Partitioning of Fixed Carbon to Sucrose and Starch in Leaf Cells
Introduction
Photosynthesis in leaf cells results in the fixation of atmospheric CO₂ into carbohydrates. The two primary end-products of photosynthetic carbon assimilation are sucrose and starch. Sucrose serves as the major transport form of carbohydrates, while starch is a temporary storage form in chloroplasts. Their partitioning is regulated by enzymatic activity, developmental stage, and environmental factors.
Fixed Carbon Assimilation
During the Calvin cycle in the chloroplast stroma, CO₂ is fixed into triose phosphates (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate). These 3-carbon compounds are the precursors for both sucrose and starch biosynthesis.
Starch Synthesis in the Chloroplast
- Some triose phosphates remain in the chloroplast and are used for starch synthesis.
- Triose phosphates are converted into glucose-6-phosphate and then to glucose-1-phosphate.
- Glucose-1-phosphate reacts with ATP to form ADP-glucose, the substrate for starch synthase.
- Starch synthase polymerizes glucose units into amylose and amylopectin, which accumulate as starch granules.
Sucrose Synthesis in the Cytosol
- Other triose phosphates are exported to the cytosol via the triose phosphate/phosphate translocator.
- In the cytosol, they are converted to fructose-6-phosphate and glucose-6-phosphate.
- These combine to form sucrose-6-phosphate via sucrose phosphate synthase.
- Sucrose-6-phosphate is dephosphorylated by sucrose phosphatase to form sucrose.
Regulation of Carbon Partitioning
The distribution of fixed carbon between starch and sucrose is influenced by:
- Light intensity: High light promotes starch accumulation.
- Time of day: Sucrose synthesis predominates during the day; starch is broken down at night to support metabolism.
- Enzyme activity: Sucrose phosphate synthase and ADP-glucose pyrophosphorylase regulate the flow towards sucrose or starch.
Physiological Significance
- Sucrose: Translocated through the phloem to non-photosynthetic tissues for growth and storage.
- Starch: Acts as a buffer against carbon starvation at night when photosynthesis ceases.
Conclusion
The partitioning of fixed carbon into sucrose and starch in leaf cells is a finely regulated process essential for plant growth, survival, and adaptation. It ensures energy availability during both day and night and supports the plant’s metabolic demands.