Question

I‘ve read that 6 turns of the Calvin cycle are required to make 1 glucose molecule.

But, 3CO2 and 3RuBP are used in one cycle and 6 triose phosphate is produced. Only 5 triose phosphate molecules are required for RuBP regeneration. The remaining 1 triose phosphate has 3 carbon atoms in it. Only 3 more carbon atoms are needed, which can be produced again in the next cycle. So, it would seem that, in total, only 2 cycles are needed to produce 1 glucose molecule. Why do most authors write that six are needed 6?

Answer 1

The Calvin cycle requires six turns to produce one glucose molecule with six carbon atoms. The remaining molecules are used to regenerate the starting molecule of the cycle.

Step-by-step explanation:

The Calvin cycle is a series of chemical reactions that occur in the chloroplasts of plants during photosynthesis. It is responsible for converting carbon dioxide into glucose, a carbohydrate molecule.

One complete turn of the Calvin cycle fixes one molecule of carbon dioxide and produces two molecules of glyceraldehyde-3-phosphate (GA3P), which can be used to form glucose or other compounds.

Therefore, it takes six turns of the Calvin cycle to produce one glucose molecule, which has six carbon atoms. The remaining five GA3P molecules are used to regenerate the molecule that starts the cycle, ribulose-1,5-bisphosphate (RuBP).