Ribulose-1,5-bisphosphate (RuBP) is a five-carbon sugar phosphate that plays a crucial role in the process of photosynthesis. It is a substrate for the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which catalyzes the first step of carbon fixation in the Calvin cycle. In the Calvin cycle, RuBP combines with carbon dioxide to form two molecules of 3-phosphoglycerate (3-PGA). This reaction is essential for converting atmospheric carbon into organic compounds, ultimately supporting plant growth and providing energy for other organisms in the ecosystem.