Photosynthesis: The Calvin Cycle

The Calvin Cycle, also known as the light-independent or dark reactions, is the second part of photosynthesis. It doesn't directly need light but relies on ATP and NADPH produced during the light-dependent reactions. This cycle takes place in the stroma, the fluid surrounding the thylakoids.

In the carbon fixation phase, the enzyme Rubisco combines carbon dioxide from the air with a five-carbon compound called RuBP. This forms a six-carbon compound that quickly splits into two molecules of three-phosphoglycerate. The video illustrates this process using three molecules of CO2.

During the reduction phase, ATP provides energy, and NADPH contributes electrons and hydrogen to reduce the intermediate compound, creating glyceraldehyde 3-phosphate (G3P). G3P is the main product of the Calvin Cycle, used to synthesize sugars. For every three CO2 molecules entering the cycle, one G3P molecule is gained.

In the regeneration phase, the remaining G3P molecules are converted back into RuBP, the starting compound, to continue the cycle. This process requires energy supplied by ATP. The video demonstrates how five G3P molecules (15 carbon atoms) are regenerated into three RuBP molecules (15 carbon atoms).

To produce one glucose molecule (which has six carbons), two G3P molecules are needed. The video emphasizes how the Calvin Cycle, powered by ATP and NADPH from light reactions, creates glucose and other organic compounds essential for the growth of plants and algae, supporting almost all life on Earth.