Question

If the Calvin cycle uses three molecules of CO2 to produce six molecules of G3P (glyceraldehydes-3-phosphate), but only one G3P molecule is used to form a carbohydrate molecule, what happens to the other carbons that were taken in?

A. Any unused CO2 is released into the environment.
B. The other five G3P molecules are used to regenerate more RuBP.
C. Any unused G3P molecule is converted and stored as starch.
D. The remaining five G3P molecules are used to reduce CO2.

Answer 1

The Calvin cycle uses five of the produced G3P molecules to regenerate RuBP, allowing continued carbon fixation and synthesis of carbohydrates.

Step-by-step explanation:

In the Calvin cycle, every three molecules of carbon dioxide result in the production of six molecules of G3P, but only one of these G3P molecules is used to form glucose, a simple carbohydrate. The remaining five molecules of G3P do not go to waste; instead, they play a critical role in the cycle. They are utilized to regenerate ribulose-1,5-bisphosphate (RuBP), which is essential for the continuation of the Calvin cycle itself. This regeneration process requires additional energy in the form of ATP.

The Calvin cycle must turn three times to produce enough carbon to form one G3P molecule that can leave the cycle to contribute to carbohydrate synthesis, and it must turn six times to create a six-carbon glucose molecule. During each turn, in addition to the one G3P leaving the cycle, the other five G3P molecules are used to reform the necessary RuBP so that the cycle may continue and fix more carbon dioxide. This is how the Calvin cycle contributes to the photosynthetic energy storage process.