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The process takes place at a biological membrane. In prokaryotes this is the plasma membrane, and in eukaryotes it is the inner of the two mitochondrial membranes. NADH and FADH₂, electron carrier molecules that were "loaded" during the citric acid cycle, are used in an intricate mechanism (involving NADH-Q reductaseThe NADH dehydrogenase (EC 1. 3) complex, also called NADH coenzyme Q oxidoreductase is the first complex in the electron transfer chain of mitochondria and catalyzes the transfer of electrons from NADH to coenzyme Q (CoQ): :NADH + CoQ + H+ → NAD+ +, cytochrome c oxidaseThe enzyme cytochrome c oxidase ( PDB , EC 1. 1 ) is a large transmembrane protein found in the mitochondrion and is the terminal electron acceptor in the electron transfer chain, taking 4 reducing equivalents from cytochrome c and converting molecular ox, and cytochrome reductaseThe Coenzyme Q cytochrome c reductase complex, sometimes called the cytochrome bc complex and at other times Complex III, is the third complex in the electron transfer chain ( PDB , EC 1. It is a transmembrane lipoprotein, and it catalyzes the reduction o) to pumpA proton pump is an integral membrane protein that is capable of moving protons across the membrane of a cell, mitochondrion, or other subcellular compartment, thereby creating a difference or gradient in both pH and electrical charge (ignoring difference H⁺For alternative meanings see proton (disambiguation). Proton Classification Subatomic particle Fermion Hadron Baryon Nucleon Proton Properties Mass: 938 MeV/ c2 Electric Charge: 1. 6 × 10−19 C Spin: 1/2 In physics, the proton is a subatomic particle across the membrane against a proton gradientMultivariate calculus In vector calculus, gradient is a vector-valued operator that acts on a scalar field. The gradient of a scalar field is a vector field called gradient field which shows its rate and direction of change. For example, consider a room..

A large protein complex called ATP synthaseAn ATP synthase is a general term for an enzyme that can synthesize adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate by utilizing some form of energy. The overall reaction sequence is: :ADP + P → ATP These enzymes is embedded in that membrane and enables protons to pass through in both directions; it generates ATPFor other uses of the initials ATP, see ATP (disambiguation Adenosine triphosphate ATP is the nucleotide known in biochemistry as the " molecular currency" of intracellular energy transfer; that is, ATP is able to store and transport chemical energy withi when the proton moves with (down) the gradient, and it costs ATP to pump a proton against (up) the gradient. Because protons have already been pumped into the intermembrane space against the gradient, they now can flow back into the mitochondrial matrix via the ATP synthase, generating ATP in the process. The reaction is:

ADP^3- + H⁺ + P_i ↔ ATP^4- + H₂O

Each NADH molecule contributes enough proton motive force to generate 3 ATP. Each FADH₂ molecule is worth 2 ATP. All together, the 10 NADH and 2 FADH₂ molecules contributed through oxidation of glucose ( glycolysis, conversion of pyruvate to acetyl-CoA, and the Krebs cycle) account for 34 of the 38 total ATP energy carrier molecules. It is worth noting that these ATP values are maximums, in reality each NADH molecule contributes between 2 and 3 ATP, while each FADH₂ contributes a maximum of 2 ATP.