The central metabolic pathway of aerobic metabolism primarily involves cellular respiration, which includes several key stages that convert glucose and other substrates into adenosine triphosphate (ATP), the energy currency of the cell. The main stages of aerobic metabolism are:
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Glycolysis: This process occurs in the cytoplasm and breaks down glucose (a six-carbon sugar) into two molecules of pyruvate (three-carbons each). This pathway produces a small amount of ATP (2 ATP molecules per glucose molecule) and reduces nicotinamide adenine dinucleotide (NAD+) to NADH.
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Pyruvate Oxidation: The pyruvate produced in glycolysis is transported into the mitochondria, where it is converted into acetyl-CoA by the pyruvate dehydrogenase complex. Carbon dioxide is released in this process, and more NADH is generated.
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Citric Acid Cycle (Krebs Cycle): The acetyl-CoA enters the citric acid cycle, which takes place in the mitochondrial matrix. Through a series of enzymatic reactions, acetyl-CoA is oxidized, leading to the production of additional NADH and flavin adenine dinucleotide (FADH2), as well as GTP (or ATP), and more carbon dioxide is released.
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Electron Transport Chain (ETC): The NADH and FADH2 produced in the earlier stages donate electrons to the electron transport chain, which is located in the inner mitochondrial membrane. As electrons move through a series of protein complexes, they release energy that is used to pump protons (H+) across the membrane, creating a proton gradient.
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Oxidative Phosphorylation: The protons flow back into the mitochondrial matrix through ATP synthase, a process driven by the proton gradient, and this flow produces ATP from ADP and inorganic phosphate. Oxygen serves as the final electron acceptor in the chain, combining with electrons and protons to form water.
Together, these processes efficiently convert the energy stored in glucose into ATP, with the complete oxidation of one glucose molecule yielding up to approximately 30-32 ATP molecules, depending on the cell type and conditions. Aerobic metabolism is highly efficient and is crucial for the energy needs of most eukaryotic cells.
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