Photosynthesis in Higher Plants Class 11 Biology NCERT Solutions

Step-by-step solution

1. 1The answer is that an external character does exist, and the character is the leaf, and it is the pattern of the veins that matters. So identify the character, say what to look for, and then explain the physiological reason, since the reason is where the marks are.
2. 2The external character is the venation of the leaf, and the way to look is first to establish that the plant is a dicot, which is checked by the broad leaf with reticulate venation and by the flower with four or five parts or a multiple of four or five, since the grasses and the sedges, which are the principal monocots, are excluded by this. Then the leaf of a C3 dicot shows a reticulate or net-like venation, in which the midrib gives off a number of branches which themselves branch again to form a network, whereas the leaf of a C4 dicot shows the parallel venation of the monocot type, in which the veins run side by side from the base to the tip of the leaf. So the simple statement is that a broad leaf with a reticulate or net-like venation is a C3 plant, and a broad leaf with a parallel venation is a C4 plant, and both are dicots, which is why the venation alone is a trap and the broadness of the leaf is the confirmation.
3. 3The reason the vein pattern betrays the pathway, and this is the part to write. In a C4 plant the vascular bundles are not scattered but are arranged in a ring, and they are of two kinds, the larger ones which are the vascular bundles proper, surrounded by a bundle sheath of large and prominent cells, and the smaller ones lying between them. The leaves of the larger bundles are the midrib and the veins, and the veins of a C4 leaf run parallel because the bundles are arranged in a ring close together and are all directed along the length of the leaf, so the reticulate network of a C3 leaf is not formed. In a C3 plant, by contrast, the vascular bundles in the leaf are not surrounded by a prominent bundle sheath of large cells but by a smaller and less conspicuous one, and the smaller veins of the leaf end freely within the lamina, that is they are not connected to one another, and this open and freely ending venation is what produces the reticulate or the net-like pattern.
4. 4The confirmation, since the venation alone would mislead, is to look at the leaf surface for the Kranz anatomy, which is visible in a leaf cleared and examined under the microscope but can often be suspected in a hand section, or more simply to look at the stem. And the physiology to state is the reason for the whole arrangement: the C4 pathway is a mechanism for concentrating the CO2 at the RuBisCO, which requires the two compartments, since the fixation by PEP carboxylase occurs in the mesophyll and the Calvin cycle in the bundle sheath, and this two-compartment arrangement with the mesophyll cells arranged radially around the bundle sheath, the Kranz anatomy, is the visible expression of that biochemical requirement. So a plant showing a broad leaf with parallel venation and a stem with the bundles in a ring surrounded by a prominent bundle sheath is a C4 plant, and a plant showing a broad leaf with reticulate venation is a C3 plant.

Step-by-step solution

1. 1The internal structure is the leaf, in transverse section, and the character is the Kranz anatomy, that is the arrangement of the mesophyll and the bundle sheath. Give the anatomy of the C4 leaf first, since it is the decisive one, then the C3 leaf for contrast, and then the reason.
2. 2The internal structure to examine is the leaf in transverse section, and the decisive character is the Kranz anatomy, the German word for a wreath, which is the ring of radially arranged mesophyll cells around the bundle sheath. In a C4 leaf the vascular bundle is of the large kind and is surrounded by two sheaths, an outer of thin-walled and 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 here is not differentiated into the palisade and the spongy layers of a C3 leaf, since the mesophyll cells of a C4 leaf are all of one kind, arranged radially, and the chloroplasts of the mesophyll cells are smaller, grana are absent or few, and are of a different type from those of the bundle sheath. The two compartments are therefore unmistakable in the section, and this is the C4 leaf.
3. 3The C3 leaf for contrast. In a C3 leaf the vascular bundle has a single bundle sheath of a single layer of relatively small, thin-walled cells containing few chloroplasts, and there is no ring of radially arranged mesophyll around it, so the mesophyll is fully differentiated into the palisade parenchyma beneath the upper epidermis and the spongy parenchyma above the lower epidermis, and the bundle sheath is not prominent. So the absence of the Kranz anatomy and the presence of a palisade and a spongy mesophyll identify the C3 leaf, and this is exactly the standard of a dorsiventral leaf of a dicot, whereas a C4 leaf is nearly isobilateral and its mesophyll is not layered.
4. 4The explanation, which is the last and the best part of the answer. The anatomy is the visible counterpart of the biochemistry, and it exists for one reason, which is to raise the concentration of the carbon dioxide around the RuBisCO and so to suppress the oxygenase activity of that enzyme and the photorespiration that follows from it. The first fixation, by the PEP carboxylase, occurs in the mesophyll cell, which is exposed to the air and which has no oxygenase enzyme, so the carbon is fixed there into the four-carbon oxaloacetic acid, which is converted to malate or aspartate. These four-carbon acids are then transported across to the bundle sheath cell, which is the second compartment, and in it the four-carbon acid is decarboxylated, so that the carbon dioxide is released locally, raising the concentration of the CO2 around the RuBisCO to a value five to ten times the atmospheric one. The RuBisCO is confined to this compartment, and the chloroplasts of the bundle sheath are large and rich in grana and have no PEP carboxylase, and the bundle sheath is also relatively impervious to the gases, so the oxygen cannot readily reach the enzyme. It is the carboxylase activity that is therefore favoured, the oxygenase activity is suppressed, the photorespiration is virtually absent, and the plant is consequently far more productive, at the cost of the extra ATP that the C4 pathway consumes in regenerating the PEP, which is a cost that is more than repaid in the high light and the high temperature of the conditions in which the C4 plants grow.

Step-by-step solution

1. 1The apparent paradox is resolved by the fact that the productivity depends not on the number of cells doing the job but on the rate and on the absence of the loss, so the answer should be built in three parts: the mechanism of the concentration, the two compensating advantages, and the balance of the extra cost against the avoided loss.
2. 2State the paradox and resolve it arithmetically first, since this is the heart of the question. The Calvin cycle is indeed carried out in a very small proportion of the cells of a C4 plant, in NCERT's example about 20 to 30 per cent, or in some species only 5 to 10 per cent of the leaf cells, the bundle sheath, and yet the plant fixes carbon at a rate several times that of a C3 plant. So the productivity per bundle sheath cell must be several times the productivity per cell of a C3 plant, and the reason is not that the enzyme is faster but that the loss has been abolished. Every molecule of CO2 fixed by the Calvin cycle of a C3 plant is accompanied by a molecule of CO2 lost again in the photorespiration, so a C3 plant must fix several molecules of CO2 for every sugar molecule it keeps, while the C4 plant keeps essentially all that it fixes.
3. 3Then the two mechanisms, and both must be given. The first is the carbon-concentrating mechanism, already described: the PEP carboxylase in the mesophyll fixes the carbon into a four-carbon acid, and the four-carbon acid is carried to the bundle sheath and decarboxylated there, so that the bundle sheath cell receives a carbon dioxide concentration of five to ten times the atmospheric one. Since the rate of the carboxylation by the RuBisCO depends on the ratio of the concentration of the carbon dioxide to that of the oxygen, a high local carbon dioxide makes the carboxylation greatly outrun the oxygenation, and the rate of photosynthesis rises with the carbon dioxide. The second mechanism is the elimination of the oxygenase activity, which is a consequence of the first but is worth stating separately, since the RuBisCO is present in the bundle sheath, where the four-carbon acid is decarboxylated and where the oxygen concentration is low because the mesophyll's photosynthetic activity has exhausted the intercellular spaces for oxygen. So the photorespiratory loss of about 25 to 30 per cent of the fixed carbon in a C3 plant is reduced to almost nothing in a C4 plant.
4. 4Then the cost, because an honest answer must state what the plant pays, and the examiner will look for it. The C4 pathway spends five ATP for every molecule of CO2 fixed, in addition to the three ATP that the Calvin cycle itself requires, so the C4 plant requires about 40 to 50 per cent more ATP than a C3 plant for the same amount of carbon fixed. The extra ATP is required for the regeneration of the PEP, the pyruvate phosphate dikinase reaction being the step that regenerates the PEP from the pyruvate and uses ATP, and the mesophyll also has to carry out the light reactions with the PSI largely cyclic, and the transport of the malate requires energy. The C4 plant is therefore only advantageous where light is intense and the temperature is high, since there the photorespiration in a C3 plant is at its worst and the extra ATP is available in abundance, whereas in moderate light and moderate temperature the photorespiration in a C3 plant is small and the C4 plant is merely wasting ATP, and this is why the C4 plants are the weeds of the hot and bright habitats and why the C3 plants dominate in the cool and the temperate ones.
5. 5Close with the productivity point that answers the question directly. The productivity of the plant is a whole-plant property and is not limited by the number of the cells doing the photosynthesis, since a plant with few photosynthetic cells and a high rate in each of them is exactly as productive as, or more productive than, a plant with many cells working slowly, and the C4 plant compensates for the smaller number of the bundle sheath cells by the very high rate in each of them and by the absence of the photorespiratory loss. So the few cells are not a disadvantage but the condition of the advantage, and it is the concentration of the carbon dioxide that makes each of those few cells highly productive, so that the carbon that is fixed in the mesophyll is not lost but is delivered to the few cells and fixed there almost without waste.

Step-by-step solution

1. 1The question asks for a single reason, and the reason is the high concentration of the carbon dioxide around the enzyme, but the answer must give the reason for the high concentration as well, and then the competitive basis of the choice. Give the two activities, the competition, the mechanism of the concentration, and the outcome.
2. 2First state what the two activities are and that they compete. The RuBisCO is a bifunctional enzyme: as a carboxylase it catalyses the addition of the carbon dioxide to the ribulose 1,5-bisphosphate, the first step of the Calvin cycle, giving the two molecules of 3-phosphoglycerate; as an oxygenase it catalyses the addition of the oxygen to the same substrate, the ribulose bisphosphate, giving one molecule of 3-phosphoglycerate and one molecule of phosphoglycolate, and the phosphoglycolate is then recycled through the photorespiratory pathway, which yields no sugar, consumes ATP and releases carbon dioxide. Since both reactions use the same substrate, the ribulose bisphosphate, and the same enzyme, they are in competition, and the balance between them is decided by the relative concentrations of the two gases at the enzyme.
3. 3Then the reason: the concentration of the carbon dioxide at the RuBisCO in a C4 plant is five to ten times the atmospheric value. This is because the C4 pathway concentrates the carbon dioxide. The first fixation, in the mesophyll cell, is made not by the RuBisCO but by the PEP carboxylase, which has no oxygenase activity at all and therefore fixes the carbon without any competition from the oxygen, into the oxaloacetic acid and thence into the malate or aspartate. These four-carbon acids are then transported into the bundle sheath cell, which is the compartment in which the RuBisCO is confined, and there they are decarboxylated, so that the carbon dioxide is released exactly where the RuBisCO is and its local concentration rises several-fold above the atmospheric figure. A second contribution to the same end is that the oxygen itself is scarce in the bundle sheath, since the photosynthetic activity of the mesophyll and of the sheath has consumed the intercellular oxygen and the bundle sheath wall is relatively impervious, so the ratio of carbon dioxide to oxygen, which is what actually decides the competition, is raised still further.
4. 4Then the outcome, and the quantitative comparison is what the examiner wants. The competitive carboxylation and oxygenation reactions of the RuBisCO depend on the ratio of the concentrations of the two gases, so a five to tenfold raise in the carbon dioxide at the enzyme shifts the balance heavily in favour of the carboxylation, and the rate of the carboxylation rises with the concentration of the carbon dioxide, as all such rates do. It is estimated that the oxygenation accounts for a substantial fraction of the enzyme's activity in a C3 plant exposed to the present atmospheric concentration, and that it is largely or almost completely switched off in a C4 plant and in a C3 plant grown under a high concentration of carbon dioxide. So the RuBisCO carries out more carboxylation in a C4 plant because, and this is the point of the question, the C4 pathway has raised the carbon dioxide at the enzyme, and it does so at the cost of extra ATP, so the enzyme is given a far more favourable ratio of carbon dioxide to oxygen than a C3 plant ever has.

Step-by-step solution

1. 1Answer in two parts, the first being a definite no with its reason, and the second the function of the accessory pigments. The two are joined by the fact that the reaction centre needs chlorophyll a, so the second part explains the first.
2. 2Part one, the answer is no. The reason is that chlorophyll a is the only pigment of the plant that can carry out the photochemical reaction of the reaction centre, since it is the chlorophyll a of the reaction centre of photosystem I and of photosystem II that transfers the absorbed energy to the reaction centre and is thereby converted into the primary electron acceptor, and no other pigment can do this. So a plant with a high concentration of chlorophyll b but no chlorophyll a would be unable to transfer the energy of the absorbed light 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, so there would be no photolysis of water, no evolution of oxygen, no reduction of NADP plus, no photophosphorylation and no synthesis of ATP, and consequently no fixation of carbon in the Calvin cycle. It would be able to absorb the light, because chlorophyll b does absorb light, and it would therefore look green, but it would be unable to use the energy, so it would not carry out photosynthesis, and this is the crisp statement to write, that the plant would absorb the light but could not convert it into chemical energy.
3. 3Part two, the reason plants have chlorophyll b and the other accessory pigments. The reason is that the accessory pigments broaden the range of the light that the plant can absorb, and this is worth quantifying. The chlorophyll a absorbs most efficiently in the blue-violet region at about 430 nanometres and in the red region at about 662 nanometres, and it absorbs the light of the green and of the yellow wavelengths only poorly, and the chlorophyll b absorbs mainly in the blue and the blue-green region at about 453 nanometres, so between the two the plant collects 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 reason is the indirect one, and it is the more important physiologically. The chlorophyll b passes the energy it has absorbed to the chlorophyll a of the reaction centre, and it does so by resonance energy transfer, that is by the excitation of the pigment and the transfer of the excitation to the neighbouring pigment, and not by an electron transfer; and this transfer is very efficient, close to a hundred per cent, so that the accessory pigment is acting simply as a light-collecting antenna, funnelling the energy it has captured to the one pigment that can use it. The third reason is the protective one, the accessory pigments protecting the chlorophyll a from the photo-oxidation, that is from the excess light, the chlorophyll b and the carotenoids absorbing the excess energy and dissipating it as heat, so that the reaction centre chlorophyll is not destroyed, and this is the reason that a plant which has been given too much light and has been unable to use the energy, as a leaf in bright sun, turns yellow and bleaches, the chlorophyll being photo-oxidised and the accessory pigments remaining.
4. 4Then name the other accessory pigments and say what each does, and the names are examinable. The carotenoids, the carotenes and the xanthophylls, are the yellow and the orange pigments, and the xanthophylls, the lutein, the zeaxanthin and the violaxanthin, and the carotenes, the beta-carotene and the lycopene, are the accessory pigments, and they absorb the violet and the blue and the blue-green light that the chlorophyll absorbs only poorly, and they pass the energy to the chlorophyll b or to the chlorophyll a, and they are also the protectants against the photo-oxidation, and the carotenes are the source of the autumn colour of the leaves when the chlorophyll is degraded and the carotenes are left exposed. So the whole family of accessory pigments exists to broaden the absorption and to protect the reaction centre, while the chlorophyll a of the reaction centre does the actual photochemistry, and it is this division of labour that the plant has evolved, with the accessory pigment doing the collecting and the chlorophyll a doing the work.

Step-by-step solution

1. 1This is a chlorophyll-destruction question, so give the cause, the sequence of the colour change, and the answer on the stability, and the last part is the crispest single mark in the question.
2. 2The cause is the loss of chlorophyll in the absence of light, and the mechanism is that the chlorophyll is a pigment of the chloroplast and that the maintenance of the chlorophyll in the leaf depends on its continued synthesis, which requires light. Since the synthesis of the chlorophyll is itself a light-dependent process, and since a leaf kept in the dark is still respiring and is still consuming the products of the photosynthesis, the chlorophyll that is present is gradually used up and is not replaced, and the leaf therefore loses its green colour. The chlorophyll is a labile and unstable pigment, and it is the chlorophyll that is destroyed first, while the other pigments, the carotenoids, are comparatively stable, so as the green colour is lost the yellow and the orange of the carotenoids, which were always present but masked by the much more abundant chlorophyll, are left exposed, and this is the yellow or the pale green colour that is seen.
3. 3Then the sequence to describe, since the question says the colour becomes yellow or pale green, and the order of the colours is informative. A leaf kept in the dark first becomes pale green, because the chlorophyll is being diluted as the yellow carotenoids become relatively more prominent, and then it becomes yellow as the last of the chlorophyll goes and the carotenoids alone remain, and it finally becomes brown, as the carotenoids too are degraded. And it should be noted that the same sequence, in the same order, is the one seen in the autumn leaf and in the yellowing leaf of a detached branch, so the darkening experiment and the autumn colouring are the same process.
4. 4The pigment that is more stable is the carotenoid, and this is the answer, and the reason is what is examinable. The chlorophyll is unstable because it is a porphyrin with a phytol side chain attached to a magnesium ion in the centre of the porphyrin ring, and it is photolabile, being readily degraded in the light and in the dark to the colourless phytol and to the porphyrin, and it is bleached by the light, so it is destroyed in the absence of light even without any photo-oxidation. The carotenoids, that is the carotenes and the xanthophylls, are comparatively stable, and it is this greater stability which is the reason that the yellow and the orange of a leaf are the pigments that remain. The chlorophyll a and the chlorophyll b differ also in their stability, and the chlorophyll b is the more stable of the two, so a leaf that has lost both but retains some chlorophyll b would be pale green rather than yellow, and it is this that accounts for the pale green stage that precedes the yellow. The other point to add, which the question invites, is that the pigments in the leaf are held in the thylakoid membrane of the chloroplast, and that in the dark the whole chloroplast is degraded, the thylakoid being dismantled, so that the pigments are freed and destroyed, and that this is why the green colour returns in a yellowing leaf that is put back in the light, since the chlorophyll is resynthesised in the light and the carotenoids, being still there, need not be made.

Step-by-step solution

1. 1The answer is the sun leaves, and the reason is the response of the chlorophyll synthesis to light, so the answer must run from the observation to the explanation and then to the functional consequence.
2. 2The darker green leaves are those of the plant kept in the sunlight, that is the sun leaves, and in the first case the leaves on the sunny side of the same plant are darker green than those on the shady side, and in the second case the leaves of the potted plant kept in the sun are darker green than those of the potted plant kept in the shade. The colour is green because the leaf contains the chlorophyll, and the depth of the green is a measure of the amount of the chlorophyll present in the leaf, so the question reduces to why the sun leaves contain more chlorophyll than the shade leaves.
3. 3The reason is that the synthesis of the chlorophyll is light-dependent, and the light intensity in the sun is much greater than in the shade, so a leaf in the bright light is able to synthesise more chlorophyll than a leaf in the dim light. The shade leaf therefore contains less chlorophyll and its leaves are thinner, since the shade plant has less light to capture and so invests less in the light-capturing tissue and more in the growth, while the sun leaf is thicker and more heavily pigmented, so that the sun leaf and the shade leaf of the same species are recognisable by the colour and by the thickness. And it is worth noting that this is a genuine acclimation rather than a genetic difference, since a plant moved from one condition to the other will adjust, and a plant that is grown first in the shade and then moved to the light will become darker green and thicker, while a plant moved from the light into the shade will become paler and thinner and the excess chlorophyll will be lost, since the plant regulates the pigment to the light it actually receives.
4. 4Then the functional consequence, and this is what makes the answer complete. A leaf with more chlorophyll absorbs more of the incident light, so the sun leaf, with its high chlorophyll content, is adapted to the strong light and is able to use more of the light that falls on it, whereas the shade leaf, with its low chlorophyll content, is adapted to the dim light of the canopy floor, for a high chlorophyll content would be a disadvantage there, since the excess pigment would absorb light that could not be used and the energy would have to be dissipated, and it would also cost the plant more nitrogen to make it. So the sun and the shade leaves of the same plant represent 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.

Step-by-step solution

1. 1Identify the regions of the graph before answering the three parts, since every part depends on knowing the shape. The graph of the rate of photosynthesis against the light intensity is a curve that rises steeply, then less steeply, and then flattens, and the whole of the three parts is about where the light is still limiting and where it has ceased to be.
2. 2Identify the regions. The curve begins at the origin, that is at zero light intensity the rate is zero, and it rises very steeply, almost as a straight line, in the first region, the low light region, and the gradient of the curve here is greatest, so that the rate of photosynthesis is directly proportional to the light intensity and every additional unit of light produces a corresponding increase in the rate. This first region is where the light is the limiting factor. The curve then bends and rises more and more slowly, the second region, the light saturation region, in which the rate continues to increase but more slowly than the light, so that the light is no longer the sole limiting factor and some other factor has begun to hold the rate down. Finally the curve becomes horizontal, the third region, the plateau, in which the rate of photosynthesis is at its maximum and is completely independent of the light, so that the light is no longer a limiting factor at all and the limit lies elsewhere.
3. 3Part (a). Light is a limiting factor in the region of the steeply rising part of the curve, which is the low light region before the point at which the curve begins to bend, that is in region A and in the region up to the point B where the curve begins to flatten, and the answer to be given is that light is the limiting factor in the region A and in the region between A and B, and it is not a limiting factor in the region of the plateau beyond the point C, since there the rate is at its maximum and unchanging with the light. If the graph is marked so that A, B and C are three points on the rising part and on the curve, then light is the limiting factor up to the point B and is not a limiting factor beyond the point C, and at the point B itself it is becoming only partly limiting.
4. 4Part (b), the limiting factors in region A, which is the low light region. In region A, at the very lowest light intensities, the limiting factors are the availability of light itself and the capacity of the light-harvesting apparatus, so the rate is low because there is little light to harvest and the antenna is not yet working at anything like its full capacity; the light is by definition the limiting factor here, and the chloroplasts are far from being saturated with light. It should also be noted that the other metabolic factors, that is the temperature, the availability of the carbon dioxide and the enzyme supply, are in excess at these low intensities, so they are not limiting, and the rate rises with the light simply because the reaction centres are being excited more frequently. And the practical point to add is that a plant growing at the very bottom of a dense canopy experiences precisely the conditions of region A, since the light there is only a small fraction of the full sunlight and photosynthesis is accordingly light-limited, which is why the density of a crop is a compromise between the shading of the lower leaves and the light of the upper ones.
5. 5Part (c), the points C and D. C represents the light saturation point, that is the light intensity at which the rate of photosynthesis has reached its maximum, the intensity above which a further increase in the light produces no increase in the rate, and at and beyond that point the light is no longer the limiting factor. D represents the maximum rate of photosynthesis, that is the plateau itself, the highest value of the rate that the plant can achieve, the point on the graph at which the curve has become horizontal, and D is therefore the maximum rate at the conditions of the experiment, the value being a function of the carbon dioxide supply, the temperature and the enzyme content of the leaf rather than of the light. So on the graph C is the saturation point and D is the maximum rate, and the value at D is the highest photosynthetic efficiency of that leaf under those conditions. It is also worth adding the closing observation that these three points are precisely what the flat part of the curve means for the farmer, since once the light saturation point C is reached, no amount of extra light will increase the crop, and the yield must then be raised by the carbon dioxide or by the temperature or by the water, and it is the water, by opening the stomata, that in practice limits the rate at D.

Step-by-step solution

1. 1The question is a three-part comparison and it should be answered as three tables, and the three are quite different in kind, so the headings should reflect that. Part (a) is a biochemical comparison, part (b) a photochemical one, and part (c) an anatomical one, and the examiner will expect the tables to be in that form.
2. 2Part (a), the C3 and the C4 pathways. The first point is the first acceptor of the CO2: in the C3 it is the ribulose bisphosphate, a five-carbon compound, and the first stable product is a three-carbon compound, the 3-phosphoglycerate, so the plant is called C3; in the C4 it is the phosphoenolpyruvate, a three-carbon compound, and the first stable product is a four-carbon compound, the oxaloacetic acid, so the plant is called C4. The second is the first fixing enzyme: in the C3 it is the RuBisCO, which is both a carboxylase and an oxygenase, whereas in the C4 the first fixation is by the PEP carboxylase, which has no oxygenase activity, the RuBisCO being used only later. The third is the number of cellular compartments: the C3 is a single cell type, the mesophyll, in which the whole cycle runs, whereas the C4 has two, the mesophyll and the bundle sheath, the two-cell arrangement. The fourth is the initial products: the C3 gives two molecules of the 3-phosphoglycerate, and the C4 gives the oxaloacetic acid, which is at once converted into the malate or the aspartate. The fifth is the CO2 concentrating mechanism: the C3 has none, so the RuBisCO works at the ambient carbon dioxide, whereas the C4 decarboxylates the four-carbon acid in the bundle sheath and so works at a carbon dioxide concentration of five to ten times the atmospheric one. The sixth is the photorespiration: it is present and substantial in the C3, losing about 25 to 30 per cent of the fixed carbon, and it is virtually absent in the C4. The seventh is the energy cost: the C3 needs 3 ATP and 2 NADPH per molecule of carbon dioxide, and the C4 needs 5 ATP and 2 NADPH, so the C4 spends about 40 to 50 per cent more ATP. The eighth is the net productivity and the habitat: the C3 is the more productive in the cool and the moderate light and temperature, and it is the plant of the temperate and the cool regions and of the greater part of the world's crop plants, while the C4 is the more productive in the high light and the high temperature and is the plant of the tropics and of the arid and the hot habitats, being the weeds of the hot and bright places. And the ninth is the number of the cells carrying the cycle: all the mesophyll cells in the C3, but only the bundle sheath cells in the C4.
3. 3Part (b), the cyclic and the non-cyclic photophosphorylation. The first point is the photosystem: the cyclic uses only the photosystem I, whereas the non-cyclic uses both the photosystem I and the photosystem II. The second is the flow of the electrons: in the cyclic, the electron that leaves the photosystem I returns to it by an alternative path, so there is no net gain or loss and no production of NADPH, whereas in the non-cyclic the electron leaves the photosystem I, goes down the chain to the ferredoxin and the NADP plus, reducing it to NADPH plus H plus, and the electron that is lost is replaced by the splitting of water at the photosystem II, so the flow is a flow and not a cycle. The third is the ATP: both synthesise ATP, but the cyclic synthesises additional ATP only and does so in quantity, without any NADPH, whereas the non-cyclic synthesises ATP along with NADPH, and the ATP to the NADPH ratio of the cyclic is very high. The fourth is the products: the cyclic produces ATP only, with no oxygen and no NADPH, whereas the non-cyclic produces ATP, NADPH and oxygen. The fifth is the water: the cyclic does not split water, so it consumes none and evolves no oxygen, whereas the non-cyclic splits it. The sixth is the antenna: the cyclic uses mainly the chlorophyll b and the PSI reaction centre, so the effective wavelength is the red and it is active even in the red light, whereas the non-cyclic uses both the chlorophyll a and b of both photosystems. And the seventh is the function: the cyclic serves to make ATP, and to generate the reducing power to build up the gradient for it, and it is the mechanism by which the plant can increase the ATP supply to match the extra demand of the Calvin cycle in a C4 plant, whereas the non-cyclic is the route by which the plant manufactures its NADPH and its oxygen.
4. 4Part (c), the anatomy of the leaf. The first point is the mesophyll: the C3 has it fully differentiated into a palisade parenchyma beneath the upper epidermis and a spongy parenchyma above the lower, whereas the C4 has it undifferentiated, the cells being all of one kind and arranged radially around the bundle sheath. The second is the bundle sheath: the C3 has one sheath of a single layer of relatively small, thin-walled cells with few chloroplasts, whereas the C4 has two sheaths, an outer of thin-walled parenchyma with chloroplasts and an inner of large, prominent, chloroplast-rich cells. The third is the Kranz anatomy: it is absent in the C3 and present in the C4, the mesophyll cells being arranged in a ring round the bundle sheath, which is the German word for wreath. The fourth is the arrangement of the vascular bundles: they are scattered in a C3 leaf and arranged in a ring in a C4 leaf, and in a C4 leaf they are of two kinds, large bundles proper with the prominent sheath and small bundles between them. The fifth is the chloroplasts: 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 and grana-rich and carry the RuBisCO. The sixth is the veins: the small veins end freely in the lamina in the C3, giving the reticulate venation, whereas in the C4 the veins run parallel, giving the parallel venation of a broad leaf. And the seventh is the symmetry: the C3 leaf is dorsiventral, with a different upper and lower surface, whereas the C4 leaf is nearly isobilateral, with the two surfaces more alike, since the mesophyll is not layered.