Question

How many molecules of water would be consumed to support one Calvin cycle?

• A. One
• B. Two
• C. Four
• D. Six

Answer 1

Answer: (B)

Two

In the entire process of photosynthesis; fixation of six carbon dioxide molecules occurs to generate a molecule of hexose and is accompanied by the release of six moles of oxygen. During the biosynthetic phase, the Calvin cycle turns six times in order to produce one molecule of hexose. The released oxygen comes from water and not from carbon dioxide. During photosynthesis, water is oxidized to release oxygen and carbon dioxide is reduced to form sugar. Thus twelve molecules of water are oxidised to release six molecules of oxygen released along with the formation of one molecule of hexose by six turns of Calvin cycle. In other words for one turn of Calvin cycle, two molecules of water need to be photo oxidised.