How many molecules of G3P are needed to synthesize one molecule of glucose?

two molecules of G3P are needed to synthesize one molecule of glucose.

Explanation:

For every three molecules of CO2 that enters the Calvin cycle (light independent reaction), one molecule of the three carbon glyceraldehyde 3-phosphate (G3P) is produced. Two molecules of G3P are needed to produce one molecule of glucose. Therefore the Calvin cycle needs to make a total of 6 turns, using 6CO2 to produce two molecules of G3P, that are needed to produce 1 molecule of glucose.