class 11 Notes

Photosynthesis occurs in chloroplasts, mainly in the mesophyll cells of leaves. It involves two coupled stages — the light reaction (in the thylakoid membranes) and the dark reaction / Calvin cycle (in the stroma). The overall equation was first proposed by Cornelius van Niel and later confirmed by Ruben and Kamen using the isotope ¹⁸O.

The light reaction takes place in the thylakoid membrane and involves two photosystems and an electron transport chain.

• PS II (P680): absorbs light at 680 nm, loses electrons, which pass through the electron transport chain (plastoquinone → cytochrome b6f → plastocyanin) to PS I. ATP is generated by proton gradient (chemiosmosis) as protons are pumped into the thylakoid lumen.
• PS I (P700): absorbs light at 700 nm, re-energises electrons, which pass through ferredoxin to NADP⁺ reductase, producing NADPH.
• Non-cyclic photophosphorylation: electrons flow from PS II → PS I and do not return to PS II. Both ATP and NADPH are produced. O₂ is released.
• Cyclic photophosphorylation: electrons from PS I cycle back through the cytochrome b6f complex to PS I via plastocyanin. Only ATP is produced (no NADPH, no O₂). This occurs when NADPH is in excess or when the Calvin cycle needs more ATP.

The Calvin cycle (Calvin–Benson cycle) takes place in the stroma of the chloroplast. It uses the ATP and NADPH from the light reaction to fix CO₂ into organic molecules. The cycle has three phases: carbon fixation, reduction, and regeneration of RuBP.

• Carbon fixation: CO₂ combines with ribulose-1,5-bisphosphate (RuBP, a 5-carbon sugar) catalysed by RuBisCO to form an unstable 6-carbon intermediate that splits into two molecules of 3-phosphoglycerate (3-PGA, a 3-carbon compound).
• Reduction: 3-PGA is phosphorylated by ATP and reduced by NADPH to glyceraldehyde-3-phosphate (G3P). One G3P exits the cycle per 3 CO₂ fixed — this is the net product.
• Regeneration of RuBP: the remaining G3P molecules are rearranged and phosphorylated by ATP to regenerate RuBP, allowing the cycle to continue.

It takes 6 turns of the Calvin cycle (fixing 6 CO₂) to produce one molecule of glucose (C₆H₁₂O₆). C3 plants (e.g. rice, wheat) use only the Calvin cycle for carbon fixation.

C4 plants (e.g. maize, sugarcane, sorghum) have evolved a carbon-concentrating mechanism to overcome photorespiration. They use two types of cells for carbon fixation:

• Mesophyll cells: CO₂ is fixed by PEP carboxylase (which has no oxygenase activity) into oxaloacetate (OAA, a 4-carbon compound) from PEP and CO₂. OAA is converted to malate or aspartate.
• Bundle sheath cells: the 4-carbon acid is transported to bundle sheath cells, where it is decarboxylated to release CO₂. The released CO₂ is then fixed by RuBisCO in the Calvin cycle. The high CO₂ concentration around RuBisCO suppresses photorespiration.
• Kranz anatomy: C4 leaves have a distinctive arrangement — bundle sheath cells surround the vascular bundles with thick walls, and mesophyll cells are arranged concentrically around them. This close packing ensures efficient CO₂ transfer.

Photorespiration is a wasteful process that occurs when RuBisCO acts as an oxygenase instead of a carboxylase — it fixes O₂ instead of CO₂ onto RuBP. This produces one molecule of 3-PGA and one molecule of 2-phosphoglycolate (a 2-carbon compound). The 2-phosphoglycolate must be salvaged through a pathway involving chloroplasts, peroxisomes and mitochondria (the C2 pathway), consuming ATP and releasing CO₂ without producing useful energy.

• Light intensity: rate increases linearly at low intensities, then plateaus at the light saturation point. At very high intensities, rate may decline due to photooxidative damage.
• Light quality: chlorophyll absorbs mainly red (660–680 nm) and blue (430–450 nm) light. Green light is mostly reflected — hence leaves appear green.
• CO₂ concentration: increasing atmospheric CO₂ from ~0.04% to ~0.1% significantly boosts the rate in C3 plants. C4 plants are already saturated at ambient CO₂ because of their CO₂-concentrating mechanism.
• Temperature: photosynthesis has a Q₁₀ of ~2 up to the optimum (~25–35 °C for most C3 plants), beyond which the rate drops due to enzyme denaturation (RuBisCO, etc.) and increased photorespiration.
• Water: water stress causes stomatal closure, reducing CO₂ entry and hence limiting the rate.

Example: How many ATP and NADPH molecules are consumed to produce one molecule of glucose in the Calvin cycle?

Solution: Producing one glucose (6 CO₂ fixed) requires 6 turns of the Calvin cycle. Each turn fixes 1 CO₂ and consumes 3 ATP + 2 NADPH. For 6 CO₂: 6 × 3 = 18 ATP and 6 × 2 = 12 NADPH are consumed. The 18 ATP come from both cyclic and non-cyclic photophosphorylation; the 12 NADPH come only from non-cyclic photophosphorylation.

Example: A C4 plant and a C3 plant are placed in an environment with high temperature, high light intensity and low CO₂ concentration. Which plant will photosynthesise more efficiently and why?

Solution: The C4 plant will be more efficient. Under these conditions (high O₂, low CO₂, high temperature), RuBisCO in C3 plants fixes O₂ instead of CO₂, leading to photorespiration and loss of fixed carbon. In C4 plants, PEP carboxylase initially fixes CO₂ into OAA in mesophyll cells; the CO₂ is then concentrated in bundle sheath cells where RuBisCO operates at high CO₂/O₂ ratio, virtually eliminating photorespiration.

Exercises (NCERT Q1–Q9)

• 1) By looking at a plant externally, can you tell whether a plant is C3 or C4? Why and how?
• 2) By looking at which internal structure of a plant you can tell whether a plant is C3 or C4? Explain.
• 3) Even though a very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why?
• 4) RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants?
• 5) Suppose there were plants that had a high concentration of Chlorophyll b, but lacked chlorophyll a, would it carry out photosynthesis? Then why do plants have chlorophyll b and other accessory pigments?
• 6) Why is the colour of a leaf kept in the dark frequently becomes yellow, or pale green? Which pigment do you think is more stable?
• 7) Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green ? Why?
• 8) Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: (a) At which point/s (A, B or C) in the curve light is a limiting factor? (b) What could be the limiting factor/s in region A? (c) What do C and D represent on the curve?
• 9) Give comparison between the following: (a) C3 and C4 pathways (b) Cyclic and non-cyclic photophosphorylation (c) Anatomy of leaf in C3 and C4 plants

Answers at a glance

1. 1
   Q1 → No. No external character of a plant is a reliable test of the C3 or the C4 pathway, and the plant is identified by its internal structure instead. The obvious candidate is the venation of the leaf, and the usual statement is that a broad leaf with a reticulate or net-like venation indicates a C3 plant and a leaf with the parallel venation indicates a C4 plant, the dicots being recognised by the broad leaf and the net-veined and by the flower with four or five parts or a multiple of four or five. That rule does not hold, and it is broken in both directions: Amaranthus is a broad-leaved dicot with reticulate venation and is a C4 plant, while dicots such as Calophyllum and Melastoma, with a parallel or a pinnate-parallel venation, are C3 plants, and among the monocots, which all show the parallel venation, both the C3 crops and the C4 crops occur, so that a grass tells one nothing about the pathway. The reason the rule fails is that the venation is a correlation and not a necessity, for nothing in the biochemistry of the C4 pathway requires any particular vein pattern, the pathway depending on the compartmentation of the leaf, the fixation by the PEP carboxylase in the mesophyll and the Calvin cycle in the bundle sheath, and the mesophyll and the bundle sheath being connected across the leaf by the plasmodesmata and not by the veins. The disposition of the vascular bundles follows the ancestry, the habit and the mechanics of the leaf, so that the vein pattern reports the phylogeny rather than the pathway, and the venation is at most a clue for the field botanist. The plant is then identified internally, which is the subject of the next question: the leaf is cut in transverse section and examined for the Kranz anatomy, the ring of radially arranged mesophyll cells about the chloroplast-rich bundle sheath, the presence of the bundle sheath being what identifies the C4 plants; or, where a section cannot be made, the two carboxylases are assayed, the PEP carboxylase being present only in a C4 plant, or the mesophyll cells are tested for the RuBisCO, which they contain in a C3 plant and which they lack in a C4 plant. So the conclusion is that a C3 or a C4 plant cannot be recognised from its external appearance, and no single external character is decisive.
2. 2
   Q2 → The internal structure to be examined is the leaf in transverse section, and the decisive character is the Kranz anatomy, the wreath-like arrangement of radially placed mesophyll cells around the bundle sheath, which is the anatomy of a C4 leaf. In such a leaf the vascular bundles are of the large kind, arranged in a ring, and each is surrounded by two sheaths, an outer of thin-walled, chloroplast-containing parenchymatous cells and an inner of large, prominent, chloroplast-rich bundle sheath cells, and around this inner sheath the mesophyll cells are arranged in a ring, radially, so that a wreath is seen in the section. The mesophyll of a C4 leaf is therefore not differentiated into a palisade and a spongy region as it is in a C3 leaf, its cells are of one kind arranged radially, and the chloroplasts of the mesophyll are smaller and have fewer grana and are of a different type from those of the bundle sheath, so the two compartments are unmistakable. In a C3 leaf, in contrast, the bundle has a single bundle sheath of one layer of relatively small, thin-walled cells containing few chloroplasts, there is no ring of radially arranged mesophyll around it, and the mesophyll is fully differentiated into the palisade parenchyma beneath the upper epidermis and the spongy parenchyma above the lower, so the leaf is a typical dorsiventral dicot leaf and is nearly isobilateral in a C4. The explanation of the anatomy is that it is the visible counterpart of the biochemistry and exists for one purpose, to raise the carbon dioxide concentration around the RuBisCO and so to suppress the oxygenase activity of that enzyme and the photorespiration that follows it. The first fixation, by the PEP carboxylase, takes place in the mesophyll cell, which is exposed to the air and which possesses no oxygenase activity, so the carbon is fixed there into the four-carbon oxaloacetic acid and thence into malate or aspartate; these four-carbon acids are then transported to the bundle sheath cell, the second compartment, where they are decarboxylated, so that the carbon dioxide is released locally and its concentration around the RuBisCO is raised to several times the atmospheric value. The RuBisCO is confined to this compartment, its chloroplasts are large and rich in grana and lack the PEP carboxylase, and the bundle sheath is relatively impervious to the gases, so the oxygen cannot readily reach the enzyme. The carboxylase activity is therefore favoured over the oxygenase activity, the photorespiration is virtually absent, and the plant is far more productive, at the cost of the extra ATP that the C4 pathway consumes in regenerating the PEP, a cost more than repaid in the high light and high temperature in which the C4 plants grow.
3. 3
   Q3 → The C4 plant is highly productive in spite of the fact that only a small proportion of its cells, the bundle sheath cells, carry out the Calvin cycle, and the reason is that the productivity of a plant is not a function of the number of the cells doing the photosynthesis but of the rate in each of them and of the absence of the losses, and the C4 plant has both. In the first place the productivity of each bundle sheath cell is several times the productivity of a cell of a C3 plant, because of the carbon-concentrating mechanism. The PEP carboxylase of the mesophyll fixes the carbon into the four-carbon oxaloacetic acid and thence into malate or aspartate, these four-carbon acids are transported to the bundle sheath, and they are decarboxylated there, so that the bundle sheath cell is supplied with a carbon dioxide concentration five to ten times the atmospheric one, and since the rate of the carboxylation by the RuBisCO depends on the ratio of the carbon dioxide to the oxygen, this high local concentration makes the carboxylation greatly outrun the oxygenation, and the photosynthetic rate rises with it. In the second place, and this follows from the first, the oxygenase activity of the RuBisCO is effectively abolished, so the photorespiration is almost non-existent, whereas in a C3 plant the photorespiration loses about a quarter to a third of the fixed carbon, and a substantial part of the carbon dioxide that a C3 plant fixes is released again in the process, so that a C3 plant must fix more carbon dioxide for every sugar molecule it keeps while the C4 plant keeps essentially all that it fixes. The plant does pay for this, and the cost should be stated: the C4 plant requires five ATP and two NADPH for every molecule of carbon dioxide fixed where the C3 plant requires three ATP and two NADPH, and the five already includes the three of the Calvin cycle, so the C4 plant spends two ATP more for the same carbon fixed, that is about two-thirds more, the greater part of the extra being spent in the regeneration of the PEP by the pyruvate phosphate dikinase step and in the mesophyll, where the light reactions are largely cyclic so as to supply it. The C4 plant is therefore advantageous only where the light is intense and the temperature high, since there the photorespiration of a C3 plant is at its worst and the extra ATP is abundantly available, whereas in moderate conditions the C4 plant is merely wasting ATP, which is why the C4 plants are the weeds of the hot and bright habitats and the C3 plants dominate the cool and temperate ones. And the resolution of the question is that the few cells are not a disadvantage at all but the very condition of the advantage, for a plant with few photosynthetic cells working at a very high rate is exactly as productive as one with many cells working slowly, and what the C4 pathway provides is that high rate, together with the elimination of the photorespiratory loss, so that the carbon fixed in the mesophyll is delivered to the few bundle sheath cells and fixed there almost without waste.
4. 4
   Q4 → The RuBisCO carries out more carboxylation in a C4 plant because the C4 pathway raises the concentration of the carbon dioxide at the RuBisCO, and the ratio of the carbon dioxide to the oxygen at the enzyme is what decides the balance of its two activities. The RuBisCO is a bifunctional enzyme: as a carboxylase it catalyses the addition of the carbon dioxide to the ribulose 1,5-bisphosphate in the first step of the Calvin cycle, giving two molecules of 3-phosphoglycerate; as an oxygenase it catalyses the addition of the oxygen to the same substrate, giving one molecule of 3-phosphoglycerate and one of phosphoglycolate, which is then recycled through the photorespiratory pathway that yields no sugar, consumes ATP and releases carbon dioxide again. Since both reactions use the same substrate and the same enzyme they are in competition, and the outcome is decided by the relative concentrations of the two gases at the enzyme. In a C4 plant that concentration is raised five to ten times above the atmospheric value, and the mechanism is the carbon-concentrating mechanism itself. The first fixation is made not by the RuBisCO but by the PEP carboxylase in the mesophyll cell, and this enzyme has no oxygenase activity at all, so it fixes the carbon without any competition from the oxygen, into the oxaloacetic acid and thence into malate or aspartate; these four-carbon acids are then transported into the bundle sheath cell, in which the RuBisCO is confined, and they are decarboxylated there, so that the carbon dioxide is released precisely where the RuBisCO is and its local concentration rises several-fold above the atmospheric value. A second contribution in the same direction is that the bundle sheath cell is relatively sealed to the gases, its walls being thick and impervious to gaseous exchange and there being no intercellular spaces, so that the carbon dioxide released there is retained around the enzyme and the oxygen of the mesophyll cannot readily reach it, so the ratio of carbon dioxide to oxygen is raised still further. Because the competitive carboxylation and oxygenation reactions depend on that ratio, a five to tenfold rise in the carbon dioxide at the enzyme shifts the balance heavily towards the carboxylation, and the rate of carboxylation also rises with the concentration of the carbon dioxide. The oxygenation therefore accounts for a substantial fraction of the enzyme's activity in a C3 plant at the present atmospheric concentration but is largely or completely switched off in a C4 plant, and also in a C3 plant grown in an atmosphere enriched with carbon dioxide, which is the same fact exploited by the glasshouses. The RuBisCO thus does more carboxylation in a C4 plant simply because the plant has gone to the considerable expense of two extra ATP for every molecule of carbon fixed, five in place of the three, in order to give that enzyme a far more favourable ratio of carbon dioxide to oxygen.
5. 5
   Q5 → Such a plant would not carry out photosynthesis. The reason is that the chlorophyll a is the only pigment of the plant that can perform the photochemical reaction of the reaction centre: it is the chlorophyll a of the reaction centre of photosystem I and of photosystem II that passes the energy of the absorbed light to the reaction centre and is thereby converted into the primary electron acceptor, and no other pigment can do this. A plant rich in chlorophyll b but lacking chlorophyll a would therefore be unable to transfer the energy of the light it absorbs to the reaction centre, since the reaction centre chlorophyll is specifically the chlorophyll a, and so it would be unable to initiate the photochemical charge separation; there would be no photolysis of water, no evolution of oxygen, no reduction of NADP plus, no photophosphorylation and therefore no synthesis of ATP, and consequently no fixation of carbon in the Calvin cycle. It would absorb the light, because the chlorophyll b does absorb it, and would therefore look green, but it would be unable to convert that light into chemical energy. Plants nevertheless do possess chlorophyll b and a range of other accessory pigments, for three reasons. The first is that the accessory pigments broaden the range of the light the plant can use: the chlorophyll a absorbs most efficiently in the blue-violet at about 430 nanometres and in the red at about 662 nanometres and absorbs the green and the yellow poorly, while the chlorophyll b absorbs mainly in the blue and the blue-green at about 453 nanometres, so between them the two pigments collect light from about 400 to about 700 nanometres, that is practically the whole of the visible spectrum, whereas the chlorophyll a alone would leave gaps. The second and physiologically more important reason is that the accessory pigment transfers the energy it has absorbed to the chlorophyll a of the reaction centre by resonance energy transfer, that is by the excitation of the pigment and the handing on of the excitation to the neighbouring pigment and not by a transfer of electrons, and this transfer is very efficient, being close to complete, so the accessory pigment acts simply as a light-collecting antenna that funnels the energy it has captured to the one pigment able to use it. The third reason is protection: the accessory pigments protect the chlorophyll a from photo-oxidation by absorbing and dissipating the energy of excess light as heat, which is why a leaf exposed to a bright sun and unable to use the energy, as on the sunlit side, turns yellow and bleaches, the chlorophyll being destroyed while the accessory pigments remain. The other accessory pigments are the carotenoids, the carotenes such as the beta-carotene and the lycopene and the xanthophylls such as the lutein, the zeaxanthin and the violaxanthin, which are the yellow and orange pigments, which absorb the violet, the blue and the blue-green light that the chlorophyll absorbs only poorly, which pass the energy on to the chlorophyll b or a, and which serve also as the protectants against the photo-oxidation, and which become the orange and the yellow colour of the autumn leaves when the chlorophyll is degraded and they are left exposed. Thus the division of labour is that the accessory pigments do the collecting and the protecting while the chlorophyll a of the reaction centre does the actual photochemistry, and it is precisely this specialisation of the chlorophyll a that makes the plant of the first part unable to photosynthesise.
6. 6
   Q6 → The colour of a leaf kept in the dark becomes yellow or pale green because the chlorophyll is lost. The mechanism is that the maintenance of the chlorophyll in the leaf depends on its continued synthesis, that synthesis itself requires light, and that a leaf kept in the dark is nevertheless still respiring and still consuming the products of the photosynthesis, so the chlorophyll that is present is gradually used up and is not replaced. The chlorophyll is a labile pigment, being a porphyrin with a magnesium ion in the centre of the ring and a phytol side chain, and it is readily degraded, to the colourless chlorophyllide and the phytol, and it is bleached by the light, so it is destroyed even in the dark and without any photo-oxidation. Since the other pigments, the carotenoids, are comparatively stable and were present all along but masked by the far more abundant green chlorophyll, the loss of the chlorophyll unmasks them, and the leaf therefore passes first through a pale green stage, in which the yellow carotenoids are becoming relatively more prominent as the chlorophyll diminishes, and then through a yellow stage, in which the last of the chlorophyll has gone and the carotenoids alone remain, and finally to a brown stage as the carotenoids too are degraded. The same sequence in the same order is seen in the autumn leaf and in the yellowing leaf of a detached branch, so the experiment in the dark and the autumn colouring are the same process. The pigment that is more stable is the carotenoid, that is the carotenes and the xanthophylls, and this is the reason that the yellow and the orange are the colours that survive. The pale green of the earlier stage is the same mixture at an earlier point, the green of the chlorophyll that is still present being simply diluted by the yellow carotenoids, and it is better not to claim that the chlorophyll b is the more stable of the two chlorophylls, since which of them is lost first varies with the species and the conditions; the stability that the question asks about is the much greater stability of the carotenoids as against the chlorophyll. Finally, the pigments lie in the thylakoid membrane of the chloroplast, and in the dark the chloroplast is itself degraded and the thylakoid dismantled, so that the pigments are freed and destroyed, and it is for this reason that a yellow leaf put back into the light becomes green again, since the chlorophyll is resynthesised in the light while the carotenoids, being still present, need not be made.
7. 7
   Q7 → The leaves that are darker green are those of the plant kept in the sunlight, and in the first case the leaves on the sunny side of the plant are darker green than those on the shady side, while in the second the leaves of the potted plant kept in the sun are darker than those of the potted plant kept in the shade. Since the green colour of a leaf is the colour of the chlorophyll it contains, the depth of the green is a measure of the amount of the chlorophyll present, and so the question reduces to asking why the sun leaves contain more chlorophyll than the shade leaves. The reason is that the synthesis of the chlorophyll is light-dependent and the intensity in the sun is far greater than in the shade, so a leaf in bright light synthesises more chlorophyll than one in dim light; the shade leaf therefore contains less chlorophyll and is also thinner, since the plant on the forest floor has less light to capture and so invests less in the light-capturing tissue and more in growth, while the sun leaf is thicker and more heavily pigmented, so that the two are recognisable by the colour and by the thickness. It is worth noting that this is a physiological acclimation and not a genetic difference, for a plant moved from one condition to the other adjusts, a plant grown first in the shade and then moved to the light becoming darker green and thicker and a plant moved from the light into the shade becoming paler and thinner as the excess chlorophyll is lost, so the plant regulates its pigment to the light it actually receives. The functional consequence completes the explanation. A leaf with more chlorophyll absorbs more of the light falling on it, so the sun leaf, with its high pigment content, is adapted to the strong light and can use a larger share of it, whereas the shade leaf is adapted to the dim light of the canopy floor, where a high pigment content would be a disadvantage, since the excess pigment would absorb light that could not be used and would have to be dissipated harmlessly, and it would cost the plant nitrogen to make. The sun and the shade leaves of the same plant are therefore two different adaptations of the same organ to two different light environments, and the darker green of the sun leaf is the visible consequence of the light-dependence of the synthesis of the chlorophyll.
8. 8
   Q8 → The graph of the rate of photosynthesis against the light intensity is a curve that rises steeply, then less steeply, and then flattens, and it is this shape that answers the three parts. The first region is the low light region, in which the curve rises almost as a straight line with its greatest gradient, so the rate of photosynthesis is directly proportional to the light intensity and every additional unit of light gives a corresponding increase in the rate, and this is the region in which the light is the limiting factor. The second is the light saturation region, in which the curve bends and the rate continues to increase but more slowly than the light, so that some factor other than the light has begun to hold the rate down. The third is the plateau, in which the rate is at its maximum and is completely independent of the light, and here the light is not limiting at all. (a) Light is the limiting factor in the steeply rising part of the curve, that is in region A and in the region between A and B, where the rate is proportional to the light, and it ceases to be the limiting factor beyond the point C where the curve has flattened, being only partly limiting at B itself where the curve begins to bend. (b) In region A, the low light region, the limiting factor is the availability of the light itself together with the capacity of the light-harvesting apparatus: the rate is low because there is little light to harvest and the antenna of the reaction centres is not yet working at anything like its full capacity, and the light is by definition the limiting factor. The other metabolic factors, the temperature, the supply of the carbon dioxide and the enzyme content of the leaf, are in excess at these low intensities and are not limiting, and the rate rises with the light simply because the reaction centres are being excited more frequently. The practical significance is that a plant at the bottom of a dense canopy lives in the conditions of region A, the light there being only a small fraction of the full sunlight, which is why the density of a crop is a compromise between the shading of the lower leaves and the light reaching the upper ones. (c) C represents the light saturation point, the light intensity at which the rate of photosynthesis has reached its maximum, so that above it no further increase in the light raises the rate, and at and beyond this point the light is no longer limiting. D represents the maximum rate of photosynthesis, that is the plateau, the highest value the plant can attain, and its value is a function of the supply of the carbon dioxide, of the temperature and of the enzyme content of the leaf rather than of the light. The flat part of the curve therefore means for the farmer that once the light saturation point C is reached no additional light will increase the crop and the yield must be raised by the carbon dioxide or the temperature or the water, and it is in practice the water, by opening the stomata and so admitting the carbon dioxide, that sets the limit at D.
9. 9
   Q9 → The three comparisons are as follows. (a) C3 and C4 pathways: the first acceptor of the carbon dioxide is the ribulose bisphosphate, a five-carbon compound, in the C3 and the phosphoenolpyruvate, a three-carbon compound, in the C4, and the first stable product is accordingly a three-carbon compound, the 3-phosphoglycerate, in the C3 and a four-carbon compound, the oxaloacetic acid, in the C4, which is the origin of the two names. The first fixing enzyme is the RuBisCO, which is both a carboxylase and an oxygenase, in the C3, whereas in the C4 the first fixation is by the PEP carboxylase, which has no oxygenase activity, the RuBisCO acting only later. The number of cellular compartments is one in the C3, the mesophyll, in which the whole cycle runs, and two in the C4, the mesophyll and the bundle sheath. The initial products are two molecules of 3-phosphoglycerate in the C3 and the oxaloacetic acid, at once converted into malate or aspartate, in the C4. The carbon dioxide concentrating mechanism is absent in the C3, so the RuBisCO works at the ambient carbon dioxide, and present in the C4, which decarboxylates the four-carbon acid in the bundle sheath and so supplies the enzyme at five to ten times the atmospheric concentration. The photorespiration is present and substantial in the C3, losing some 25 to 30 per cent of the fixed carbon, and virtually absent in the C4. The energy cost is 3 ATP and 2 NADPH per molecule of carbon dioxide in the C3 and 5 ATP and 2 NADPH in the C4, the five already including the three of the Calvin cycle, so the C4 spends two ATP more, that is about two-thirds more, for the same carbon fixed. The net productivity and the habitat follow: the C3 is the more productive in cool and moderate light and temperature and is the plant of the temperate regions and of most of the world's crops, while the C4 is the more productive in high light and high temperature and is the plant of the tropics, of the arid regions and of the hot habitats. And the cycle is carried out by all the mesophyll cells in the C3 but only by the bundle sheath cells in the C4. (b) Cyclic and non-cyclic photophosphorylation: the cyclic uses only the photosystem I and the non-cyclic uses both the photosystem I and the photosystem II; in the cyclic the electron leaving the photosystem I returns to it by an alternative path, so there is no net gain or loss and no NADPH is made, whereas in the non-cyclic the electron passes down the chain to the ferredoxin and reduces the NADP plus to NADPH plus H plus, and the electron lost is replaced by the splitting of water at the photosystem II, so that the flow is a flow and not a cycle; both synthesise ATP, but the cyclic makes additional ATP only and in quantity, with no NADPH, so its ATP to NADPH ratio is very high, whereas the non-cyclic makes ATP along with NADPH; the products of the cyclic are ATP only, with no oxygen and no NADPH, and those of the non-cyclic are ATP, NADPH and oxygen; the cyclic consumes no water and evolves no oxygen, and the non-cyclic splits water; the reaction centre of each photosystem is a chlorophyll a, the P700 of PS I and the P680 of PS II, while the antenna about it is built chiefly of the chlorophyll b, so that the cyclic, which works with PS I alone, is driven by the P700 and is active even in the far red, whereas the non-cyclic uses the P680 and the P700 together; and the function of the cyclic is to make ATP and to generate the reducing power that builds the gradient for it, being the mechanism by which the plant can raise the ATP supply to meet the extra demand of the Calvin cycle in a C4 plant, whereas the non-cyclic is the route by which the plant makes its NADPH and its oxygen. (c) Anatomy of the leaf: the mesophyll is fully differentiated into a palisade parenchyma and a spongy parenchyma in the C3 and is undifferentiated, its cells being of one kind arranged radially round the bundle sheath, in the C4; the bundle sheath is a single layer of relatively small, thin-walled cells with few chloroplasts in the C3 and two sheaths, an outer of thin-walled chloroplast-bearing parenchyma and an inner of large, prominent, chloroplast-rich cells, in the C4; the Kranz anatomy is absent in the C3 and present in the C4; the vascular bundles are scattered in the C3 leaf and arranged in a ring, and of two kinds, in the C4; the C3 mesophyll chloroplasts have well-developed grana, whereas in the C4 the mesophyll chloroplasts are small and have few or no grana and lack the PEP carboxylase while the bundle sheath chloroplasts are large, grana-rich and carry the RuBisCO; the small veins end freely in the lamina giving the reticulate venation in the C3 and run parallel giving the parallel venation of a broad leaf in the C4; and the C3 leaf is dorsiventral with a different upper and lower surface whereas the C4 leaf is nearly isobilateral.

MCQ Questions

1 Mark Questions

1. 1
   Name the gas released during photosynthesis. (Ans: O₂)
2. 2
   Which pigment is essential for photosynthesis?
3. 3
   What is photolysis of water? (Ans: splitting of water in the light reaction, releasing O₂, protons and electrons)
4. 4
   Which enzyme fixes CO₂ in the Calvin cycle? (Ans: RuBisCO (Ribulose bisphosphate carboxylase-oxygenase))

2 Mark Questions

1. 1
   Differentiate between light and dark (carbon) reactions on any two points.
2. 2
   Why do C4 plants have a higher photosynthesis rate at high temperatures than C3 plants? (Ans: CO₂ concentration mechanism through bundle sheath cells and no/minimal photorespiration)
3. 3
   Write any two plant factors that affect the rate of photosynthesis. (Ans: chlorophyll amount, leaf area, enzymes, stomatal opening)

3 Mark Questions

1. 1
   Describe the electron transport chain of photosynthesis (Z-scheme). (Ans: PSII → plastoquinone → cytochrome → plastocyanin → PSI → ferredoxin/NADP⁺; ATP and NADPH produced)
2. 2
   Describe the three stages of the Calvin cycle. (Ans: carboxylation (RuBP + CO₂ → 2(3-PGA)), reduction (PGA → G3P using ATP & NADPH), regeneration (RuBP)
3. 3
   What is photorespiration and why is it called a wasteful process? (Ans: RuBisCO fixes O₂ instead of CO₂, consuming ATP and releasing CO₂ without producing sugar)

5 Mark Questions

1. 1
   Describe the C4 pathway (Hatch–Slack) in detail, explaining the role of mesophyll and bundle-sheath cells and why it avoids photorespiration. (Ans: PEP carboxylase fixes CO₂ to OAA in mesophyll; pyruvate/CO₂ release in bundle sheath where Calvin cycle runs)
2. 2
   Describe the molecular structure of chlorophyll and the absorption spectrum of photosynthesis, explaining why leaves appear green and how complementary pigments help. (Ans: porphyrin ring with Mg²⁺ and phytol tail; absorption peaks in blue and red, green reflected)