NCERT Solutions for Class 11 Biology Chapter 11: Photosynthesis in Higher Plants

Concept — what decides C₃ or C₄. The difference between C₃ and C₄ plants lies in leaf anatomy and the first stable product of CO₂ fixation (PGA, 3 carbons, or OAA, 4 carbons) — features that are inside the leaf, not on its outer surface. Both kinds have green leaves, stomata, veins and similar outer form.

• Answer: Not reliably. By external appearance alone one cannot say with certainty whether a plant is C₃ or C₄, because the distinguishing structures (Kranz anatomy, the arrangement of bundle sheath and mesophyll cells) are internal and the biochemical pathway cannot be seen.
• Why: both types carry out the Calvin cycle and have normal green leaves; the C₄ pathway is an additional CO₂-fixing step in the mesophyll with the Calvin cycle confined to the bundle sheath.
• External clues (only suggestive, not proof): C₄ plants are adapted to dry tropical regions, tolerate higher temperatures and high light, and have high productivity of biomass (e.g., maize, sorghum, sugarcane); a vigorous, fast-growing sun-loving plant of a hot dry habitat is more likely to be C₄, while many crops of temperate climates are C₃.
• How to confirm: cut a thin vertical section of the leaf and look under a microscope for Kranz anatomy, or study the first product of fixation (OAA vs PGA) and the response to temperature, light and CO₂.

Extra depth. NCERT section 11.8 itself says that the presence of the bundle sheath around vascular bundles, seen in leaf sections, helps identify C₄ plants. This is why the textbook asks you to cut sections of maize or sorghum leaves and collect leaves of diverse species. Habitat clues can mislead, since some plants of dry habitats use other CO₂-fixation strategies and many C₃ plants also thrive in sun.

Conclusion: a plant cannot be reliably classed as C₃ or C₄ from external features; the decisive evidence is internal — Kranz leaf anatomy and the first CO₂-fixation product.

Concept — Kranz anatomy. The internal structure that identifies a C₄ plant is the leaf anatomy around the vascular bundles. In C₄ plants the vascular bundles are surrounded by particularly large bundle sheath cells arranged as a wreath; this is called ‘Kranz’ anatomy (Kranz = wreath).

• C₄ leaf: bundle sheath cells form one or more layers around the vascular bundles; they are large, have a large number of chloroplasts, thick walls impervious to gaseous exchange and no intercellular spaces. Mesophyll cells surround the bundle sheath.
• Division of labour: mesophyll cells fix CO₂ into OAA (PEPcase); the C₄ acid is carried to the bundle sheath, which releases CO₂ and runs the Calvin cycle (RuBisCO).
• C₃ leaf: no Kranz anatomy — the bundle sheath is poorly developed or absent; CO₂ is fixed and the Calvin cycle runs in the mesophyll cells (palisade and spongy), which are alike in function.
• How to see it: cut a thin vertical section of the leaf (as suggested for maize or sorghum), stain if needed and observe under a microscope; the presence of the large bundle sheath around the vascular bundles identifies C₄ plants.

Extra depth. The wall of bundle sheath cells is impervious to gases, so CO₂ released inside stays concentrated near RuBisCO — the structural basis for the absence of photorespiration in C₄ plants. Kranz anatomy is therefore an anatomical adaptation that supports the biochemical C₄ cycle.

Conclusion: the Kranz anatomy — large chloroplast-rich bundle sheath cells forming a wreath around the vascular bundles — marks a C₄ plant; its absence indicates a C₃ plant.

Concept — a CO₂ concentrating mechanism. In a C₄ plant only the bundle sheath cells run the Calvin pathway, but these cells work under very favourable conditions because the C₄ cycle in the mesophyll acts as a CO₂ pump.

1. High CO₂ at the RuBisCO site: the C₄ acid from the mesophyll is broken down in the bundle sheath, releasing CO₂, which increases the intracellular CO₂ concentration around RuBisCO.
2. No photorespiration: because CO₂ is plentiful, RuBisCO works as a carboxylase and its oxygenase activity is minimised, so there is no wasteful release of fixed carbon and no ATP wasted in photorespiration.
3. Efficient first fixation: PEPcase of the mesophyll captures CO₂ very effectively (it does not bind O₂), and passes it on continuously.
4. Specialised anatomy: bundle sheath cells have many chloroplasts, thick gas-tight walls and no intercellular spaces, so CO₂ is held inside them.
5. Tolerance of high temperature and high light: C₄ plants show higher rates at higher temperatures and light intensities (the temperature optimum is about 30–40°C), so they keep photosynthesising when C₃ plants slow down.
6. Result: greater productivity of biomass and better yields.

Extra depth. The C₄ pathway costs extra ATP (to regenerate PEP) but the saving from avoiding photorespiration is larger under hot, bright conditions. The few bundle sheath cells are therefore not a limitation: they are like small, well-supplied factories fed by the mesophyll.

Conclusion: C₄ plants are highly productive because the C₄ cycle pumps CO₂ into the bundle sheath cells, eliminating photorespiration and letting the Calvin cycle work at high efficiency even in hot, bright conditions.

Concept — competitive binding. RuBisCO’s active site can bind both CO₂ and O₂; the binding is competitive, and the relative concentration of CO₂ and O₂ determines which of the two binds. RuBisCO has a much greater affinity for CO₂ when the CO₂:O₂ ratio is nearly equal.

• In C₄ plants, C₄ acids from the mesophyll are broken down in the bundle sheath cells to release CO₂; this increases the intracellular concentration of CO₂ where RuBisCO is located (RuBisCO is present in the bundle sheath, not in the mesophyll).
• With a high CO₂ : O₂ ratio, CO₂ outcompetes O₂ for the active site, so RuBisCO functions as a carboxylase — RuBP + CO₂ → 2 PGA — and its oxygenase activity is minimised.
• In C₃ plants there is no such CO₂ pump; some O₂ binds RuBisCO, RuBP is converted to one phosphoglycerate and one phosphoglycolate, and photorespiration occurs (no sugar, no ATP, release of CO₂).
• Hence C₄ plants have no photorespiration, better productivity and higher temperature tolerance.

Extra depth. NCERT notes that RuBisCO is the most abundant enzyme in the world. Its dual nature means the same active site can be steered by gas concentration — which is exactly what the C₄ plant exploits. The function of photorespiration is still not known.

Conclusion: in C₄ plants the bundle sheath cells have a high CO₂ concentration (from decarboxylation of C₄ acids), so CO₂ outcompetes O₂ and RuBisCO acts predominantly as a carboxylase.

Concept — chlorophyll a is the key pigment. Of the four leaf pigments (chlorophyll a, chlorophyll b, xanthophylls, carotenoids) only chlorophyll a forms the reaction centre of both photosystems — P700 in PS I and P680 in PS II are special chlorophyll a molecules. All other pigments are accessory pigments of the light-harvesting antenna.

• Answer: No — a plant with chlorophyll b but no chlorophyll a could not carry out photosynthesis. Chlorophyll b and the other accessory pigments can absorb light but can only transfer the energy to chlorophyll a; without a chlorophyll a reaction centre there is no excitation and ejection of electrons, no electron transport, no ATP and NADPH.
• Evidence from spectra: the absorption peaks of chlorophyll a in the blue and red regions match the peaks of the photosynthesis action spectrum — chlorophyll a is the chief pigment.
• Why then chlorophyll b and other accessory pigments? (i) They absorb light at other wavelengths and pass the energy to chlorophyll a, so a wider range of the incoming light is used for photosynthesis; (ii) they make photosynthesis more efficient; (iii) they protect chlorophyll a from photo-oxidation.
• This explains why the action spectrum does not overlap one-to-one with the absorption spectrum of chlorophyll a — some photosynthesis takes place at other wavelengths too.

Extra depth. Each photosystem has all pigments (except one chlorophyll a molecule) forming the antenna; the one remaining chlorophyll a molecule is the reaction centre, which differs in PS I (700 nm) and PS II (680 nm). A plant lacking chlorophyll a would lack both reaction centres, so neither photosystem could work.

Conclusion: such a plant could not photosynthesise because chlorophyll a forms the reaction centres; chlorophyll b and the other accessory pigments are present to widen the range of light absorbed and to protect chlorophyll a from photo-oxidation.

Concept — green and yellow pigments. The green colour of a leaf is due to chlorophyll a (blue-green) and chlorophyll b (yellow-green), which are masked in the green leaf; the yellow/orange colours come from xanthophylls (yellow) and carotenoids (yellow to yellow-orange). A leaf kept in the dark loses chlorophyll and the yellow pigments become visible.

• In the dark the green pigment is lost and not made: chlorophyll breaks down and its formation needs light, so the green colour fades. The leaf looks yellow or pale green.
• Why yellow? The xanthophylls and carotenoids (yellow, yellow-orange) present in the thylakoids do not disappear at the same rate and become visible when chlorophyll is lost.
• Which pigment is more stable? The carotenoids/xanthophylls (yellow pigments) are more stable than chlorophyll — that is why the yellow colour remains. NCERT also notes that accessory pigments protect chlorophyll a from photo-oxidation, i.e., chlorophyll is the vulnerable pigment.
• The same change is seen in ageing leaves and in leaves lacking light for long (etiolation), when chlorophyll is lost first.

Extra depth. Chlorophyll synthesis in many plants needs light; in its absence the colour of the leaf is mainly that of the remaining yellow pigments. That chlorophyll is easily damaged is also seen in strong light — NCERT says that increase of incident light beyond a point causes the breakdown of chlorophyll. The statement about the greater stability of carotenoids is the standard textbook inference answer to this question.

Conclusion: the leaf turns yellow or pale green in the dark because chlorophyll is lost, unmasking the xanthophylls and carotenoids, which are the more stable pigments.

Concept — leaf colour and light. This is an observation question. The depth of green depends on the amount of chlorophyll (and the number of chloroplasts) per unit leaf area, and the amount of chlorophyll responds to the light available. Light is rarely limiting for most plants, but in shade it is.

• Observation (expected): leaves on the shady side (or potted plants in the shade) are darker green; leaves in strong sunlight are lighter green.
• Reason 1 — more chlorophyll to capture scarce light: in shade, light intensity is low and light may be the limiting factor (Fig. 11.10, region A); the plant makes more chlorophyll per leaf area, which absorbs more of the available light and raises the rate of photosynthesis.
• Reason 2 — strong light breaks down chlorophyll: NCERT states that increase in incident light beyond a point causes breakdown of chlorophyll and a decrease in photosynthesis, so sun leaves tend to carry less chlorophyll and look lighter.
• Reason 3 — light saturation: light saturates at about 10 per cent of full sunlight, so sun leaves do not need extra pigment; they are limited by CO₂ or temperature instead.

Extra depth. Compare leaves of the same plant growing on the shady and sunny sides, or potted plants kept in sun and shade for some days, and record the colour; a chlorophyll extract in acetone could be compared by eye for quantitative support. The explanation above rests on the NCERT statements about light as a limiting factor and about chlorophyll breakdown in excess light; other adaptive changes of shade leaves are beyond the textbook.

Conclusion: the leaves (or plants) in the shade are darker green, as they have more chlorophyll to capture the limited light, whereas strong sunlight keeps chlorophyll content lower by breaking it down.

Concept — reading the light curve. The graph shows the rate of photosynthesis against light intensity: a rising straight-line part (A), a bend (B) and a flat part (C). The dotted line from the bend to the x-axis is D, and the dotted line to the y-axis is E (maximum rate). At low light intensities there is a linear relationship between incident light and CO₂ fixation; at higher intensities the rate stops rising as other factors become limiting.

1. (a) Light is the limiting factor at A (the rising linear part, where the rate increases in proportion to light intensity). Beyond B the curve flattens, so light is no longer limiting in the region C.
2. (b) In region A the limiting factor is light intensity — the plant has enough CO₂, water and optimal temperature but not enough light; hence, if light is increased the rate increases. Other factors are present at non-limiting levels in this region (by Blackman’s Law the factor nearest its minimal value controls the rate).
3. (c) C represents the plateau, where light saturation has been reached: increase in light intensity gives no further rise because other factors (CO₂ concentration, temperature, water, enzymes) become limiting. D represents the light intensity at which saturation occurs (about 10 per cent of full sunlight, NCERT), with E the maximum rate.

Extra depth. Since saturation occurs at only about 10 per cent of full sunlight, light is rarely limiting in nature except for plants in shade or in dense forests; increase of light beyond a point causes breakdown of chlorophyll and a decrease in photosynthesis (not shown in this curve). The reading of C and D follows the graph’s marking: C is the stretch of the curve to the right of the bend and D is the dotted line dropped from the bend to the x-axis.

Conclusion: light limits photosynthesis in region A; the plateau C marks light saturation at the intensity D, beyond which CO₂, temperature or other factors limit the rate.

Concept — three comparisons. (a) compares the two CO₂-fixation pathways, (b) the two modes of ATP synthesis in the light reaction, and (c) the leaf structure that supports them.

(a) C₃ pathway versus C₄ pathway

(b) Cyclic versus non-cyclic photophosphorylation

(c) Leaf anatomy of C₃ and C₄ plants

Extra depth. NCERT notes that C₄ plants use the Calvin cycle as the main biosynthetic pathway, so the two pathways are not alternatives but C₄ is an add-on step; likewise cyclic photophosphorylation probably meets the extra ATP demand of the Calvin cycle (3 ATP versus 2 NADPH per CO₂).

Conclusion: C₃ plants fix CO₂ into PGA in mesophyll cells and show photorespiration, C₄ plants fix it into OAA and run the Calvin cycle in Kranz bundle sheaths; non-cyclic photophosphorylation gives ATP, NADPH and O₂ whereas cyclic gives only ATP.