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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?

The leaves on the shady side or those kept in the shade have leaves that are darker green. This is because light is a limiting factor for photosynthesis. In order to increase the rate of photosynthesis, the leaves present in 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. 

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Give comparison between C3 and C4 pathways.


Cpathways

Cpathways

1.

The primary acceptor of CO2 is RUBP – a six-carbon compound.

1.

The primary acceptor of CO2 is phosphoenol pyruvate – a three-carbon compound.

2.

The first stable product is 3-phosphoglycerate.

2.

The first stable product is oxaloacetic acid.

3.

It occurs only in the mesophyll cells of the leaves.

3.

It occurs in the mesophyll and bundle-sheath cells of the leaves.

4.

It is a slower process of carbon fixation and photo-respiratory losses are high.

4.

It is a faster process of carbon fixation and photo-respiratory losses are low.

5.

Occurs in all the plants

5. 

Occurs only in C4 plants

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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-and other accessory pigments?


The plant having high concentration of chlorophyll-b but lacking chlorophyll-a will not carry out photosynthesis.
Chlorophyll-a molecules forms the reaction centre and acts as the main pigment molecules. The other pigments like chlorophyll-b and other photosynthetic pigments such as carotenoids and xanthophylls act as accessory pigments. Their role is to absorb the different wavelength of light and make the process of photosynthesis efficient. Carotenoids and xanthophylls also protect the chlorophyll molecule from photo-oxidation. 

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Figure 13.10 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?



Figure 13.10 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?

(a)  Since the rate of photosynthesis doe not increase with the increase in the incident light therefore light is a limiting factor in the region B.

(b) The other limiting factors may be water, temperature, and the concentration of carbon dioxide.

(c) Point D represents the optimum point and gives the light intensity at which the maximum photosynthesis is recorded.
Ponit c represents the region where the rate of photosynthesis remains constant even after incresing the incident light. 

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Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable?

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-molecules stops and degrades slowly. This changes the colour of the leaf gradually to light green. The xanthophyll and carotenoid pigments become predominant as they are not degraded, causing the leaf to become yellow. These pigments are more stable as light is not essential for their production. 
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