The VIDYANKER team has thoughtfully prepared the NCERT Solutions for Class 11 Biology Chapter 11, "Photosynthesis in Higher Plants" These solutions are designed to help you tackle the NCERT textbook questions with ease. We recommend going through the chapter's theory before diving into the solutions for a deeper understanding. Feel free to share these NCERT Solutions for Class 11 Biology with others learning is always better when shared!
NCERT TEXTBOOK QUESTIONS SOLVED
1. By looking at a plant externally can you tell whether a plant is C3 or C4 ? Why and how?
Ans. By examining a plant from the outside, it is usually not possible to ascertain with absolute certainty whether a plant is C₃ or C₄, since the principal differences between C₃ and C₄ plants are internal pertaining to their leaf anatomy, photosynthetic pathways, and enzyme systems. But in certain instances, such hints as habitat and leaf structure can be informative. Hot and dry habitats are the typical environments for C₄ plants, with narrow leaves containing fewer stomata to minimize water loss. Internally, C₄ plants have a characteristic Kranz anatomy where bundle sheath cells have a wreath-like distribution around vascular bundles, which is not externally visible. So to determine a plant as C₃ or C₄ accurately, one would have to look into the internal structure of leaves microscopically or conduct biochemical tests to examine their photosynthetic enzymes.
2. By looking at which internal structure of a plant can you tell whether a plant is C3 or C4? Explain.
Ans. Because leaves of C4 plants possess distinctive anatomy referred to as Kranz anatomy. This distinguishes them from C3 plants. Certain cells, referred to as bundle-sheath cells, enclose the vascular bundles. These contain numerous chloroplasts. They are thick-walled and lack intercellular spaces. Thus, we could identify if a plant is C3 or C4 based on 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. Because, by C4 cycle, a plant is able to photosynthesise even in the presence of extremely low concentration of CO2 (up to 10 parts per million), the xeric conditions induced partial closure of stomata would not have much impact. Thus, the plants are able to adapt to develop at low water content, high temperature and high light intensities. This cycle is particularly adapted to such plants that develop in dry tropical and subtropical climates. Furthermore, the rate of photosynthesis is also higher because of the lack of photorespiration in these plants. It can be imagined that both C4 cycle and photorespiration are the product of evolution or could have been one of the causes of evolution for the adaptation of plants to various environments. C4 plants are roughly twice as efficient as C3 plants in converting solar energy to production of dry matter.
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 not found in C4 plant mesophyll cells. It is found in the bundle-sheath cells that are around the vascular bundles. The Calvin cycle in C4 plants takes place in the bundle-sheath cells. Phosphoenol pyruvate, a three-carbon molecule, is the main CO2 acceptor in the mesophyll cells. It is reduced to the four-carbon molecule oxaloacetic acid (OAA). OAA is then reduced to malic acid. Malic acid is channeled to the bundle-sheath cells where decarboxylation and CO2 fixation by the Calvin cycle happens. This averts the enzyme RuBisCo from being an 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. Chlorophyll-a molecules are antenna molecules. They become excited by light absorption and release electrons in cyclic and non-cyclic photophosphorylations. They constitute the reaction centres of photosystems I and II. Chlorophyll-b and other pigments like carotenoids and xanthophylls are accessory pigments. Their function is to capture energy and transfer it to chlorophyll-a. Carotenoids and xanthophylls also prevent the chlorophyll molecule from photo-oxidation. Hence, chlorophyll-a is crucial in photosynthesis. If any plant lacked chlorophyll-a but had a high level of chlorophyll-b, then the plant would not carry out photosynthesis.
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. As leaves need light in order to undergo photosynthesis, the hue of a leaf maintained in the dark turns from a darker to lighter green. At times, it also becomes yellow. The synthesis of the chlorophyll pigment needed for photosynthesis is proportional to the availability of light. Without light, the synthesis of chlorophyll-a molecules halts and they are slowly degraded. This shifts the colour of the leaf step by step to light green. In doing this, the carotenoid and xanthophyll pigments become dominant, making the leaf yellow. They are more stable since light is not required to produce them. They are always found in plants.
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 leaves of the shaded side are also darker green than those that are exposed to sunlight for two reasons:
(i) The chloroplasts are found predominantly in the cells of the mesophyll along their walls for receiving maximum amount of falling light.
(ii)The chloroplasts position themselves in a vertical orientation along the lateral walls of high intensity light and along tangential wails in intermediate light.
8. The given 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 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) At point A
(b) In the region A’, light can be a limiting factor.
(c) C represents the stage beyond which light is not a limiting factor. D represents the stage beyond which intensity of light has no effect on the 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
Ans. (a) C3 and C4 pathways
| C3 pathways | C4 pathways |
|---|---|
| Primary acceptor of CO2 is RUBP (5C) | The primary acceptor of CO2 is phosphoenol pyruvate (3C). |
| The first stable product is 3 phosphoglycerate. | The first stable product is oxaloacetic acid. |
| It occurs only in the mesophyll cells of the leaves. | It occurs in the mesophyll and bundle-sheath cells of the leaves. |
(b) Cyclic and non-cyclic photophosphorylation
| Cyclic photophosphorylation | Non-cyclic photophosphorylation |
|---|---|
| It occurs only in photosystem I. | It occurs both in photosystems I and II. |
| It involves only the synthesis of ATP. | It involves the synthesis of ATP and NADPH2. |
| In this process, electrons move in a closed circle. | In this process, electrons do not move in a closed circle. |
(c) Anatomy of leaf in C3 and C4 plants
| Anatomy of leaf in C3 | Anatomy of leaf in C4 |
|---|---|
| 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 |
