photophosphorylation

photophosphorylation The formation of ATP from ADP and inorganic phosphate using light energy in photosynthesis (compare oxidative phosphorylation). There are two pathways, noncyclic and cyclic photophosphorylation, which occur in the thylakoid membranes of the chloroplasts. In noncyclic photophosphorylation electrons derived from the photolysis of water are raised to higher energy levels in photosystems I and II and pass along an electron transport chain of carrier molecules (see ferredoxin; plastocyanin; plastoquinone) to NADP reductase. This enzyme transfers electrons to NADP⁺ to make NADPH, which provides reducing power for the light-independent reactions of photosynthesis. In cyclic photophosphorylation the electrons from photosystem I that are raised to a higher energy level are recycled through the electron carrier system back to photosystem I. Both pathways of electron flow cause H⁺ ions to be pumped by a group of cytochromes, the cytochrome b₆–f complex, across the thylakoid membrane. This creates a proton gradient that drives the phosphorylation of ADP to ATP by the enzyme ATP synthetase (see chemiosmotic theory).