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

Plants are the primary source of food for all animals, which also includes human beings. This food, which the green plants need is made or synthesized by them through photosynthesis, and hence termed as autotrophs. This chapter's core is the process of photosynthesis in higher plants (green plants), which is defined as a physicochemical process, which consumes light energy to run the synthesis of organic compounds.

This chapter aims to make the students aware of the structure of the photosynthetic machinery and all those reactions that convert light energy into chemical energy. Photosynthesis in green plants is an indispensable process because of two reasons. The first reason is that it serves as the basis of existence for all known food chains on earth. The second reason is that through this process oxygen is released by the green plants into the atmosphere, which is the only survival instrument for all forms of life. Delve into the brief on the topics of this chapter to find out more about them.

• What do we know about Photosynthesis?
• Early Experiments
• At which place does photosynthesis occur?
• Number of Pigments involved in the process of Photosynthesis
• Meaning of Light Reaction
• The Electron Transport
• Parts where ATP and NADPH are used?
• About the C4 Pathway
• Photorespiration
• Factors impacting the process of Photosynthesis

Chapter 13 'Photosynthesis in Higher Plants' of class 11 Biology decodes the complex chemicals and reactions that play a vital part in changing the light energy into chemical energy when photosynthesis occurs in green plants, which have pigment chlorophyll present in them. In addition to this, the chapter also discusses the technicality (organization of photosynthetic machinery) of the photosynthesis process.

Q1. By looking at a plant externally can you tell whether a plant is C 3 or C 4 ? Why and how?
Answer: External morphology does not necessarily differentiate plants from plants. The leaves are isobilateral but all isobilateral leaves are not in nature. Anatomically, plants can be differentiated from plants as plants possess leaves with Kranz anatomy.

Q2. By looking at which internal structure of a plant can you tell whether a plant is C 3 or C 4 ? Explain.
Answer: By  looking  at  the  internal  structure  of the leaf, it  can be revealed whether the plant is C3 or C4. In C4 plants, a leaf has Kranz anatomy wherein the undifferentiated mesophyll occurs in concentric layers around vascular bundles. Each vascular bundle is surrounded by a bundle sheath of large-sized chloroplast-containing cells. The chloroplasts of the bundle sheath cells are devoid of grana but contain starch grains. The chloroplasts of the mesophyll cells are granular and starch grains do not occur in them. In C3 plants, bundle sheath cells are neither very large, nor contain chloroplasts. The mesophyll is differentiated into palisade and spongy parenchyma. All chloroplasts are granular and possess starch grains.

Q3. 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: The productivity of a plant is directly proportional to the rate of photosynthesis which in turn is dependent on the amount of carbon dioxide present in a plant. In C4 plants a mechanism for increasing the concentration of carbon dioxide is present. Here, the Calvin cycle occurs in the bundle-sheath cells. The C4 compound malic acid from the mesophyll cells is broken down inside the bundle- sheath cells. This causes the release of carbon dioxide gas. The increase in Carbon dioxide gas ensures that the enzyme RuBisCo does not act as an oxygenase, but as a carboxylase. This prevents losses by photorespiration thus increasing the rate of photosynthesis. Hence, it can be concluded that even though very few cells in a  plant carry out the biosynthetic – Calvin pathway, yet they are highly productive.

Q4. RuBisCo is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCo carries out more carboxylation in C 4 plants?
Answer: 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 C4 plants functions only as a carboxylase and not as an oxygenase.

Q5. 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: Chlorophyll-a molecules are essential for the process of photosynthesis as they act as antenna molecules. These molecules get excited by absorbing photons and emitting electrons during cyclic and non-cyclic photophosphorylation. Chlorophyll-a  molecules form the reaction centers for both photosystems I and II. Due to these reasons, photosynthesis is absent in plants lacking chlorophyll a. Chlorophyll b, carotenoids and xanthophylls are accessory pigments. The major functions of these pigments are as follows

1. Absorption of light rays of different wavelengths and transfer of this energy to reaction centres.

    2.Carotenoids and xanthophylls also protect the chlorophyll molecule from photo-oxidation.

Thus, it can be concluded that if any plant were to lack chlorophyll-a and contain a high concentration of chlorophyll-b, then this plant would not undergo photosynthesis.

Q6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable?

Answer: A leaf kept in dark for long becomes yellow or pale green because of the disintegration of chlorophylls. Light is essential for the production of chlorophyll pigment. In the absence of light, the production of chlorophyll-a molecules stops, and they get broken slowly. It results in the change in color of the leaf to light green. This causes xanthophyll and carotenoid pigments to become predominant, causing the leaf to become yellow. Carotenoids which provide yellow color to the leaf are more stable.

Q7. 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?

Answer: The leaves in shade are much greener than the ones exposed to sunlight because of the following reasons.

1. The chloroplasts of the mesophyll cells are irregularly arranged, and their orientation is in vertical files along the walls in strong light.
2. Photo-oxidation of chloroplasts occurs in bright light and its non-oxidation occurs in shaded areas.

Q8. Figure 13.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions:

1. At which point/s (A, B or C) in the curve is light a limiting factor?

Answer:

At the A point, light is the limiting factor because the rate of photosynthesis is minimum here.

2. What could be the limiting factor/s in region A ?

Answer: In region A light is a limiting factor. The other limiting factors in this region can be temperature, CO 2and H
c. What do C and D represent on the curve?

Answer:

C represents a stage beyond which light is not a limiting factor and D is the line beyond which the intensity of light does not affect the rate of photosynthesis.

Q9. Give comparison between the following:

1. C 3 and C 4 pathway

Answer:

2. Cyclic and non-cyclic photophosphorylation

Answer:

c. Anatomy of leaf in C 3 and C 4 plant

Answer: