Here's why:
* The Calvin cycle starts with the fixation of carbon dioxide (CO2) by the enzyme Rubisco. This reaction produces an unstable six-carbon molecule that immediately splits into two molecules of 3-PGA.
* 3-PGA is a three-carbon molecule. It's the first stable product of the Calvin cycle, meaning it exists long enough for the cycle to continue.
* The subsequent steps of the Calvin cycle involve the conversion of 3-PGA into glucose. This process requires energy and reducing power supplied by ATP and NADPH, which are produced in the light-dependent reactions of photosynthesis.
