In the Calvin cycle, the immediate product after CO2 fixation is which molecule?

• A. Two molecules of 3-PGA
• B. One molecule of G3P
• C. Two molecules of RuBP
• D. Two molecules of CO2

Answer: (A)

When CO2 is fixed in the Calvin cycle, it is added to a five-carbon molecule called RuBP (ribulose-1,5-bisphosphate) by the enzyme Rubisco. This collision creates a six-carbon intermediate that is inherently unstable, so it immediately splits into two three-carbon molecules of 3-phosphoglycerate (3-PGA). That’s why the immediate product of the fixation step is two molecules of 3-PGA. The next steps then use ATP and NADPH to convert these 3-PGA molecules into glyceraldehyde-3-phosphate (G3P), which can be used to regenerate RuBP or synthesize sugars. The other options don’t fit because RuBP is the acceptor, not the product of fixation; CO2 is the substrate, not the product; and G3P is produced after reduction of 3-PGA, not directly from fixation.