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. All organisms produce a phosphate compound,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.
non-cyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems in the thylakoid membrane. First, a photon is absorbed by chlorophyll pigments surrounding the reaction core center of Photosystem II. The light excites an electron in the pigment P680 at the core of Photosystem II, which is transferred to the primary electron acceptor, pheophytin, leaving behind high-energy P680+.[1] The energy of P680+ is used in two steps to split a water molecule into 2H+ + 1/2 O2 + 2e- (photolysis or light-splitting). An electron from the water molecule reduces P680+ back to P680, while the H+ and oxygen are released. The electron transfers from pheophytin to plastoquinone (PQ), which takes 2e- (in two steps) from pheophytin, and two H+ Ions from the stroma to form PQH2. This plastoquinol is later oxidized back to PQ, releasing the 2e- to the cytochrome b6f complex and the two H+ ions into the thylakoid lumen. The electrons then pass through Cyt b6 and Cyt f to plastocyanin, using energy from Photosystem I [1] to pump hydrogen ions (H+) into the thylakoid space. 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 Photosystem II complex replaced its lost electrons from H2O, so electrons are not returned to Photosystem II as they would in the analogous cyclic pathway. Instead, they are transferred to the Photosystem I complex, which boosts their energy to a higher level using a second solar photon. 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
NADP+ + 2H+ + 2e- → NADPH + H+
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.
The concentration of NADPH in the chloroplast may help regulate which pathway electrons take through the light reactions. 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.
