Stages of Photosynthesis

Photosynthesis occurs in two stages. These stages are called the light reactions and the dark reactions. The light reactions take place in the presence of light. The dark reactions do not require direct light, however dark reactions in most plants occur during the day.

Light reactions occur mostly in the thylakoid stacks of the grana. Here, sunlight is converted to chemical energy in the form of ATP (free energy containing molecule) and NADPH (high energy electron carrying molecule). Chlorophyll absorbs light energy and starts a chain of steps that result in the production of ATP, NADPH, and oxygen (through the splitting of water). Oxygen is released through the stomata. Both ATP and NADPH are used in the dark reactions to produce sugar.

Dark reactions occur in the stroma. Carbon dioxide is converted to sugar using ATP and NADPH. This process is known as carbon fixation or the Calvin cycle. The Calvin cycle has three main stages: carbon fixation, reduction, and regeneration. In carbon fixation, carbon dioxide is combined with a 5-carbon sugar [ribulose1,5-biphosphate (RuBP)] creating a 6-carbon sugar. In the reduction stage, ATP and NADPH produced in the light reaction stage are used to convert the 6-carbon sugar into two molecules of a 3-carbon carbohydrate, glyceraldehyde 3-phosphate. Glyceraldehyde 3-phosphate is used to make glucose and fructose. These two molecules (glucose and fructose) combine to make sucrose or sugar. In the regeneration stage, some molecules of glyceraldehyde 3-phosphate are combined with ATP and are converted back into the 5-carbon sugar RuBP. With the cycle complete, RuBP is available to be combined with carbon dioxide to begin the cycle over again.