Respiration in biology is the energy‑generating process in which cells oxidize substrates and transfer electrons through membrane‑bound carriers to a terminal electron acceptor, generating ATP.
Mechanisms and Variants
During respiration, reduced cofactors such as NADH and FADH₂ donate electrons to an electron transport chain embedded in a membrane. As electrons flow through a series of carriers—including flavoproteins, iron–sulfur proteins, quinones and cytochromes—protons are pumped across the membrane, creating an electrochemical gradient. ATP synthase uses this proton motive force to phosphorylate ADP to ATP. In aerobic respiration, molecular oxygen serves as the final electron acceptor and is reduced to water; this pathway yields high ATP per molecule of glucose and is typical of many bacteria, archaea and all eukaryotic mitochondria. Anaerobic respiration uses alternative acceptors such as nitrate, sulfate, ferric iron or carbon dioxide; although less energetic, these pathways sustain growth in oxygen‑limited environments. Facultative anaerobes like Escherichia coli can switch between aerobic respiration and anaerobic respiration or fermentation depending on the availability of oxygen and suitable acceptors. Obligate anaerobes rely exclusively on non‑oxygen acceptors, whereas microaerophiles require low oxygen concentrations. The chemiosmotic theory, proposed by Peter Mitchell, explains how proton gradients drive ATP synthesis in both prokaryotes and mitochondria.
Respiration Strategies
The diversity of respiratory metabolism reflects ecological adaptations. Denitrifying bacteria reduce nitrate to nitrogen gas, contributing to the nitrogen cycle. Sulfate‑reducing bacteria convert sulfate to hydrogen sulfide in anaerobic sediments, while iron‑oxidizing and iron‑reducing microorganisms cycle ferric and ferrous iron in soils and mines. Methanogenic archaea use carbon dioxide and hydrogen to produce methane under strictly anoxic conditions. Even in human physiology, aerobic respiration in mitochondria supplies most cellular ATP; during intense exercise, when oxygen is limited, cells switch to fermentation to regenerate NAD⁺, illustrating the interplay between respiration and other pathways.
Respiration is central to cellular energy conservation and influences global biogeochemical cycles. Understanding its mechanisms provides insight into microbial ecology, bioenergy production and human physiology.
Related Terms: Oxidative Phosphorylation, Electron Transport Chain, Aerobic Respiration, Anaerobic Respiration, Fermentation