In which cell of leaf, pyruvate is converted to PEP in C4 pathway?a)Epidermal cellsb)Mesophyll cellsc)Bundle sheath cellsd)Guard cellsCorrect answer is option 'B'. Can you explain this answer?

Question

Answer

In C4​ plants, initial fixation of CO2​ occurs in mesophyll cells. The primary acceptor of CO2​ is phosphoenolpyruvate (PEP). CO2​ is again fixed inside bundle sheath cells through the Calvin cycle. The pyruvate is sent back to mesophyll cells where it is changed to phosphoenolpyruvate (PEP).

Answer

In C4 plants, initial fixation of CO2 occurs in mesophyll cells. The primary acceptor of CO2 is PEP. CO2 is again fixed inside bundle sheath cells through the Calvin cycle. The pyruvate is sent back to mesophyll cells where it is changed to PEP. See ncert diagram.

Answer

Conversion of Pyruvate to PEP in C4 pathway in Mesophyll cells of leaves

The C4 pathway is a carbon fixation pathway found in certain plants. It is an adaptation to overcome the inefficiencies of the traditional C3 pathway in high-temperature and low-carbon dioxide environments. In the C4 pathway, carbon dioxide is initially fixed into a four-carbon compound called oxaloacetate (OAA) in mesophyll cells, and then further processed in bundle sheath cells to produce phosphoenolpyruvate (PEP) before entering the traditional C3 pathway.

1. Mesophyll Cells
The first step of the C4 pathway occurs in the mesophyll cells, which are located in the leaf tissue. Mesophyll cells have a high concentration of chloroplasts and are responsible for photosynthesis. In these cells, carbon dioxide is initially fixed by an enzyme called phosphoenolpyruvate carboxylase (PEP carboxylase) to form oxaloacetate (OAA).

2. Conversion of Pyruvate to PEP
After the initial fixation of carbon dioxide, oxaloacetate is converted to malate or aspartate and transported to the bundle sheath cells. In the bundle sheath cells, malate or aspartate is decarboxylated to release carbon dioxide and regenerate pyruvate.

Pyruvate is then converted back into phosphoenolpyruvate (PEP) through a series of enzymatic reactions. The enzyme responsible for this conversion is pyruvate orthophosphate dikinase (PPDK). This conversion is necessary to replenish the PEP levels in the mesophyll cells, which are continuously used up in the initial carbon dioxide fixation.

3. Role of Mesophyll Cells
Mesophyll cells play a crucial role in the C4 pathway as they are responsible for the initial fixation of carbon dioxide and the conversion of pyruvate to PEP. The high concentration of chloroplasts in these cells allows for efficient photosynthesis and carbon dioxide fixation. The C4 pathway evolved as an adaptation to optimize carbon fixation in environments with high temperatures and low carbon dioxide levels.

Conclusion
In conclusion, the conversion of pyruvate to PEP in the C4 pathway occurs in the mesophyll cells of leaves. These cells are responsible for the initial fixation of carbon dioxide and the subsequent conversion of pyruvate back to PEP. The C4 pathway is an efficient adaptation in certain plants to overcome the limitations of the traditional C3 pathway and optimize carbon fixation in specific environmental conditions.

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