Which molecule leaves the Calvin cycle to be used in biosynthesis for the plant?

• A. Glyceraldehyde-3-phosphate (G3P).
• B. RuBP.
• C. 3-PGA.
• D. Glucose.

Answer: (A)

The molecule that leaves the Calvin cycle for biosynthesis is glyceraldehyde-3-phosphate. Inside the cycle, CO2 is fixed and ultimately converted into a three-carbon sugar called glyceraldehyde-3-phosphate using ATP and NADPH. This triose phosphate is the exportable product, and it moves to the cytoplasm where it serves as a building block for making sugars like sucrose and starch and for other biosynthetic pathways. Each turn of the cycle produces one molecule of glyceraldehyde-3-phosphate that can exit, while the rest are used to regenerate RuBP so the cycle can continue.

Ribulose-1,5-bisphosphate (RuBP) is the carbon dioxide acceptor that gets regenerated; it isn’t exported for biosynthesis. 3-phosphoglycerate is an earlier intermediate that remains in the cycle until it is reduced to glyceraldehyde-3-phosphate. Glucose is a downstream product formed from glyceraldehyde-3-phosphate through additional steps, not the molecule that directly leaves the cycle in a single export event.