Fixation of CO2 in C4 plants

C4 plants are the plants that have a unique photosynthetic pathway in which they fix CO2 into a four-carbon compound before it is used in the Calvin cycle. This pathway is more efficient than the C3 pathway used by most plants, especially in hot and dry conditions.

Steps involved in CO2 fixation in C4 plants

1. CO2 fixation in mesophyll cells: In C4 plants, CO2 is initially fixed into a four-carbon compound oxaloacetate (OAA) in the mesophyll cells of leaves.

2. Formation of malate: The OAA is then converted into malate, which is transported to the bundle sheath cells.

3. Release of CO2: In the bundle sheath cells, malate is decarboxylated to release CO2, which is then used in the Calvin cycle to produce sugars.

4. Regeneration of PEP: The remaining three-carbon compound pyruvate is transported back to the mesophyll cells where it is converted back to PEP (phosphoenolpyruvate), which is required for the initial CO2 fixation.

ATP requirement for CO2 fixation in C4 plants

The initial fixation of CO2 in C4 plants requires the enzyme PEP carboxylase, which uses one molecule of ATP to fix one molecule of CO2 into a four-carbon compound. In addition, the transport of malate from mesophyll cells to bundle sheath cells requires energy in the form of ATP and NADPH.

Therefore, the total number of ATP molecules required for fixing one molecule of CO2 in C4 plants is 5. This is because:

- One ATP molecule is used by PEP carboxylase for CO2 fixation
- Two ATP molecules are used for the conversion of OAA to malate
- Two ATP molecules are used for the transport of malate from mesophyll cells to bundle sheath cells.

Conclusion

In summary, C4 plants have a unique photosynthetic pathway in which the initial fixation of CO2 requires the enzyme PEP carboxylase and one molecule of ATP. The transport of malate from mesophyll cells to bundle sheath cells also requires energy in the form of ATP. The total number of ATP molecules required for fixing one molecule of CO2 in C4 plants is 5.