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

Q1. By looking at a plant externally, can you tell whether a plant is C₃ or C₄? Why and how?

Answer:

No, you can't determine whether a plant is C₃ or C₄ by simply observing it from the outside. Both plants can have the same leaf size and shape. For instance, wheat (a C₃ plant) and maize (a C₄ plant) can appear the same, but they employ different mechanisms for photosynthesis. To properly identify them, you must observe their internal structures.

• External appearance cannot be trusted: Leaf size and shape may be the same in both forms.
• Internal structure is the key: You must examine the anatomy of the leaf to identify the type.

Q2. By looking at which internal structure of a plant, can you tell whether a plant is C₃ or C₄? Explain.

Answer:

The main internal feature that helps differentiate C₃ and C₄ plants is known as Kranz anatomy.

• C₄ plants possess a distinct cell arrangement to effectively capture carbon dioxide.
• C₄ plants alone possess Kranz anatomy: Efficient fixation of carbon is permitted.
• Different structures of the cells: The Chloroplast arrangement between the two groups is quite different.

This can be observed under a microscope. Here’s how they differ:

Q3. Even though a very few cells in a C₄ plant carry out the biosynthetic Calvin pathway, they are highly productive. Can you discuss why?

Answer:

C₄ plants are very productive due to their adaptation of a unique method of better capturing carbon dioxide, thus improving their rate of photosynthesis. This is how they do it:

• Concentration of CO₂: C₄ plants fix CO₂ in a four-carbon molecule in the mesophyll cells before exporting it to the bundle sheath cells. The mechanism provides a sufficient CO₂ supply for photosynthesis at all times.
• Lower photorespiration: With high CO₂ levels, RuBisCo (the carbon fixation enzyme) performs better as a carboxylase than as an oxygenase, and this reduces energy loss via photorespiration.

So, even with fewer cells being invested in the Calvin cycle, the ability to capture CO₂ so efficiently enables C₄ plants to survive under hot and dry conditions.

• Efficient use of CO₂ enhances productivity: This leads to higher rates of photosynthesis.

• Minimised energy loss through photorespiration: This makes them more efficient than C₃ plants under certain conditions.

Q4. RuBisCo is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCo carries out more carboxylation in C₄ plants?

Answer:

RuBisCo is an enzyme which can bind both carbon dioxide (CO₂) and oxygen (O₂). But in the case of C₄ plants, it performs a carboxylase role specifically under certain conditions:

• Increased concentration of CO₂: There is a continuous CO₂ supply along with the limitation of oxygen within bundle sheath cells of C₄ plants. This facilitates the RuBisCo to work towards the fixation of CO₂ instead of interacting with O₂.
• Lack of oxygen from photolysis: As there is no oxygen generated during light reactions in these cells, RuBisCo is mainly involved in carboxylation, thereby increasing the efficiency of photosynthesis.

Thus, the special cellular environment in C₄ plants facilitates RuBisCo to be most effective for carbon fixation.

• High CO₂ concentration favours carboxylation over oxygenation: This results in more efficient photosynthesis.
• Low oxygen availability reduces photorespiration hazards: Increasing overall productivity.

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:

Plants with chlorophyll b alone cannot perform photosynthesis effectively because chlorophyll a plays a vital role in the process:

• Role of chlorophyll a: It is the primary pigment for the absorption of light energy and starting the process of photosynthesis by donating electrons to be utilised in energy formation.
• Process of accessory pigments and chlorophyll b: Chlorophyll b helps in the absorption of other wavelengths of light and transfers that energy to chlorophyll a, but is unable to carry out its vital function.

Thus, in the absence of chlorophyll a, there cannot be photosynthesis, even with the presence of chlorophyll b.

• Chlorophyll a is required for photosynthesis: No chlorophyll a = no photosynthesis.
• Chlorophyll b is helpful but can't replace chlorophyll a's role: It helps to trap light but doesn't initiate the process.

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:

When leaves are stored in the dark, they tend to become yellow or pale green for several reasons:

1. Degradation of chlorophyll: Chlorophyll needs light to be produced; in the absence of light, it begins to break down. When chlorophyll degrades, the green colour disappears.
2. Exposure of accessory pigments: When chlorophyll is reduced, other pigments such as carotenoids are exposed, which causes leaves to turn yellow.
3. This process shows how important light is in keeping healthy green leaves.
4. Chlorophyll breaks down in the dark, causing yellowing.
5. Accessory pigments such as carotenoids become apparent, adding to the yellow colour.

Q7. Look at the 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:

Shaded leaves are darker green than sun-exposed leaves for several reasons:

1. Chloroplast orientation and packing: Chloroplasts in shaded places can be packed more densely or have a different orientation within mesophyll cells to capture more light.
2. Reduced photo-oxidation in shade: In intense sunlight, some chlorophyll can be damaged or oxidised; however, shaded leaves have more intact chlorophyll since they receive milder light.
3. Therefore, leaves that are shaded tend to be darker green because they have healthier chlorophyll.
4. Increased chlorophyll in shaded leaves makes the colour darker.
5. Lower damage from excessive sunlight preserves levels of chlorophyll.

Q8. Figure 11.10 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?

Answer:

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

Q8. Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: (b)

(b) 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, CO₂ and H₂O.

Q8. Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions:

(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 a comparison between the following:

(a) C₃ and C₄ pathway

Answer:

Q9. Give a comparison between the following:

(b) Cyclic and non-cyclic photophosphorylation

Answer:

Q9. Give a comparison between the following:

(c) Anatomy of a leaf in C₃ and C₄ plant

Answer: