Principles of Biology (POB) Exam 2 Practice

The Calvin Cycle needs a five-carbon CO2 acceptor that can be rebuilt each turn. This molecule is RuBP (ribulose-1,5-bisphosphate). When CO2 is fixed, RuBP reacts with CO2 to form two molecules of 3-PGA. The cycle then uses ATP and NADPH to convert these into glyceraldehyde-3-phosphate (G3P). Most of the G3P is subsequently rearranged and phosphorylated using ATP to regenerate RuBP, allowing the cycle to keep capturing CO2. So RuBP is the molecule that gets regenerated to accept CO2 in every turn. In contrast, G3P is a product used for making sugars, NADPH is a reducing agent consumed in the reduction steps, and ATP provides energy during regeneration but is consumed, not regenerated for CO2 capture.