How many ATP molecules are consumed for every 3 CO2 molecules fixed in the Calvin Cycle?

During the Calvin Cycle, which is a fundamental part of photosynthesis occurring in the chloroplasts of plant cells, carbon dioxide is converted into glucose using the energy from ATP and the reducing power of NADPH produced in the light-dependent reactions.

For every 3 molecules of carbon dioxide (CO2) that are fixed, there are a series of enzymatic reactions that must occur in the cycle. These reactions involve the production of 1 molecule of glyceraldehyde-3-phosphate (G3P), which is a three-carbon sugar and a precursor to glucose and other carbohydrates.

To fully understand the energy requirements, we need to recognize the process:

1. The cycle starts with the carbon fixation of 3 CO2 molecules, creating an unstable 6-carbon compound that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).

2. Each of these 3-PGA molecules is then phosphorylated by ATP and reduced by NADPH.

3. Specifically, the reduction process (moving from 3-PGA to G3P) consumes ATP, where each of the 6 molecules of 3-PGA formed from the initial fixation of CO2 uses a molecule of ATP. Thus, 6 ATP are