138. Which one of the following is NOT true regarding phosphoenolpyruvate carboxykinase type of C4
photosynthesis?
1. Aspartate is transported from mesophyll cell to bundle sheath cell
2. Alanine is transported from bundle sheath cell to mesophyll cell
3. Oxaloacetate is formed both in mesophyll cell to bundle sheath cell
4. Malate is transported from mesophyll cell to bundle sheath cell

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Question:

Which one of the following is NOT true regarding phosphoenolpyruvate carboxykinase type of C4 photosynthesis?

1. Aspartate is transported from mesophyll cell to bundle sheath cell

2. Alanine is transported from bundle sheath cell to mesophyll cell

3. Oxaloacetate is formed both in mesophyll cell to bundle sheath cell

4. Malate is transported from mesophyll cell to bundle sheath cell

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Correct Answer:

2. Alanine is transported from bundle sheath cell to mesophyll cell

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Detailed Explanation:

In C4 plants, the phosphoenolpyruvate carboxykinase (PEPCK) pathway plays a crucial role in carbon fixation by efficiently transferring CO2 to bundle sheath cells, where the Calvin cycle takes place. Let’s review each statement:

1. Aspartate is transported from mesophyll cell to bundle sheath cell:

• True: In the C4 pathway, aspartate is one of the primary metabolites that carries fixed carbon from the mesophyll cells (where CO2 is initially fixed) to the bundle sheath cells. The mesophyll cells convert phosphoenolpyruvate (PEP) into oxaloacetate, which is then converted into aspartate for transport.

2. Alanine is transported from bundle sheath cell to mesophyll cell:

• False: Alanine is not typically transported from bundle sheath cells to mesophyll cells. Instead, malate or aspartate is usually transported from mesophyll to bundle sheath cells. The exchange is mostly between aspartate, malate, and other intermediates, not alanine. This makes this statement incorrect.

3. Oxaloacetate is formed both in mesophyll cell to bundle sheath cell:

• True: Oxaloacetate (OAA) is initially formed in mesophyll cells through the enzyme PEP carboxylase by adding CO2 to phosphoenolpyruvate (PEP). This oxaloacetate can then be converted to malate or aspartate and transported to the bundle sheath cells. The reaction involving oxaloacetate occurs both in mesophyll and bundle sheath cells.

4. Malate is transported from mesophyll cell to bundle sheath cell:

• True: Malate is a key metabolite in C4 photosynthesis. After carbon fixation, malate is one of the compounds that carries CO2 into the bundle sheath cells, where the Calvin cycle can take place.

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Conclusion:

In C4 photosynthesis, the main metabolic intermediates involved in transporting carbon between mesophyll and bundle sheath cells include malate, aspartate, and oxaloacetate. The statement that alanine is transported from bundle sheath cells to mesophyll cells is incorrect, as it is not typically part of the C4 transport mechanism.