What is the net product of the Calvin cycle after fixing three CO2 molecules?

What is the net product of the Calvin cycle after fixing three CO2 molecules?

• A. One molecule of glyceraldehyde-3-phosphate leaves the cycle.
• B. Two molecules of 3-PGA leave the cycle.
• C. Glucose leaves the cycle.
• D. RuBP is regenerated but nothing leaves.

Answer: (A)

In the Calvin cycle, carbon fixed from CO2 is ultimately turned into carbohydrates using energy from ATP and reducing power from NADPH. After three CO2 molecules are fixed, the net product that exits the cycle is one molecule of glyceraldehyde-3-phosphate. This three-carbon sugar phosphate can be used to build sugars like sucrose or starch, or feed into glucose synthesis after additional turns of the cycle. The cycle also regenerates RuBP so it can continue, but most of the fixed carbon remains in the cycle to rebuild RuBP. The other options don’t fit the net outcome. While 3-PGA appears early in the pathway, it is converted downstream and does not leave the cycle net in a single turn. Glucose would require multiple turns (two G3P units) to assemble, so it doesn’t leave after just three CO2 fixed. And RuBP is regenerated as part of the cycle, but that doesn’t mean nothing leaves the cycle. The correct net product that exits after fixing three CO2 is glyceraldehyde-3-phosphate.

In the Calvin cycle, carbon fixed from CO2 is ultimately turned into carbohydrates using energy from ATP and reducing power from NADPH. After three CO2 molecules are fixed, the net product that exits the cycle is one molecule of glyceraldehyde-3-phosphate. This three-carbon sugar phosphate can be used to build sugars like sucrose or starch, or feed into glucose synthesis after additional turns of the cycle. The cycle also regenerates RuBP so it can continue, but most of the fixed carbon remains in the cycle to rebuild RuBP.

The other options don’t fit the net outcome. While 3-PGA appears early in the pathway, it is converted downstream and does not leave the cycle net in a single turn. Glucose would require multiple turns (two G3P units) to assemble, so it doesn’t leave after just three CO2 fixed. And RuBP is regenerated as part of the cycle, but that doesn’t mean nothing leaves the cycle. The correct net product that exits after fixing three CO2 is glyceraldehyde-3-phosphate.