Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water. This process takes place in the chloroplasts, primarily within the leaves of plants and is necessarily to start energy flow in ecosystems.
The overall balanced chemical reaction for photosynthesis can be summarised as: 6CO2 + 6H2O → C6H12O6 + 6O2
Photosynthesis consists of two main stages: light-dependent reactions and light-independent reactions (Calvin cycle).
Light-dependent reactions
The light-dependent reactions occur in the thylakoid membranes of the chloroplasts. These reactions require light energy, which is absorbed by chlorophyll and other pigments. The key inputs for these reactions are light, water and ADP and NADP+ molecules.
Light energy is used to split water molecules (photolysis), releasing oxygen as a by-product. The energy from light is also used to convert ADP into ATP and to reduce NADP+ into NADPH, both of which are energy carriers for the next stage of photosynthesis. The net outputs of the light-dependent reactions are oxygen, ATP and NADPH.
Light-independent reactions
The light-independent reactions, also known as the Calvin cycle, take place in the stroma of the chloroplasts. These reactions do not directly require light but use the ATP and NADPH produced during the light-dependent reactions.
The main input is carbon dioxide, which enters the cycle and is fixed into a stable intermediate compound. Through a series of enzyme-driven steps, ATP provides energy, and NADPH donates electrons to reduce carbon compounds, eventually synthesising glucose. The net outputs of the Calvin cycle are glucose, ADP, NADP+ and inorganic phosphate, which are recycled for use in further light-dependent reactions.
