NCERT Solutions Class 11 Biology Chapter 13

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

Ans-

No, one cannot tell whether a plant is C3 or C4 by looking at only external features. There is no morphological difference between leaves of C3 and C4 plants, so it is almost impossible to differentiate these plants just by external investigation. C4 plant can be differentiated from C3 plant by anatomical examination only. The vascular bundle of C4 plants are surrounded by large cells called bundle sheath cells. Such structure found especially in C4 plants is called ‘Kranz Anatomy’. Thus, the study of vertical section of leaves under compound microscope is necessary to differentiate C3 plants from C4 plants.

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

Ans-

The major anatomical difference between C3 and C4 plant is the Kranz anatomy (Kranz is a German word for “wreath”). The leaves of C4 plant show the presence of Kranz anatomy, which is absent in C3 plants. Generally, the cross-sections of C3 leaves under a microscope shows only one type of cells that contains chloroplast. These are loosely arranged and are called mesophyll cells. But in case of C4 plants, apart from being loosely arranged mesophyll cells, a tightly arranged and chloroplast containing cells are found surrounding the vascular bundle. These cells are called bundle sheath cells. This anatomical feature of C4 plant is called Kranz anatomy.

Q.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-

Calvin cycle occurs in all photosynthetic plants and the main enzyme involved is RuBP carboxylase-oxygenase (RuBisCO). It is during the Calvin cycle that the sugar is synthesised by fixing carbon dioxide. The rate at which carbon dioxide is fixed determines the productivity of the plant. In case of C3 plants, all the mesophyll cells contain the enzyme RuBisCO, while in C4 plants only bundle sheath cell contain RuBisCO, thus it can be said that in C4 plant only a few cells carry out Calvin cycle and fix carbon dioxide as compared to C3 plants. Still, productivity is higher in C4 plants. The reason is the difference in the anatomy of these two plants.

RuBisCO binds to both carbon dioxide and oxygen but it has a higher affinity for carbon dioxide. Under normal condition, it binds to carbon dioxide and carries out the Calvin cycle for carbon fixation. But at high concentration of oxygen, RuBisCo catalyses the binding of oxygen to RuBP forming phosphoglycolate and initiates photorespiratory pathway. This pathway results in the release of carbon dioxide consuming ATP. There is no net synthesis of ATP or sugar molecule in the photorespiratory pathway. Photorespiration occurs only in C3 plants.

In C4 plant, the Kranz anatomy indirectly helps in the suspension of photorespiratory pathways in two ways-

1. It lacks RuBisCO in its mesophyll cells thus minimising the chances of photorespiration pathway.
2. The C4 acid formed in mesophyll cell is transported to bundle sheath cells, where they are decarboxylated to C3 acid and carbon dioxide molecule. The C3 acid is again transported back to mesophyll cells and help in the formation of more C4 acid. The carbon dioxide released in bundle sheath cell increases its concentration and thus minimizing the chances to oxygenase activity of RuBisCO enzyme.

Thus the productivity of C4 plant is higher as compared to C3 plants as they bypass the photorespiration pathway and fix more carbon under similar condition.

Q.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 is an enzyme that acts both as a carboxylase and oxygenase. However, it carries out more carboxylation in C4 plants due to following two reasons:

1. C4 plants contain RuBisCO in bundle sheath cells but lack the same in mesophyll cells. This minimises the chances of an increase in oxygen concentration near the enzyme, in turn inhibiting its oxygenation activity.
2. The C4 acid formed in mesophyll cell is transported to bundle sheath cells, where it is decarboxylated to C3 acid and carbon dioxide molecule. The C3 acid is again transported back to mesophyll cells and helps in the formation of more C4 acid. The carbon dioxide released in the bundle sheath cell increases its concentration and thus accelerates the carboxylation activity of the enzyme.

Q.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 plant having a high concentration of chlorophyll b, but lacking chlorophyll a will be unable to carry out photosynthesis efficiently. The chlorophyll a molecules form the reaction centers of both Photosystem I and Photosystem II. The excited reaction center chlorophyll molecule transfers its energy to various acceptors in cyclic and non-cyclic photophosphorylation and thus NADPH₂ molecule is produced. A plant lacking chlorophyll a molecule will be defective in light reaction of photosynthesis and in turn will not be able to carry out fixation of carbon dioxide.

Plants contain chlorophyll b and other accessory pigments for performing the following two functions:

• They form the antenna molecule, absorb light energy and transfer it to reaction center chlorophyll a molecule, thus increasing the efficacy of the photosystem.
• They surround the reaction center chlorophyll a molecule and thus protect it from photo-oxidative damage.

Q.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?

Photosynthesis is a light-dependent process and in the presence of light, plants synthesise the photosynthetic molecules like chlorophyll a and b. Due to their short life and also due to photo-oxidative damage of chlorophyll by light, plants need a continuous synthesis of chlorophyll molecules. When a plant is shifted from light to dark, it stops synthesising the chlorophyll molecule and the existing chlorophyll molecule loses its stability and gets degraded.

Under these conditions, the colour of accessory pigments like xanthophylls (yellow) and carotenoids (yellow to yellow-orange) predominates which do not require light for their synthesis. Thus, pigments carotenoids and xanthophyll are more stable and the leaves turn yellow or pale green under dark.

Q.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-

The rate of photosynthesis depends upon the external environmental factors like water supply, temperature, the concentration of carbon dioxide and light intensity as well as internal factors like chlorophyll and concentration of RuBisCO. To increase the photosynthetic efficiency plants can modulate internal factors, but they cannot modulate external factors.

When a plant is shifted from shade to light, it senses an increase in light intensity, which stimulate the plant to increase the rate of photosynthesis by increasing the chlorophyll content in leaves.

Thus, the leaves of the same plant on the shady side as compared to the sunny side of the potted plant kept in sunlight possess higher chlorophyll, thereby appearing darker green in colour.

Q.8 Figure 13.10 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 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) Keeping all the other factors apart and considering light as the only factor affecting the rate of photosynthesis, in the above curve at point “A”, light is the limiting factor because the photosynthesis rate is minimum at this point.

(b) Apart from light, other environmental factors such as the concentration of carbon dioxide, water supply and temperature may also be the limiting factors. Also, the internal factors such as chlorophyll content in leaves may be one of the limiting factors affecting the rate of photosynthesis at the point “A”.

(c) Stage ‘C’ is representing that beyond it light is not a limiting factor.

At stage ‘D’, the rate of photosynthesis becomes saturated. This suggests that above it, the intensity of light does not play a role in increasing the rate of photosynthsis.

Q.9 Give a comparision between the following:

(a) C3 and C4 pathways
(b) Cyclic and non-cyclic photophosphorylation
(c) Anatomy of leaf in C3 and C4 plants

Ans-

(a) C₃ and C₄ pathways

(b) Cyclic and non-cyclic photophosphorylation

(c) Anatomy of leaf in C₃ and C₄ plants