What is the main function of transketolase in Stage 3 of the Calvin Cycle?

Transketolase plays a pivotal role in the Calvin Cycle, particularly in the regeneration of ribulose 1,5-bisphosphate (RuBP), which is essential for the cycle to continue. In Stage 3 of the Calvin Cycle, the primary focus is on regenerating materials that allow the cycle to sustain ongoing carbon fixation. Without sufficient RuBP, the cycle cannot effectively capture CO2, hindering overall photosynthesis.

Transketolase catalyzes the transfer of two-carbon units in the form of ketose sugars, facilitating the conversion of intermediates derived from glyceraldehyde 3-phosphate (G3P) into ribulose 5-phosphate (Ru5P), which is then converted to RuBP. This process is crucial because RuBP is the molecule that reacts with CO2 to initiate the carbon fixation process. Therefore, transketolase's function in regenerating RuBP ensures that the cycle can perpetuate, allowing continuous CO2 incorporation and sugar production.

The other choices relate to different functions or stages that are not directly associated with the specific role of transketolase in the Calvin Cycle's regeneration phase, making them less relevant within this context.