Primary CO2 Acceptor in C3 and C4 Pathway

In both C3 and C4 pathways, the primary CO2 acceptor is the molecule called phosphoenolpyruvate (PEP).

C4 Plants Pathway

C4 plants have an additional step in their photosynthetic pathway that helps them to overcome the limitations of C3 pathway. In C4 plants, the first step of CO2 fixation occurs in the mesophyll cells, where the CO2 is converted into a 4-carbon compound called oxaloacetate. This process is catalyzed by the enzyme PEP carboxylase, which has a higher affinity for CO2 than Rubisco, the enzyme responsible for CO2 fixation in C3 plants.

The oxaloacetate is then transported to the bundle sheath cells, where it is decarboxylated to release CO2 and pyruvate. The CO2 released in the bundle sheath cells is then fixed by Rubisco into the Calvin cycle, just like in C3 plants. However, the concentration of CO2 in the bundle sheath cells is much higher than in the mesophyll cells, which reduces the rate of photorespiration and increases the rate of photosynthesis.

PEP Carboxylase

PEP carboxylase is the enzyme that catalyzes the first step of CO2 fixation in C4 plants. This enzyme has a higher affinity for CO2 than Rubisco, which allows it to fix CO2 even when the concentration of CO2 is low. PEP carboxylase is also insensitive to the inhibitory effects of O2 and has a higher temperature optimum, which makes it more efficient in hot and dry environments.

Conclusion

In conclusion, PEP is the primary CO2 acceptor in both C3 and C4 pathways. C4 plants have an additional step in their photosynthetic pathway that helps them to overcome the limitations of C3 pathway, which involves the use of PEP carboxylase to fix CO2 in the mesophyll cells and the concentration of CO2 in the bundle sheath cells to reduce photorespiration.

The primary acceptor in C3 plants is a five-carbon ketose sugar and in C4 plants, it is a three-carbon molecule. Complete answer: In the C3 cycle, carbon dioxide is reduced to form a first three carbon stable compound, 3-phosphoglyceric acid 3-PGA.