NCERT Solutions for Class 11 Biology Chapter 13: Photosynthesis in Higher Plants

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

Ans. It is not possible to distinguish whether a plant is C₃ or C₄ by observing its leaves and other morphological features externally. Unlike C₃plants, the leaves of C₄ plants have a special anatomy called Kranz anatomy and this difference can only be observed at the cellular level. For example, although wheat and maize are grasses, wheat is a C₃ plant, while maize is a C₄ plant.

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

Ans. Bundle sheath cells are specialised cells that wrap the vascular bundles in C₄ plants. Chloroplasts are found in every cell of the bundle sheath.
Leaves’ mesophyll cells do not differentiate into the spongy and palisade parenchyma, which have fewer intercellular spaces, but chloroplasts are present in the mesophyll cells. The bundle sheath of C₃ plants, on the other hand, lack chloroplasts, and the mesophyll cells of the leaves are divided into spongy and palisade parenchyma. As a result, we can detect whether a plant is C₃ or C₄ by looking at its internal structure.

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?

Ans. The productivity of a plant is measured by the rate of its photosynthesis. The amount of carbon dioxide present in a plant is directly proportional to the rate of photosynthesis. C₄ plants have a mechanism for increasing the concentration of carbon dioxide. In C₄ plants, the Calvin cycle occurs in the bundle-sheath cells. The C₄ compound (malic acid) from the mesophyll cells is broken down in the bundle sheath cells. As a result, CO₂ is released. The increase in CO₂ensures that the enzyme RuBisCo does not act as an oxygenase, but as a carboxylase. This prevents photorespiration and increases the rate of photosynthesis. This is the reason that C₄
plants are highly productive.

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?

Ans. RuBisCo functions as oxygenase only when there is a higher concentration of oxygen and a lower concentration of carbon dioxide. Both these conditions are not found in bundle sheath cells of C₄ plants. These do not have a photolytic evolution of oxygen. They receive a regular supply of carbon dioxide even when the
stomata are closed. Therefore, RuBisCo of C₄plants functions only as a carboxylase and not as oxygenase.

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?

Ans. The plants which do not possess chlorophyll a, will not carry out photosynthesis because chlorophyll a molecule acts as reaction center. Chlorophyll a P680 acts as reaction center of PS-II and chlorophyll a P700 acts as reaction center of PS-I. However, the plants possess chlorophyll b and other accessory pigments to absorb shorter wavelengths of light so that all the effective wavelength of visible light can be utilized in light reaction.

6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable?

Ans. The chlorophyll is the pigment, responsible for the green colour of leaf. Light is essential for the production of chlorophyll and in absence of light or in dark there is no production of chlorophyll and chlorophyll present leaf is degraded. During this process, the xanthophyll and carotenoid pigments become predominant, causing the leaf to become yellow or pale green. These pigments are more stable as light is not essential for their production.

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?

Ans. Light is essential for photosynthesis. Leaves receive less light for photosynthesis when they are in shade. Therefore, the leaves or plants in shade perform less photosynthesis as compared to the leaves or plants kept in sunlight. In order to increase the rate of photosynthesis, the leaves present in shade have more chlorophyll pigments. This increases the chlorophyll content with increase in the amount of light absorbed by the leaves which increases the rate of photosynthesis. Therefore, the leaves or plants in shade are greener than the leaves for plants kept in the sun.

8. Figure 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 is light a limiting factor?

(b) What could be the limiting factor/s in region A?

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

Ans. (a) Light is essential for photosynthesis. But, it becomes a limiting factor for plants growing in shade or under tree canopies. In the given graph, light is a limiting factor at the point where photosynthesis is the minimum. The least value for photosynthesis is in region A. Hence, light is a limiting factor in this region.

(b) Light is a limiting factor in region A. Water, temperature, and the concentration or carbon dioxide could also be limiting factors in this region.

(c) Point D represents the optimum point and gives the light intensity at which the maximum photosynthesis is recorded. The rate of photosynthesis remains constant after this point, even though the intensity of light increases in region C.

9. Give comparison between the following:

(a) C₃ and C₄ pathways

(b) Cyclic and non-cyclic photophosphorylation

(c) Anatomy of leaf in C₃ and C₄plants

Ans. Differences between:

(a) C₃ and C₄pathways:

C₃ Pathway	C₄ Pathway
Photosynthesis occurs in mesophyll tissues.	Photosynthesis occurs both in mesophyll and bundle sheath cells.
Kranz anatomy is absent.	Kranz anatomy is present.
The carbon dioxide acceptor is RuBisco.	The carbon dioxide acceptor is PEP carboxylase.
Photorespiration loss is high.	Photorespiration does not take place.

(b) Cyclic and non-cyclic photophosphorylation:

Cyclic Photophosphorylation	Non-cyclic Photophosphorylation
It occurs only in photosystem I.	It occurs in photosystems I and II.
It involves only the synthesis of ATP.	It involves the synthesis of ATP and NADPH₂.
In this process, photolysis of water does not take place.	The photolysis of water occurs.
Oxygen is not produced.	Oxygen gas is produced.
In this process, electrons move in a closed circle.	In this process, electrons do not move in a closed circle.

Leaf Anatomy of C₃ Plants	Leaf Anatomy of C₄ Plants
Bundle-sheath cells are absent.	Bundle-sheath cells are present.
RuBisCo is present in the mesophyll cells.	RuBisCo is present in the bundle-sheath cells.
The first stable compound produced is 3-phosphoglycerate – a three-carbon compound.	The first stable compound produced is oxaloacetic acid – a four-carbon compound.
Photorespiration occurs.	Photorespiration does not occur.