11 Biology

Photosynthesis

NCERT Solution

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

Answer: Plants which are adapted to dry climates follow the C4 pathway. While this knowledge can be helpful in telling a C4 or a C3 plant, external features hardly provide a clue for this. However, histological study of bundle sheath in the leaves can easily give clue about the plant type.

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

Answer: Bundle sheath; around the vascular bundle.

Question 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?

Answer: C4 plants avoid photorespiration and thus avoid the wastage of ATP. Thus there is more efficient utilization of the resources. This helps a C4 plant to produce more food compared to a C3 plant.


Question 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?

Answer: In C4 plants, RuBisCO is not wasted in oxygenation and hence it carries out more carboxylation.

Question 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: Pigments; other than chlorophyll a are also capable of absorbing light energy. Hence, photosynthesis will take place even in the absence of chlorophyll a. Other accessory pigments work as additional pigments to trap solar energy.

Question 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: Chlorophyll is unable to absorb energy in the absence of light and thus loses its stability. This leads to yellowing of leaves. The yellowing of leaves also shows that xanthophylls is more stable.



Question 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?

Answer: The leaves which face more light are darker green than those which face less light. This happens because of increased concentration of chlorophyll in the presence of light.

Question 8: The following figure shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions:

graph of factors affectig photosynthesis

(a) At which point/s (A, B or C) in the curve is light a limiting factor?

Answer: At point A

(b) What could be the limiting factor/s in region A?

Answer: Carbon dioxide and water; apart from light.

(c) What do C and D represent on the curve?

Answer: C is the stage beyond which light is not a limiting factor. D is the line beyond which intensity of light has no effect on the rate of photosynthesis.



Question 9: Give comparison between the following:

(a) C3 and C4 pathways

Answer: The first carbon fixation product is PGA (3-carbon atom) in C3 pathway, while it is oxaloacetic acid (4-carbon atom) in C4 pathway. The C4 pathway is more efficient than the C3 pathway; in terms of making glucose.

(b) Cyclic and non-cyclic photophosphorylation

Answer: Both the photosystems are involved in non-cyclic photophosphorylation, while only PS I is functional in cyclic photophosphorylation. Non-cyclic photophosphorylation happens in the lamella of grana, while cyclic photophosphorylation occurs in the lamella of stroma. Both ATP and NADPH+H+ are synthesized in non-cyclic photophosphorylation, while only ATP is synthesized in cyclic photophosphorylation.

(c) Anatomy of leaf in C3 and C4 plants

Answer: Bundle sheath is present in C4 leaves, while it is absent in C3 leaves. The cells of C4 leaves have a large number of chloroplasts, thick walls and no intercellular spaces.