Question

Is photorespiration present at low light intensities in C3 and C4 plants?

Answer 1

C3 reactions:

1. The reactions of the Calvin cycle are known as dark reactions as they do not depend on light directly.
2. It is the initial phase of the Calvin cycle where the fixation of CO₂ occurs.
3. The second phase of C3 reactions comprises the reduction of 3PGA (3-phosphoglycerate) to G3P (glyceraldehyde-3-phosphate) by ATP and NADPH which further get converted to ATP and NADP+.
4. RuBP is regenerated in the process which helps in fixing carbon.

C4 reactions:

1. These reactions are called light reactions as they are directly dependent on light.
2. The atmospheric CO₂ is fixed in the mesophyll cells by PEP carboxylase to produce oxaloacetate.
3. The oxaloacetate is shuttled to malate for entering into the bundle sheath cells.
4. The malate is converted to pyruvate inside the bundle sheath cells releasing CO₂.
5. This CO₂ is used to produce sugars and enters into the Calvin cycle.

Photorespiration:

1. The process of oxygenation of RuBP (ribulose bisphosphate) by the enzyme Rubisco (Ribulose bisphosphate carboxylase/oxygenase) in presence of light is known as photorespiration.
2. The enzyme rubisco can perform oxygenation and carboxylation reactions.
3. The rate of photorespiration increases as the temperature increases and the concentration of CO₂ decreases and the concentration of O₂ increases.
4. In the case of C3 plants as the concentration of O₂ increases, along with the increase in temperature and high-intensity light, the rate of photorespiration also increases.
5. Whereas in C4 plants, photorespiration is absent as they have evolved a mechanism to increase CO₂ concentration around the enzyme RuBisCO in the bundle sheath cell.
6. Hence, these plants show better yield and can tolerate higher temperatures well.