Process of sucrose synthesis in C4 plants

In C4 plants, sucrose synthesis occurs in the bundle sheath cells, which are specialized cells surrounding the vascular bundles in the leaves. The process of sucrose synthesis in C4 plants involves several steps:

1. Carbon fixation: In C4 plants, carbon dioxide is initially fixed into a 4-carbon compound called oxaloacetate in the mesophyll cells. This process is catalyzed by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase).

2. Transfer of 4-carbon compound: The 4-carbon compound is then transferred to the bundle sheath cells, where it is decarboxylated to release carbon dioxide and regenerate phosphoenolpyruvate (PEP).

3. Calvin cycle: The released carbon dioxide is then fixed into a 3-carbon compound, 3-phosphoglycerate, in the Calvin cycle in the bundle sheath cells. This process is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco).

4. Sucrose synthesis: The 3-carbon compound, 3-phosphoglycerate, is then converted into triose phosphates, such as glyceraldehyde-3-phosphate (G3P), through a series of enzymatic reactions. These triose phosphates are then used to synthesize sucrose through the action of enzymes such as sucrose phosphate synthase (SPS) and sucrose phosphate phosphatase (SPP).

5. Transport of sucrose: Once synthesized, sucrose is transported from the bundle sheath cells to other parts of the plant, such as the roots, stems, and seeds, where it serves as a source of energy and carbon for growth and development.

Overall, the process of sucrose synthesis in C4 plants involves the coordination of carbon fixation, transfer of intermediates between different cell types, and enzymatic reactions to convert carbon compounds into sucrose for transport and storage.

Step 1: Carbon Dioxide (CO2) Fixation in Mesophyll Cells

- Atmospheric CO2 diffuses into mesophyll cells through stomata.
- CO2 is fixed by the enzyme phosphoenolpyruvate carboxylase (PEPC) into oxaloacetate (OAA).

Step 2: Conversion of OAA to Malate and Aspartate

- OAA is reduced to malate by malate dehydrogenase.
- Malate is transported out of the mesophyll cells into the bundle sheath cells.
- Some malate is converted to aspartate by aspartate aminotransferase.

Step 3: Decarboxylation of Malate and Aspartate in Bundle Sheath Cells

- Malate is decarboxylated by the enzyme NADP-malic enzyme, releasing CO2 and pyruvate.
- Some aspartate is decarboxylated by the enzyme aspartate aminotransferase, also releasing CO2 and pyruvate.
- The released CO2 is fixed again by Rubisco, enhancing CO2 concentration in the bundle sheath cells.

Step 4: Regeneration of Malate and Aspartate

- Pyruvate is converted back to malate by pyruvate-phosphate dikinase (PPDK).
- Some malate is converted to aspartate by aspartate aminotransferase.

Step 5: Transport of Malate and Aspartate Back to Mesophyll Cells

- Malate and aspartate are transported back to the mesophyll cells.

Step 6: Decarboxylation and Regeneration of PEP

- Malate and aspartate are decarboxylated to release CO2.
- The CO2 fixed in the mesophyll cells is now recycled and concentrated in the bundle sheath cells.
- The pyruvate produced from decarboxylation is used to regenerate phosphoenolpyruvate (PEP) by pyruvate kinase.

Step 7: Sucrose Synthesis

- The fixed CO2 is assimilated into pyruvate in the mesophyll cells.
- Pyruvate is transported to the bundle sheath cells.
- Pyruvate and fructose-1,6-bisphosphate (FBP) from the Calvin cycle are used to synthesize sucrose by the enzyme sucrose phosphate synthase (SPS).

Summary:

C4 plants have a specialized leaf anatomy that allows for the concentration of CO2 in the bundle sheath cells. The CO2 is fixed initially in the mesophyll cells, but then transported to the bundle sheath cells where it is decarboxylated and refixed. This process enhances CO2 availability for Rubisco and reduces photorespiration, resulting in increased photosynthetic efficiency.