Cellular respiration is the metabolic process by which organisms convert biochemical energy from nutrients, such as glucose, into adenosine triphosphate (ATP), the primary energy source for cellular activities. This process is generally divided into four main stages: glycolysis, which breaks down glucose into pyruvate in the cytoplasm; pyruvate oxidation; the Krebs cycle (or citric acid cycle), which completes the oxidation of the carbons; and finally, oxidative phosphorylation, which occurs across the inner mitochondrial membrane. Oxygen acts as the final electron acceptor, and the electron transport chain utilizes the released energy to pump protons, generating the electrochemical gradient necessary for ATP synthesis. The overall net reaction equation can be summarized as the combustion of glucose, yielding carbon dioxide, water, and substantial amounts of usable energy.