Q1. By looking at a plant externally, can you tell whether a plant is $C_3$ or $C_4$? Why and how?
Answer: No, you generally cannot tell whether a plant is $C_3$ or $C_4$ just by its external appearance. The main differences between $C_3$ and $C_4$ plants lie in their internal leaf anatomy (Kranz anatomy) and metabolic pathways, not in external features.
However, you can make an educated guess based on the plant’s habitat and adaptability. $C_4$ plants usually grow in dry, tropical regions, tolerate high temperatures, and thrive under intense sunlight, whereas $C_3$ plants are more common in cool, moist climates.

Q2. By looking at which internal structure of a plant can you tell whether a plant is $C_3$or $C_4$? Explain.
Answer: The internal structure used to distinguish $C_4$ plants from $C_3$ plants is their leaf anatomy, specifically the presence of Kranz anatomy.

• $C_4$ plants: Have Kranz anatomy, where large bundle sheath cells form a circular arrangement around the vascular bundles. These cells have many chloroplasts, thick walls, and no intercellular spaces, helping concentrate $\mathrm{CO_2}$.
• $C_3$ plants: Lack Kranz anatomy; their mesophyll cells are arranged uniformly around the vascular bundles without distinct bundle sheath layers.

Q3. Even though very few cells in a $C_4$ plant carry out the Calvin (biosynthetic) pathway, they are highly productive. Why?
Answer: $C_4$ plants are highly productive because they have a $\mathrm{C}{\mathrm{O}}_2$$\mathrm{CO_2}$​ concentrating mechanism that eliminates the wasteful process of photorespiration.
In $C_4$ plants, $\mathrm{CO_2}$ is first fixed in mesophyll cells into a $C_4$ acid, which is then transported to the bundle sheath cells. There, the acid is broken down to release a high concentration of $\mathrm{CO_2}$ around the enzyme RuBisCO.
This ensures that RuBisCO acts mainly as a carboxylase (not oxygenase), minimizing photorespiration and improving the efficiency of photosynthesis, leading to higher productivity and yield.

Q4. RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why does RuBisCO carry out more carboxylation in $C_4$ plants?
Answer: In $C_4$plants, RuBisCO carries out more carboxylation because of a high internal concentration of $\mathrm{CO_2}$ around it.
Initially, $\mathrm{CO_2}$ is fixed in the mesophyll cells by the enzyme PEP carboxylase, which has no affinity for $\mathrm{O_2}$. The resulting $C_4$ acid is transported to the bundle sheath cells, where it releases $\mathrm{CO_2}$.
This elevated $\mathrm{CO_2}:\mathrm{O_2}$ ratio ensures that RuBisCO binds primarily to $\mathrm{CO_2}$, functioning efficiently as a carboxylase, and photorespiration is minimized.

Q5. Suppose there were plants that had a high concentration of chlorophyll b but lacked chlorophyll a. Would they carry out photosynthesis? Then why do plants have chlorophyll b and other accessory pigments?
Answer: Plants that lack chlorophyll a would not be able to carry out photosynthesis, because chlorophyll a is the primary pigment responsible for converting light energy into chemical energy during the process.

Accessory pigments such as chlorophyll b, xanthophylls, and carotenoids help by broadening the range of light wavelengths that can be absorbed for photosynthesis. They capture additional light energy and transfer it to chlorophyll a for conversion into chemical energy. These pigments also protect chlorophyll a from photo-oxidation caused by excessive light.

Q6. Why does the colour of a leaf kept in the dark frequently become yellow or pale green? Which pigment is more stable?
Answer: When a leaf is kept in the dark, it becomes yellow or pale green because chlorophyll synthesis requires light, and existing chlorophyll molecules degrade faster in darkness. As chlorophyll breaks down, the normally hidden carotenoids and xanthophylls (yellow pigments) become visible.
These carotenoids and xanthophylls are more stable pigments than chlorophyll and remain in the leaf tissue even when chlorophyll disappears.

Q7. Compare leaves of a plant on the shady side with those on the sunny side. Which of them are darker green, and why?
Answer: Leaves growing on the shady side of a plant, or plants kept in shade, have darker green leaves. This is because, under low light conditions, plants synthesize more chlorophyll to capture as much light as possible. The higher chlorophyll content gives the leaves a darker green colour, helping them maintain an adequate rate of photosynthesis despite limited light.

Q8. 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?
Answer: 
(a) At which points (A, B, or C) in the curve is light a limiting factor? : Light is a limiting factor in regions A and B, where the rate of photosynthesis increases proportionally with increasing light intensity.

(b) What could be the limiting factors in region D?: In region D, photosynthesis reaches a plateau, meaning light is no longer limiting. Other factors become limiting, such as $\mathrm{CO_2}$ concentration or temperature, which affect the rate of dark reactions.

(c) What do points C and D represent on the curve?:

• Point C: Marks the onset of light saturation, where further increases in light do not significantly increase photosynthesis.
• Point D: Represents the maximum rate of photosynthesis ($V_{max}$) achievable under the given conditions.

Q9. Give comparisons between the following:

(a) $C_3$ and $C_4$ pathways

(b) Cyclic and Non-cyclic Photophosphorylation

(c) Anatomy of leaf in $C_3$ and $C_4$ plants