During cellular respiration, the chemical bonds in organic molecules, primarily glucose, are broken down to release energy, which the cell uses to produce ATP (adenosine triphosphate). The overall process can be summarized in several key stages, primarily occurring in the mitochondria of eukaryotic cells:
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Glycolysis: This occurs in the cytoplasm where one molecule of glucose (a six-carbon sugar) is broken down into two molecules of pyruvate (three-carbon compounds). During this process, a small amount of ATP is produced, and high-energy electron carriers (NADH) are generated.
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Pyruvate Oxidation: The pyruvate produced in glycolysis is transported into the mitochondria, where it is converted into acetyl-CoA. This process releases CO2 and generates NADH from NAD+.
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Citric Acid Cycle (Krebs Cycle): Acetyl-CoA enters the citric acid cycle, where it is further broken down. This cycle generates additional NADH and FADH2 (another electron carrier), and it releases CO2 as a byproduct.
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Oxidative Phosphorylation: This is the final stage of cellular respiration, occurring in the inner mitochondrial membrane. Here, NADH and FADH2 donate electrons to the electron transport chain, which ultimately leads to the production of a large amount of ATP. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water.
Throughout this process, covalent bonds in glucose and other substrates are broken to release energy, which is captured in the form of ATP. The breakdown products, such as carbon dioxide and water, are then exhaled or expelled from the cell as waste.