USA Biology Olympiad (USABO) Practice Exam

RUBP is utilized and PGA is formed during which process in photosynthesis?

1. Light Reactions

2. Dark Reaction

3. Photorespiration

4. Calvin Cycle

The correct answer is: Calvin Cycle

RUBP, or ribulose bisphosphate, is a critical component in the process of photosynthesis known as the Calvin Cycle. In this cycle, RUBP combines with carbon dioxide to form a 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (PGA). This reaction is facilitated by the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO). The Calvin Cycle occurs in the stroma of chloroplasts and does not require light directly, as it takes place in what is often referred to as the "dark reactions" of photosynthesis. However, it is important to note that the Calvin Cycle relies on the ATP and NADPH generated during the light reactions to proceed. Thus, while RUBP utilization and PGA formation directly relate to the Calvin Cycle, the metabolic pathway does not occur during the light reactions, photorespiration, or standalone “dark reactions” that do not refer specifically to the Calvin Cycle.