lumen. these protons then flow down their electrochemical potential gradient through an enzyme called atp-synthase, creating atp by the phosphorylation of adp to atp. the electrons from the initial light reaction reach photosystem i, then are raised to a higher energy level by light energy and then received by an electron acceptor and reduce nadp+ to nadph. the electrons lost from photosystem ii get replaced by the oxidation of water, which is "split" into protons and oxygen by the oxygen-evolving complex (oec, also known as woc, or the water-oxidizing complex). to generate one molecule of diatomic oxygen, 10 photons must be absorbed by photosystems i and ii, four electrons must move through the two photosystems, and 2 nadph are generated (later used for carbon dioxide fixation in the calvin cycle). in prokaryotes chemiosmotic coupling between the energy of sunlight, bacteriorhodopsin and phosphorylation (chemical energy) during photosynthesis in the halophilic archaeal organism halobacterium salinarum (syn. h. halobium). the archaeal cell wall is omitted. bacteria and archaea also can use chemiosmosis to generate atp. cyanobacteria, green sulfur bacteria, and purple bacteria synthesize atp by a process called photophosphorylation. these bacteria use the energy of light to create a proton gradient using a photosynthetic electron transport chain. non-photosynthetic bacteria such as e. coli also contain atp synthase. in fact, mitochondria and chloroplasts are the product of endosymbiosis and trace back to incorporated prokaryotes. this process is described in the endosymbiotic theory. the origin of the mitochondrion triggered the origin of eukaryotes, and the origin of the plastid the origin of the archaeplastida, one of the major eukaryotic supergroups. chemiosmotic phosphorylation is the third pathway that produces atp from inorganic phosphate and an adp molecule. this process is part of oxidative phosphorylation. emergence of chemiosmosis early cell powered by external proton gradient near a deep-sea hydrothermal vent. as long as the membrane (or passive ion channels within it) is permeable to protons, the mechanism can function without ion pumps. thermal cycling model a stepwise model for the emergence of chemiosmosis, a key element in the origin of life on earth, proposes that primordial organisms used thermal cycling as an energy source (thermosynthesis), functioning essentially as a heat engine: self-organized convection in natural waters causing thermal cycling → added β-subunit of f1 atp synthase (generated atp by thermal cycling of subunit during suspension in convection cell: thermosynthesis) → added membrane and fo atp synthase moiety (generated atp by change in electrical polarization of membrane during thermal cycling: thermosynthesis) → added metastable, light-induced electric dipoles in membrane (primitive photosynthesis) → added quinones and membrane-spanning light-induced electric dipoles (today's bacterial photosynthesis, which makes use of chemiosmosis). external proton gradient model deep-sea hydrothermal vents, emitting hot acidic or alklaine water, would have created external proton gradients. these provided energy that primordial organisms could have exploited. to keep the flows separate, such an organism could have wedged itself in the rock of the hydrothermal vent, exposed to the hydrothermal flow on one side and the more alkaline water on the other. as long as the organism's membrane (or passive ion channels within it) is permeable to protons, the mechanism can function without ion pumps. such a proto-organism could then have evolved further mechanisms such as ion pumps and atp synthase. meteoritic quinones a proposed alternative source to chemiosmotic energy developing across membranous structures is if an electron acceptor, ferricyanide, is within a vesicle and the electron donor is outside, quinones transported by carbonaceous meteorites pick up electrons and protons from the donor. they would release electrons across the lipid membrane by diffusion to ferricyanide within the vesicles and release protons which produces gradients above ph 2, the process is conducive to the development of proton gradients.