photophosphorylation

thumb|300px|Photophosphorylation in the light-dependent reactions of [[photosynthesis, which occurs at the thylakoid membrane in chloroplasts and cyanobacteria.]] In the process of photosynthesis, the phosphorylation of ADP to form ATP using the energy of sunlight is called photophosphorylation. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions, driven by the main source of energy available to living organisms, which is sunlight. All organisms produce ATP, which is the universal energy currency of life. In photophosphorylation, light energy is used to pump protons across a biological membrane, mediated by flow of electrons through an electron transport chain. This stores energy in a proton gradient. As the protons flow back through an enzyme called ATP synthase, ATP is generated from ADP and inorganic phosphate. ATP is essential in the Calvin cycle to assist in the synthesis of carbohydrates from carbon dioxide and NADPH.

The scientist Charles Barnes first used the word 'photosynthesis' in 1893. This word is taken from two Greek words, , which means light, and , which in chemistry means making a substance by combining simpler substances. So, in the presence of light, synthesis of food is called 'photosynthesis'.