Talk about the refreshing of RUBP step.

One of the 6 G3P molecules leaves the cycle for the synthesis of sugars and other cellular materials. The other 5 G3P molecules remain in the cycle to regenerate the RUBP (initial reagent of the cycle). The regeneration of RUDP requires 3 ATP molecules. The RUBP is ready to receive CO2 and the cycle starts again. A carbon enters the cycle at each time. The cycle occurs three times, producing one molecule of glycerate 3 - phosphate (G3P). Each time the RUBP is regenerated. Most of the fixed carbon is converted into starch and sucrose.