Oxygen

colorless, tasteless, odorless gas that constitutes 21 per cent of the atmosphere and is required for the utilisation of energy in aerobic organisms

Description
In its molecular dioxygen form, O2, oxygen is essential for cellular respiration in all aerobic organisms C6H12O6 + 6 O2 → 6 CO2 + 6 H2O [+ 2880 kJ x mol-1] and is used in mitochondria during oxidative phosphorylation for the generation of adenosine triphosphate (ATP).

In vertebrates, O2 is taken up through the internally enlarged surfaces of the lungs, whose alveolar membranes represent a thin barrier between the environment and the blood. Oxygen is bound by the hemoglobin of erythrocytes, which change their color from purple to red upon oxygen binding, and is transported systemically with the blood stream to drive aerobic processes. Impaired oxygen binding and transport can be induced chemically, e.g., by competition of carbon monoxide for hemoglobin, leading to asphyxiation and occurs genetically in most hemoglobinopathies, leading to pathologicial proliferation of erythrocytes and other associated effects.