Cellular Respiration
The metabolic process by which cells harvest energy from glucose as ATP through glycolysis, the citric acid cycle, and oxidative phosphorylation.
Cellular respiration is the set of metabolic reactions by which cells extract energy from organic molecules — most commonly glucose — and store it in ATP, the universal energy currency of life. The overall reaction is the reverse of photosynthesis: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy, with roughly 30–32 molecules of ATP produced per glucose under ideal conditions. The process has three main stages.
Glycolysis occurs in the cytoplasm and needs no oxygen: one glucose (6 carbons) is split into two molecules of pyruvate (3 carbons each), producing a net 2 ATP and 2 NADH. This ancient pathway is nearly universal — it works in bacteria, plants, and animals, evidence of its deep evolutionary origin. Under anaerobic conditions, pyruvate is converted to lactate (in animals) or ethanol (in yeast) to regenerate NAD⁺; this fermentation yields only the 2 ATP of glycolysis.
When oxygen is available, pyruvate enters the mitochondrion, is converted to acetyl-CoA, and feeds the citric acid cycle, which oxidizes the carbon skeleton to CO₂ while harvesting NADH and FADH₂. These reduced carriers then deliver electrons to the electron transport chain embedded in the inner mitochondrial membrane. As electrons pass down the chain, the released energy pumps protons across the membrane; the resulting gradient drives ATP synthase, which couples the flow of protons back across the membrane to ATP production — a mechanism called chemiosmosis, proposed by Peter Mitchell in 1961.
The electron transport chain is where most of the ATP is made (about 26–28 of the 30–32) and where molecular oxygen acts as the final electron acceptor, forming water. Because the chain is driven by redox reactions, respiration is ultimately an oxidation–reduction process. Its efficiency — about 34–40% of the chemical energy of glucose captured as ATP — is high for a biological machine, and the rest is released as heat, which is why living tissues stay warm.
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atp biochemistry metabolism mitochondria
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