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.
Give comparison between C3 and C4 pathways.
C3 pathways |
C4 pathways |
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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 |
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.
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.