<P> The complete breakdown of glucose in the presence of oxygen is called cellular respiration . The last steps of this process occur in mitochondria . The reduced molecules NADH and FADH are generated by the Krebs cycle, glycolysis, and pyruvate processing . These molecules pass electrons to an electron transport chain, which uses the energy released to create a proton gradient across the inner mitochondrial membrane . ATP synthase then uses the energy stored in this gradient to make ATP . This process is called oxidative phosphorylation because it uses energy released by the oxidation of NADH and FADH2 to phospolyrize ADP into ATP . </P> <P> The light reactions of photosynthesis generate ATP by the action of chemiosmosis . The photons in sunlight are received by the antenna complex of Photosystem II, which excites electrons to a higher energy level . These electrons travel down an electron transport chain, causing protons to be actively pumped across the thylakoid membrane into the thylakoid 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 NAPDH are generated (later used for carbon dioxide fixation in the Calvin Cycle). </P> <P> 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 believed to have been formed when early eukaryotic cells ingested bacteria that could transfer energy using chemiosmosis . This is called the endosymbiotic theory . </P> <P> Chemiosmotic phosphorylation is the third pathway that produces ATP from inorganic phosphate and an ADP molecule . This process is part of oxidative phosphorylation . </P>

Which of the following events occurs first during the process of chemiosmosis