Why is the Calvin cycle described as a cycle?

The Calvin cycle is referred to as a cycle because it regenerates ribulose bisphosphate (RuBP) at the end of the process, allowing for the continuous fixation of carbon dioxide. This regeneration is crucial since RuBP is necessary for the initial step of carbon fixation, where CO2 is incorporated into an organic molecule. Each turn of the cycle ultimately produces a three-carbon sugar, but to sustain ongoing carbon fixation, RuBP must be replenished. The cycle of reactions involves the fixation of CO2, the reduction phase, and the regeneration of RuBP, creating a loop that can continue as long as there is a supply of CO2 and the necessary enzymes and ATP/NADPH from the light reactions of photosynthesis.