Photorespiration

CHAPTER: 10 (E)

Photorespiration

Salient Features of photorespiration:

•      It takes place only in the presence of intense light.

•      It is a wasteful process as ATP and NADPH₂ are used.

•      With increase in temperature and oxygen concentration, the affinity of RuBP carboxylase for oxygen increase and for CO₂  decreases. Hence, RuBP functions as oxygenase rather than carboxylase. 

•      It occurs in chloroplasts, peroxisomes and mitochondria.

•      Increased in temperature leads to more photorespiration that means more loss of photosynthetically fixed carbon.

•      Photorespiration reduces the potential yield of C-3 plants .

•      It is not an essential process.

•      It occurs usually in C-3 plants like tomato , wheat, oat etc and absent in C-4 plants like maize, Sugarcane etc.

  

Factors affecting rate of photosynthesis:

1. External Factors:

1. Light

1. Intensity of light:

•      The rate of photosynthesis increases with the increase in light intensity until a saturation point. A very high intensity of light decreases the rate of photosynthesis due to photorespiration.

2. Quality of light:

•      Rate of photosynthesis varies in different wavelengths of light. It occurs only in the visible part of spectrum ( i.e. 380-760nm wavelenths).

•      Plants show maximum photosynthesis in red light, which is followed by the blue light .

•      Green light is less effective in photosynthesis .

•      Photosynthesis usually do not takes place in UV-rays and infrared .

2. Temperature:

•      In general, the rate of photosynthesis increases with a rise in temperature, over a range from 6 degree centigrade to 37 degree centigrade. Temperature below 6 degree centigrade and above 37 degree centigrade show adverse effect on the rate of photosynthesis.

3. Concentration of CO₂ :

•      Affects markedly as CO₂ is one of the raw material for photosynthesis.

4. Water:

•      Water is used as raw material in photosynthesis. Plan utilize about 1% of the water in photosynthesis.

5. Oxygen:

•      Warburg(1920) while working on Chlorella reported that higher concentration of oxygen in mesophyll cells has inhibiting effect on photosynthesis. This phenomenon of the inhibition of photosynthesis by oxygen is called Warburg’s effect.

6. Mineral Elements:

•      Some elements such as Mg, Fe , Cu etc are essential for photosynthesis. Mg is one of the component of chlorophyll while Fe is required for the synthesis of Chlorophyll.

2. Internal Factors:

1. Chlorophyll contents: Directly related

2. Anatomy of leaf:

•      The rate of photosynthesis is influenced by various anatomical structures of leaves.

•      These include thickness of cuticle, position, number and distribution of stomata, arrangement of pallisade and spongy parenchyma, presence of intercellular spaces etc.

•      They affect the rate of photosynthesis by influencing the diffusion of CO₂ and absorption of light.

3. Leaf age:

•      In young leaves, photosynthesis does not start immediately but the rate of photosynthesis gradually increases as the leaves mature. The rate of synthesis declines as the leaves become old.

4. Accumulation of photosynthetic products:

•      Retards the photosynthesis

5. Demand for photosynthesis:

•      In growing plants they require more foods due to which rate of photosynthesis increases.