Figure 2.1 Photosynthetic carbon reduction (PCR cycle, also termed the Calvin-Benson cycle) utilises ATP and NADPH produced by thylakoid electron transport to drive CO₂ fixation by Rubisco (a). CO₂ is incorporated into a 5-carbon sugar phosphate to produce two 3-carbon sugar phosphates which can either be exported from the chloroplast for sucrose synthesis, be recycled to make more 5-carbon acceptors, or be used to make starch. The appearance of radioactive carbon in 3-carbon sugar phosphates and then in starch and sucrose following photosynthesis in ¹⁴CO₂ was evidence for the pathway of photosynthesis. (b) (Original drawing courtesy Robert Furbank).