<P> Hydrogen ions, or protons, will diffuse from an area of high proton concentration to an area of lower proton concentration, and an electrochemical concentration gradient of protons across a membrane can be harnessed to make ATP . This process is related to osmosis, the diffusion of water across a membrane, which is why it is called "chemiosmosis". </P> <P> ATP synthase is the enzyme that makes ATP by chemiosmosis . It allows protons to pass through the membrane and uses the free energy difference to phosphorylate adenosine diphosphate (ADP), making ATP . The generation of ATP by chemiosmosis occurs in mitochondria and chloroplasts, as well as in most bacteria and archaea, an electron transport chain pumps H+ ions in the thylakoid spaces through thylakoid membranes . The energy from the electron movement through electron transport chains cross through ATP synthase which allows the proton to pass through them and use this free energy difference to photophosphorylate ADP making ATP . </P> <P> Peter D. Mitchell proposed the chemiosmotic hypothesis in 1961 . The theory suggests essentially that most adenosine triphosphate (ATP) synthesis in respiring cells comes from the electrochemical gradient across the inner membranes of mitochondria by using the energy of NADH and FADH formed from the breaking down of energy - rich molecules such as glucose . </P> <P> Molecules such as glucose are metabolized to produce acetyl CoA as an energy - rich intermediate . The oxidation of acetyl coenzyme A (acetyl - CoA) in the mitochondrial matrix is coupled to the reduction of a carrier molecule such as nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). The carriers pass electrons to the electron transport chain (ETC) in the inner mitochondrial membrane, which in turn pass them to other proteins in the ETC . The energy available in the electrons is used to pump protons from the matrix across the stroma, storing energy in the form of a transmembrane electrochemical gradient . The protons move back across the inner membrane through the enzyme ATP synthase . The flow of protons back into the matrix of the mitochondrion via ATP synthase provides enough energy for ADP to combine with inorganic phosphate to form ATP . The electrons and protons at the last pump in the ETC are taken up by oxygen to form water . </P>

During chemiosmosis a hydrogen ion gradient is linked to the production of atp