Class 11 Biology - Photosynthesis in Higher Plants

Exercises

1. By looking at a plant externally, can you tell whether a plant is C3 or C4? Why and how?

Answer: No, it is usually not possible to distinguish between C3 and C4 plants by external appearance alone. C4 plants are generally adapted to dry, tropical regions with high light intensities and temperatures (e.g., maize, sorghum, sugarcane), but these external environmental conditions are not exclusive indicators. The true distinction lies in the internal leaf anatomy.

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2. By looking at which internal structure of a plant you can tell whether a plant is C3 or C4? Explain.

Answer: By observing the leaf anatomy (specifically the presence of Kranz anatomy), we can distinguish between the two.

• C4 Plants: Have large, thick-walled bundle sheath cells arranged in a "wreath" like manner around the vascular bundles. These cells contain a large number of chloroplasts and lack intercellular spaces.
• C3 Plants: Lack specialized bundle sheath cells with chloroplasts. Photosynthesis occurs primarily in the mesophyll cells.

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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?

Answer: C4 plants are highly productive because they have a mechanism to minimize photorespiration.

1. CO₂ is first fixed in the mesophyll cells into a C4 acid (OAA) by PEPcase, which has a high affinity for CO₂.
2. The C4 acid is transported to the bundle sheath cells and broken down to release a high concentration of CO₂.
3. This ensures that the enzyme RuBisCO functions primarily as a carboxylase, drastically reducing the wasteful oxygenase activity (photorespiration). This allows C4 plants to photosynthesize efficiently even under high light and temperature.

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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?

Answer: In C4 plants, RuBisCO is restricted to the bundle sheath cells. The Hatch and Slack pathway acts as a CO₂ pump, continuously transporting and releasing CO₂ into these cells. This creates a high intracellular concentration of CO₂ around RuBisCO, ensuring it binds with CO₂ rather than O₂, thus favoring carboxylation over oxygenation.

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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?

Answer: No, photosynthesis would not occur effectively. Chlorophyll a is the reaction centre pigment where the actual photochemical reaction (electron excitation) takes place. Role of accessory pigments (Chl b, xanthophylls, carotenoids):

1. They absorb light at different wavelengths and transfer the trapped energy to chlorophyll a, making photosynthesis more efficient.
2. They protect chlorophyll a from photo-oxidation (damage by excessive light).

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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?

Answer: Leaves turn yellow or pale green (etiolation) in the dark because chlorophyll is unstable and requires light for its synthesis. In the absence of light, chlorophyll degrades, revealing the underlying carotenoids and xanthophylls, which are more stable and give the yellow/orange color.

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7. Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Which of them has leaves that are darker green? Why?

Answer: Leaves on the shady side (or plants in the shade) are often darker green. This is because shade plants produce more chlorophyll (especially Chl b) and have larger, thinner leaves with more chloroplasts per cell to maximize the capture of limited light available for photosynthesis.

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8. Figure 11.10 shows the effect of light on the rate of photosynthesis. Answer the following:

(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?

Answer: (a) Point A and the beginning of region B. (b) Light intensity is the limiting factor in region A (the rate increases linearly with light). (c) C represents the light saturation point (beyond which light is no longer limiting). D represents the maximum rate of photosynthesis achieved under those specific conditions.

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9. Give comparison between the following: (b) Cyclic and non-cyclic photophosphorylation

Answer:

• Non-cyclic: Involves both PS II and PS I. It involves the splitting of water and results in the production of both ATP and NADPH.
• Cyclic: Involves only PS I. Electrons are cycled back to PS I. It results only in the production of ATP; no water is split and no NADPH is formed. It occurs primarily in the stroma lamellae.