What is RuBP?

RuBP, or ribulose bisphosphate, is indeed a five-carbon sugar. This molecule plays a critical role in the process of photosynthesis, specifically in the Calvin Cycle, which is responsible for the fixation of carbon dioxide. In this cycle, RuBP combines with carbon dioxide to form an unstable six-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound. This is a key step in converting light energy into chemical energy stored in glucose and other carbohydrates.

Understanding the function of RuBP is essential for grasping how plants convert atmospheric carbon into organic compounds, emphasizing the importance of this five-carbon sugar in the overall process of photosynthesis. The other choices do not accurately define RuBP, as it is not a three-carbon molecule, a pigment, or an enzyme.