The total number of carbon atoms present in the Calvin cycle is not changed during the reduction phase, because at the beginning of the reduction phase, there are 6 PGA molecules, each with 3 carbon atoms, for a total of 18 carbon atoms, and at the end of the reduction phase, there are 6 PGAL molecules, each with 3 carbon atoms, for a total of 18 carbon atoms. Thus, although the molecule present at the beginning of the reduction phase differs from that which is present at the end of the reduction phase, the number of carbon atoms remains unchanged.