RuBP, or ribulose-1,5-bisphosphate, is the five-carbon compound regenerated in the Calvin cycle.
RuBP is the cycle’s carbon dioxide acceptor. The enzyme Rubisco attaches carbon dioxide to RuBP, creating an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate, also called PGA. Subsequent reactions use ATP and NADPH to reduce these compounds to G3P.
The cycle must rebuild RuBP so that another round of carbon fixation can begin. For every three carbon dioxide molecules fixed, six G3P molecules are produced; five are rearranged and phosphorylated to regenerate three RuBP molecules, while one G3P represents the net carbohydrate gain. This regeneration stage also requires ATP.
PGA and G3P are real Calvin-cycle intermediates, but neither is the regenerated five-carbon acceptor: PGA has three carbons, and G3P also has three. Glucose is a six-carbon sugar that can be assembled later from G3P, but it is not the molecule regenerated within the cycle.