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

Q1: By looking at a plant externally can you tell whether a plant is C₃ or C₄? Why and how?
Ans: One cannot distinguish whether a plant is C₃ or C₄ by observing its leaves and other morphological features externally.
C3 vs C4 plants Unlike C₃ plants, the leaves of C₄ plants have a special anatomy called Kranz anatomy and this difference can only be observed at the cellular level. For example, although wheat and maize are grasses, wheat is a C₃ plant, while maize is a C₄ plant.

Q2: By looking at which internal structure of a plant can you tell whether a plant is C₃ or C₄? Explain.
Ans: The leaves of C₄ plants have a special anatomy called Kranz anatomy. This makes them different from C₃ plants. Special cells, known as bundle-sheath cells, surround the vascular bundles. These cells have a large number of chloroplasts. They are thick-walled and have no intercellular spaces. Therefore, we can tell whether a plant is C₃ or C₄ through its internal structure.

Q3: Even though very few cells in a C₄ plant carry out the biosynthetic–Calvin pathway, yet they are highly productive. Can you discuss why?
Ans: In C₄ plants photorespiration does not occur because they have a mechanism that increases the concentration of CO₂ at the enzyme site. This takes place when the C₄ acid from the mesophyll is broken down in the bundle sheath cells to release CO₂ that results in increasing the intracellular concentration of CO₂. In turn, this ensures that the RuBisCO functions as a carboxylase minimising the oxygenase activity. Thus, the photosynthesis rate increases and makes C₄ plants more productive.

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?
Ans: The enzyme RuBisCo is absent from the mesophyll cells of C₄ plants. It is present in the bundle-sheath cells surrounding the vascular bundles. In C₄ plants, the Calvin cycle occurs in the bundle-sheath cells.
Rubisco The primary CO₂ acceptor in the mesophyll cells is phosphoenol pyruvate – a three-carbon compound. It is converted into the four-carbon compound oxaloacetic acid (OAA). OAA is further converted into malic acid. Malic acid is
transported to the bundle-sheath cells, where it undergoes decarboxylation and CO₂ fixation occurs by the Calvin cycle. This prevents the enzyme RuBisCo from acting 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?
Ans: Plants that do not possess chlorophyll will not carry out photosynthesis because it is the primary pigment and acts as the reaction centre. It performs the primary reactions of photosynthesis or conversion of light into chemical or electrical energy. Other photosynthetic pigments are called accessory pigments. They absorb light energy of different wavelengths and hence broaden the spectrum of light absorbed by photosynthetic pigments. These pigments hand over the absorbed energy to chlorophyll a.

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?
Ans: Since leaves require light to perform photosynthesis, the colour of a leaf kept in the dark changes from a darker to a lighter shade of green. Sometimes, it also turns yellow. The production of the chlorophyll pigment essential for photosynthesis is directly proportional to the amount of light available. In the absence of light, the production of chlorophyll-a molecules stops and they get broken slowly. This changes the colour of the leaf gradually to light green. During this process, the xanthophyll and carotenoid pigments become predominant, causing the leaf to become yellow. These pigments are more stable as light is not essential for their production. They are always present in plants.

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?
Ans: Light is a limiting factor for photosynthesis. Leaves get less light for photosynthesis when they are in shade. Therefore, the leaves or plants in the shade perform less photosynthesis than the leaves or plants kept in sunlight.
In order to increase the rate of photosynthesis, the leaves present in the shade have more
chlorophyll pigments. This increase in chlorophyll content increases the amount of light absorbed by the leaves, which in turn increases the rate of photosynthesis.
Therefore, the leaves or plants in the shade are greener than the leaves or plants kept in the sun.

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? 
(b) What could be the limiting factor/s in region A? 
(c) What do C and  D represent on the curve?

Ans:

(a) Generally, light is not a limiting factor. It becomes a limiting factor for plants growing in shade or under tree canopies. In the given graph, light is a limiting factor at the point where photosynthesis is the minimum. The least value for photosynthesis is in region A. Hence, light is a limiting factor in this region.
(b) Light is a limiting factor in region A. Water, temperature, and the concentration of carbon dioxide could also be limiting factors in this region.
(c) Point D represents the optimum point and gives the light intensity at which the maximum photosynthesis is recorded. The rate of photosynthesis remains constant after this point, even though the intensity of light.

Q9: Give a 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

 (b) Cyclic and non-cyclic photophosphorylation

 (c) Anatomy of the leaves in C₃ and C₄ plants