NCERT Solutions Class 11 Biology Chapter 13 Photosynthesis in Higher Plants

NCERT Solutions for Class 11 Biology for 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?

Answer: 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.

2. By looking at which internal structure of a plant can you tell whether a plant isC3 or C4? Explai

Answer :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 presence of Kranz anatomy, which is absent in C3 plants. Generally, the cross sections of C3 leaves under 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 loosely arranged mesophyll cells, a tightly arranged and chloroplast containing cells are found surrounding the vesicular bundle.These cells are called bundle sheath cells. This anatomical feature of C4 plant is called Kranz anatomy.

Figure: Cross-section of C4 plants depicting Kranz anatomy 

3. Even though a very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productiv Can you discuss why?

Answer: Calvin cycle occurs in all photosynthetic plants and the main enzyme involved is RuBP carboxylase-oxygenase (RuBisCO). It is during Calvin cycle that the sugar is synthesized by fixing CO2. The rate at which CO2 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 cells contain RuBisCO, thus it can be said that in C4 plant only few cells carry out Calvin cycle and fix CO2 as compared to C3 plants. Still, the productivity is higher in C4 plants. The reason is the difference in the anatomy of these two plants. RuBisCO binds to both CO2 and O2 but it has higher affinity for CO2. Under normal condition, it binds to CO2 and carries out calvin cycle for carbon fixation. But at high oxygen concentration, RuBisCo catalyses the binding of oxygen to RuBP forming phosphoglycolate and initiates photorespiratory pathway. This pathway results in release of CO2 consuming ATP.There is no net synthesis of ATP or sugar molecule in photorespiratory pathway. Photorespiration occurs only in C3 plants.

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

(1) It lacks RuBisCO in it’s mesophyll cells thus minimizing the chances of photorespiration pathway.

(2) The C4 acid formed in mesophyll cells is transported to bundle sheath cells, where they are decarboxylated to C3 acid and CO2 molecule. The C3 acid is again transported back to mesophyll cells and help in formation of more C4 acid. The CO2 released in bundle sheath cells increases it’s concentration and thus minimizing the chances to oxygenase activity of RuBisCO enzyme.

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

The schematic representation of basic C4 photosynthesis

(Ref: NCERT Biology, Textbook for Class XI, Chapter No. 13, Photosynthesis in Higher Plants, Page no. 219)

4. RuBisCO is an enzyme that acts both as a carboxylase and oxygena Why do you think RuBisCO carries out more carboxylation in C4 plants?

Answer: 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:

a. C4 plants contain RuBisCO in bundle sheath cells but lacks the same in mesophyll cells. This minimizes the chances of increase in oxygen concentration near the enzyme, in turn inhibiting its oxygenation activity.
b. The C4 acid formed in mesophyll cell is transported to bundle sheath cells, where it is decarboxylated to C3 acid and CO2 molecule. The C3 acid is again transported back to mesophyll cells and helps in formation of more C4 acid. The CO2 released in bundle sheath cell increases its concentration and thus accelerates the carboxylation activity of enzyme.

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: The plant having high concentration of chlorophyll b, but lacking chlorophyll a will be unable to carry out photosynthesis efficiently. The chlorophyll a molecules forms 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 CO2 fixation. 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 surrounds the reaction center chlorophyll a molecule and thus protect it from photo-oxidative damage.

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?

Answer:Photosynthesis is a light dependent process and in the presence of light, plants synthesiz the photosynthetic molecules like chlorophyll a and Due to their short life and also due to photo-oxidative damage of chlorophyll by light, plant need continuous synthesis of chlorophyll molecules. When a plant is shifted from light to dark, it stops synthesizing the chlorophyll molecule and the existing chlorophyll molecule get degraded. Under these conditions the leaves loses its green color and the color of accessory pigments like xanthophylls (yellow) and carotenoids (yellow to yellow-orange) predominates which do not require light for their synthesis. Thus leaves turn yellow under dark.

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 sh Which of them has leaves that are darker green ? Why?

Answer:The rate of photosynthesis depends upon the external environmental factors like water supply, temperature, CO2 concentration and light intensity as well as internal factors like chlorophyll and RuBisCO concentration. 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 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 or the potted plant kept in sunlight possess higher chlorophyll, thereby appears darker green.

8. Figure shows the effect of light on the rate of photosynthesi Based on the graph, answer the following questions:

(Ref: NCERT Biology, Textbook for Class XI, Chapter No. 13, Photosynthesis in Flowering Plants, Page no. 222)

(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?

Answer:

(a) Keeping all the other factors apart and considering light as only factor affecting the rate of photosynthesis, the region “A” in the above curve is the region where light is the limiting factor because the photosynthesis rate is lowest in this region.

(b) Apart from light, other environmental factors such as CO2 concentration, water supply and temperature may also be the limiting factor. Also the internal factor, such as chlorophyll content in leaves may be one of the limiting factors affecting the rate of photosynthesis at the point “A”.

(c) The value at the X-axis, D, is that intensity of light where the rate of photosynthesis becomes saturated. This suggests that, the intensity of light “D” is the saturating intensity above which it plays no role in increasing the photosynthetic rate as other factors become limiting. The area “C” of the curve shows saturation of rate of photosynthesis.

9. Give comparison between the following:

(a) C3 and C4 pathways

(b) Cyclic and non-cyclic photophosphorylation

(c) Anatomy of leaf in C3 and C4 plants

Answer:

(a) C3 and C4 pathways

(b) Cyclic and non-cyclic photophosphorylation

(c) Anatomy of leaf in C3 and C4 plants