<P> Most of the ATP synthesized in the mitochondria will be used for cellular processes in the cytosol; thus it must be exported from its site of synthesis in the mitochondrial matrix . The inner membrane contains an antiporter, the ADP / ATP translocase, which is an integral membrane protein used to exchange newly synthesized ATP in the matrix for ADP in the intermembrane space . This translocase is driven by the membrane potential, as it results in the movement of about 4 negative charges out of the mitochondrial membrane in exchange for 3 negative charges moved inside . However, it is also necessary to transport phosphate into the mitochondrion; the phosphate carrier moves a proton in with each phosphate, partially dissipating the proton gradient . </P> <P> The citric acid cycle is regulated mainly by the availability of key substrates, particularly the ratio of NAD to NADH and the concentrations of calcium, inorganic phosphate, ATP, ADP, and AMP . Citrate--the molecule that gives its name to the cycle--is a feedback inhibitor of citrate synthase and also inhibits PFK, providing a direct link between the regulation of the citric acid cycle and glycolysis . </P> <P> In the presence of air and various cofactors and enzymes, fatty acids are degraded to acetyl - CoA . The pathway is called beta - oxidation . Each cycle of beta - oxidation shortens the fatty acid chain by two carbon atoms and produces one equivalent each of NADH and one FADH . The NADH and FADH are used to generate ATP by oxidative phosphorylation . Dozens of ATP equivalents are generated by the beta - oxidation of a single long acyl chain . The acetyl - CoA produced by beta - oxidation can be subsequently metabolized by the citric acid cycle, yielding further equivalents of ATP . </P> <P> In oxidative phosphorylation, the key control point is the reaction catalyzed by cytochrome c oxidase, which is regulated by the availability of its substrate--the reduced form of cytochrome c . The amount of reduced cytochrome c available is directly related to the amounts of other substrates: </P>

Why is atp the main source of energy