By Kevin Beck — Updated Mar 24, 2022
Cellular respiration is the biochemical cascade that powers eukaryotic cells by converting glucose into adenosine triphosphate (ATP). The process comprises four distinct stages: glycolysis, the link (bridge) reaction, the Krebs cycle, and the electron transport chain (ETC). While glycolysis can proceed without oxygen, the remaining three stages are strictly aerobic and take place inside mitochondria.
Prokaryotic cells lack membrane-bound organelles, so all of their glucose metabolism occurs in the cytoplasm. In contrast, eukaryotic cells compartmentalize the final three stages within mitochondria, enabling efficient ATP production.
Only the link reaction, Krebs cycle, and ETC are mitochondrial events. Glycolysis— the splitting of glucose into two pyruvate molecules—takes place entirely in the cytoplasm.
Mitochondria are double‑membrane organelles. Their inner membrane folds into cristae, creating the site of the ETC. The matrix, the space within the inner membrane, hosts the Krebs cycle and houses mitochondrial DNA, which is inherited maternally.
Glycolysis: Cytoplasm – Ten enzyme‑catalysed reactions convert one glucose molecule into two pyruvate molecules, producing a net gain of two ATP and two NADH. This step is anaerobic.
Link (Bridge) Reaction: Mitochondrial Matrix – Each pyruvate is decarboxylated to form acetyl‑CoA, releasing CO₂ and producing one NADH per pyruvate.
Krebs Cycle: Mitochondrial Matrix – Acetyl‑CoA condenses with oxaloacetate to form citrate. Over a series of reactions, citrate is oxidized back to oxaloacetate, generating two ATP (one per acetyl‑CoA), three NADH, and one FADH₂.
Electron Transport Chain: Inner Mitochondrial Membrane – NADH and FADH₂ donate electrons to the ETC, driving the synthesis of 32–34 ATP via oxidative phosphorylation. The proton motive force created across the inner membrane drives ATP synthase.
Collectively, cellular respiration yields 36–38 ATP per glucose molecule—approximately two from glycolysis and 34–36 from the mitochondrial stages.
