What is RuBP and what is its role in photosynthesis?

What is RuBP and what is its role in photosynthesis?

• A. Ribulose-1,5-bisphosphate is the CO2 acceptor that combines with CO2 to form two molecules of 3-PGA.
• B. It is the enzyme that fixes CO2 during photosynthesis.
• C. It is the molecule that leaves the cycle as glyceraldehyde-3-phosphate.
• D. It is the oxygenated byproduct produced during photorespiration.

Answer: (B)

RuBP is Ribulose-1,5-bisphosphate, a five-carbon sugar that serves as the CO2 acceptor in the Calvin cycle. The crucial step is that the enzyme RuBisCO fixes carbon dioxide by attaching it to RuBP, producing an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate. So RuBP’s role is to capture CO2 and provide the substrate for carbon fixation, not to be the enzyme itself. It is also not the molecule that leaves the cycle directly as glyceraldehyde-3-phosphate; after a few more steps, glyceraldehyde-3-phosphate exits to form sugars. And it is not the oxygenated byproduct of photorespiration; the oxygenated product is 2-phosphoglycolate, formed when RuBisCO adds O2 instead of CO2.

RuBP is Ribulose-1,5-bisphosphate, a five-carbon sugar that serves as the CO2 acceptor in the Calvin cycle. The crucial step is that the enzyme RuBisCO fixes carbon dioxide by attaching it to RuBP, producing an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate. So RuBP’s role is to capture CO2 and provide the substrate for carbon fixation, not to be the enzyme itself. It is also not the molecule that leaves the cycle directly as glyceraldehyde-3-phosphate; after a few more steps, glyceraldehyde-3-phosphate exits to form sugars. And it is not the oxygenated byproduct of photorespiration; the oxygenated product is 2-phosphoglycolate, formed when RuBisCO adds O2 instead of CO2.