The Process of photosynthesis: Calvin cycle (Light-independent reactions)

The second stage of photosynthesis is the Calvin cycle, also known as the light-independent reactions or the dark reactions. This stage occurs in the stroma of the chloroplasts. Here's a step-by-step breakdown:

a. Carbon fixation: In the Calvin cycle, carbon dioxide (CO2) molecules from the atmosphere are combined with a five-carbon sugar called ribulose bisphosphate (RuBP) using the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase). This forms an unstable six-carbon compound that immediately breaks down into two molecules of 3-phosphoglycerate (3-PGA).

b. Reduction and carbohydrate synthesis: ATP and NADPH generated during the light-dependent reactions are used to convert 3-PGA into a three-carbon sugar called glyceraldehyde 3-phosphate (G3P). Some G3P molecules are used to regenerate RuBP, while others are further processed to produce glucose and other carbohydrates.

c. Regeneration of RuBP: To sustain the Calvin cycle, some G3P molecules are converted back into RuBP using additional ATP from the light-dependent reactions. This step allows the cycle to continue and ensures a constant supply of RuBP for carbon fixation.

Overall, the light-dependent reactions capture light energy and convert it into chemical energy in the form of ATP and NADPH, while the Calvin cycle uses this energy, along with carbon dioxide, to synthesize glucose and other organic compounds. Oxygen is released as a byproduct of photosynthesis, contributing to the oxygen content of the atmosphere and supporting aerobic life on Earth.