<P> The citric acid cycle is an 8 - step process involving 18 different enzymes and co-enzymes . During the cycle, acetyl - CoA (2 carbons) + oxaloacetate (4 carbons) yields citrate (6 carbons), which is rearranged to a more reactive form called isocitrate (6 carbons). Isocitrate is modified to become α - ketoglutarate (5 carbons), succinyl - CoA, succinate, fumarate, malate, and, finally, oxaloacetate . </P> <P> The net gain of high - energy compounds from one cycle is 3 NADH, 1 FADH, and 1 GTP; the GTP may subsequently be used to produce ATP . Thus, the total yield from 1 glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH, and 2 ATP . </P> <P> In eukaryotes, oxidative phosphorylation occurs in the mitochondrial cristae . It comprises the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the boundary of inner membrane by oxidizing the NADH produced from the Krebs cycle . ATP is synthesized by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP . The electrons are finally transferred to exogenous oxygen and, with the addition of two protons, water is formed . </P> <P> The table below describes the reactions involved when one glucose molecule is fully oxidized into carbon dioxide . It is assumed that all the reduced coenzymes are oxidized by the electron transport chain and used for oxidative phosphorylation . </P>

Where does water come from in cell respiration
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