ribulose bisphosphate

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ribulose bisphosphate (RuBP) A five-carbon sugar that is combined with carbon dioxide to form two three-carbon intermediates in the first stage of the light-independent reactions of photosynthesis (see Calvin cycle). The enzyme that mediates the carboxylation of ribulose bisphosphate, ribulose bisphosphate carboxylase/oxygenase (see rubisco), is also involved in photorespiration.