Molecular Oxygen

∞ generated and posted on 2016.01.29 ∞

O2, that is, the pure form of this element as it most commonly exists on Earth.

Also described simply as oxygen, molecular oxygen is a gas at ambient temperatures found on Earth. It is more soluble in water at lower versus higher temperatures. Furthermore, molecular oxygen readily dissolves in lipid bilayers so therefore can cross membranes via passive diffusion rather than requiring protein facilitation (it, however, is less soluble in lipid bilayers than carbon dioxide). Molecular oxygen represents the most oxidized of the common forms of oxygen as well as a somewhat stable form of oxidized oxygen.

Molecular oxygen is used in oxidative phosphorylation as the final electron acceptor for the involve electron transport chain and as such is quite effective at receiving such otherwise energy-spent electrons. In the course of serving this role, molecular oxygen receives four electrons and four protons, thereby becoming reduced to four molecules of water (so-called metabolic water). In the reverse reaction water donates electrons, particularly to photosystem II found in cyanobacteria and chloroplasts, resulting in the generation of molecular oxygen by this oxygenic process.

Molecular oxygen serves as a poison for organisms that do not possess protective mechanisms against its properties as an oxidizer. Such microorganisms are described as strict or obligate anaerobes. Strict or obligate anaerobes predate organisms that are able to survive in an oxygen-rich atmosphere or fluids since it was only upon the invention of modern oxygenic photosynthesis, by cyanobacteria, that appreciable amounts of molecular oxygen because available in environments. Prior to this point more or all of Earth's environments were anaerobic.