Here's how it works:
1. Light-dependent reactions: Sunlight is captured by chlorophyll and used to split water molecules, releasing electrons and producing ATP (energy) and NADPH (a reducing agent).
2. Calvin cycle (light-independent reactions):
* CO2 from the atmosphere enters the Calvin cycle.
* The enzyme RuBisCO catalyzes the reaction between CO2 and a 5-carbon sugar called ribulose bisphosphate (RuBP).
* This creates an unstable 6-carbon molecule that immediately splits into two 3-carbon molecules called 3-phosphoglycerate (3-PGA).
* This is where the carbon from CO2 is "fixed"—it's now incorporated into an organic molecule.
The 3-PGA molecules then go through a series of steps using the energy from ATP and the reducing power of NADPH to eventually regenerate RuBP and produce glucose, a simple sugar that plants use for energy and growth.
So, in summary, carbon dioxide (CO2) is fixed in photosynthesis by being incorporated into the organic molecule 3-phosphoglycerate (3-PGA).
