respiration

Respiration broadly refers to the processes by which cells obtain energy from nutrients and the gas exchange between an organism and its environment. In cellular respiration, cells oxidize glucose using oxygen to produce ATP, with carbon dioxide and water as byproducts. The overall equation is glucose plus oxygen yields carbon dioxide plus water and energy.

Most cellular respiration takes place in mitochondria and proceeds through glycolysis in the cytoplasm, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation via the electron transport chain. Glycolysis yields a small amount of ATP and NADH; the later stages generate most ATP by transferring electrons and driving a proton-motive force.

When oxygen is scarce, cells may rely on anaerobic pathways. In animals, lactic acid fermentation regenerates NAD+ to sustain glycolysis, yielding lactate. In yeast and some bacteria, alcoholic fermentation produces ethanol and carbon dioxide. These pathways make far less ATP than aerobic respiration.

Physiological respiration describes breathing and gas exchange. Air is inhaled into lungs or other exchanges sites, where oxygen diffuses into blood and carbon dioxide diffuses out. In blood, oxygen is carried by hemoglobin; carbon dioxide is transported mainly as bicarbonate. Gas exchange occurs across thin barriers in moist tissues.

Breathing is regulated by the brainstem, which adjusts rate and depth in response to carbon dioxide, pH, and other signals. Physical activity, illness, and environmental conditions influence ventilation.

Across life, respiration is adapted to habitats and biology; some organisms use gills or skin for gas exchange, others rely on lungs or tracheae. Cellular respiration is universal among aerobic organisms.