Photophosphorylation

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 primary source of energy available to living organisms, which is sunlight.

ATP is essential in the Calvin cycle to assist in the synthesis of carbohydrates from carbon dioxide and NADPH.

In all living organisms, a series of redox reactions is used to produce a transmembrane electrochemical potential gradient, or a so-called proton motive force (pmf).

If donor and acceptor (the reactants) are of higher free energy than the reaction products, the electron transfer may occur spontaneously.

[citation needed] The other pathway, non-cyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems in the thylakoid membrane.

This creates a H+ gradient, making H+ ions flow back into the stroma of the chloroplast, providing the energy for the (re)generation of ATP.

The excited electrons are transferred to a series of acceptor molecules, but this time are passed on to an enzyme called ferredoxin-NADP+ reductase, which uses them to catalyze the reaction This consumes the H+ ions produced by the splitting of water, leading to a net production of 1/2O2, ATP, and NADPH + H+ with the consumption of solar photons and water.

When the chloroplast runs low on ATP for the Calvin cycle, NADPH will accumulate and the plant may shift from noncyclic to cyclic electron flow.

In 1950, first experimental evidence for the existence of photophosphorylation in vivo was presented by Otto Kandler using intact Chlorella cells and interpreting his findings as light-dependent ATP formation.