By Liz Veloz – Updated Aug 30, 2022
Cellular respiration is the cornerstone of cellular energy production. By oxidizing glucose into carbon dioxide and water, cells generate adenosine triphosphate (ATP), the universal energy currency. Oxygen is the final electron acceptor, making respiration a controlled “burning” reaction that releases usable energy.
Every cell relies on ATP to perform life‑sustaining functions. If cells didn’t continually replenish ATP through respiration, we would exhaust almost our entire body weight in ATP within a single day.
Cellular respiration unfolds in three tightly regulated phases: glycolysis, the citric acid (Krebs) cycle, and oxidative phosphorylation.
Enzymes are specialized proteins that accelerate chemical reactions without being consumed. Each step of respiration is orchestrated by a distinct set of enzymes that facilitate the transfer of electrons—redox reactions—in which one molecule is oxidized and another reduced.
The first phase occurs in the cytoplasm and comprises nine enzyme‑catalyzed reactions. Key players include the dehydrogenase family and the coenzyme NAD⁺. Dehydrogenases oxidize glucose, stripping two electrons and transferring them to NAD⁺, which becomes NADH. This process cleaves glucose into two pyruvate molecules that proceed to the next stage.
In the mitochondria—often called the cell’s power plants—pyruvate is converted into acetyl‑CoA, a high‑energy substrate. Mitochondrial enzymes then drive a series of reactions that rearrange bonds and perform additional redox transfers. Each turn of the cycle yields NADH, FADH₂, and a small amount of ATP, and it releases CO₂ as a waste product.
The final step takes place across the inner mitochondrial membrane. Oxygen acts as the terminal electron acceptor, driving a chain of electron carriers. The resulting proton gradient powers ATP synthase, producing up to 38 ATP molecules per glucose molecule—a remarkable efficiency for a biological system.
