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. Noncyclic photophosphorylation involves a linear flow of electrons derived from the photolysis of water; the electrons are transferred through photosystem II to photosystem I via an electron transport chain (ETC) consisting of plastoquinone, a cytochrome complex, and plastocyanin. The cytochromes form the cytochrome b₆–f complex across the thylakoid membrane. The ETC pumps hydrogen ions (H⁺) into the thylakoid space, and these diffuse back to the stroma through the enzyme complex ATP synthase located in the thylakoid membrane, thus synthesizing ATP by the chemiosmotic mechanism. Electrons continue to flow to photosystem I, which raises their energy levels sufficiently for the reduction of NADP⁺ to NADPH by the enzyme NADP⁺ reductase. This involves a second ETC but no production of ATP. Thus, ATP provides the energy, and NADPH the reducing power for the light-independent reactions of photosynthesis (see calvin–bassham–benson cycle). In cyclic photophosphorylation the electrons from photosystem I that are raised to a higher energy level are recycled through the ETC back to photosystem I, and the resulting proton gradient across the thylakoid membrane again drives the phosphorylation of ADP to ATP by the enzyme ATP synthetase, but there is no production of NADPH. Some photosynthetic bacteria, such as the green sulphur bacteria and purple sulphur bacteria, have only a single photosystem, and cyclic electron flow is their only way of generating ATP. Cyclic electron flow also occurs in organisms with two photosystems, such as cyanobacteria and plants, and is an adaptation for protecting the photosynthetic machinery of plants against stress caused by high light intensity. In prokaryotes the components of photophosphorylation are arranged in folds of the plasma membrane.