<P> In lactic acid fermentation, each pyruvate molecule is directly reduced by NADH . The only byproduct from this type of fermentation is lactate . Lactic acid fermentation is used by human muscle cells as a means of generating ATP during strenuous exercise where oxygen consumption is higher than the supplied oxygen . As this process progresses, the surplus of lactate is brought to the liver, which converts it back to pyruvate . </P> <P> If oxygen is present, then following glycolysis, the two pyruvate molecules are brought into the mitochondrion itself to go through the Krebs cycle . In this cycle, the pyruvate molecules from glycolysis are further broken down to harness the remaining energy . Each pyruvate goes through a series of reactions that converts it to acetyl coenzyme A. From here, only the acetyl group participates in the Krebs cycle--in which it goes through a series of redox reactions, catalyzed by enzymes, to further harness the energy from the acetyl group . The energy from the acetyl group, in the form of electrons, is used to reduce NAD+ and FAD to NADH and FADH, respectively . NADH and FADH contain the stored energy harnessed from the initial glucose molecule and is used in the electron transport chain where the bulk of the ATP is produced . </P> <P> The last process in aerobic respiration is oxidative phosphorylation, also known as the electron transport chain . Here NADH and FADH, which contain the energy in the form of electrons, deliver their electrons to the inner membranes of the mitochondrion to power the production of ATP . Oxidative phosphorylation contributes the majority of the ATP produced, compared to glycolysis and the Krebs cycle . While the ATP count is glycolysis and the Krebs cycle is two ATP molecules, the electron transport chain contributes, at most, twenty - eight ATP molecules . A contributing factor is due to the energy potentials of NADH and FADH . As they are brought from the initial process, glycolysis, to the electron transport chain, the energy stored in them are now utilized . A second contributing factor is that cristae, the inner membranes of mitochondria, increase the surface area and therefore the amount of proteins in the membrane that assist in the synthesis of ATP . Along the electron transport chain, there are separate compartments, each with their own concentration gradient of H + ions, which are the power source of ATP synthesis . To convert ADP to ATP, energy must be provided . That energy is provided by the H+ gradient . On one side of the membrane compartment, there is a high concentration of H+ ions compared to the other . The shuttling of H+ to one side of the membrane is driven by the exergonic flow of electrons throughout the membrane . These electrons are supplied by NADH and FADH as they transfer their potential energy . Once the H+ concentration gradient is established, a proton - motive force is established, which provides the energy to convert ADP to ATP . The H+ ions that were initially forced to one side of the mitochondrion membrane now naturally flow through a membrane protein called ATP synthase, a protein that converts ADP to ATP with the help of H+ ions . </P>

The majority of the atp molecules derived from the breakdown of glucose are generated by