What is the first stable product of carbon fixation in C3 plants?

The first stable product of carbon fixation in C3 plants is 3-phosphoglycerate. In the Calvin cycle, the enzyme RuBisCO attaches carbon dioxide to the five-carbon molecule ribulose-1,5-bisphosphate (RuBP). The resulting six-carbon intermediate is extremely unstable and immediately splits into two molecules of 3-phosphoglycerate, commonly abbreviated 3-PGA.

This three-carbon product gives C3 photosynthesis its name. 3-PGA is not yet a sugar that can leave the cycle. During the reduction stage, ATP adds energy and phosphate, while NADPH supplies reducing power, converting 3-PGA into glyceraldehyde-3-phosphate (G3P). Some G3P can later contribute to glucose and other carbohydrates, while most is recycled to regenerate RuBP.

A common mix-up is confusing the first stable product with G3P, the first carbohydrate output that can exit the cycle. Oxaloacetate is the initial product of carbon fixation in C4 and CAM pathways, where phosphoenolpyruvate carboxylase captures carbon dioxide before the Calvin cycle operates. RuBP, meanwhile, is the carbon dioxide acceptor, not the product.