The appearance of oxygen in the Earth's atmosphere about 2.4 billion years ago (Great Oxidation) allowed the selection and development of specialized cellular structures and multicellular organisms. The energy efficiency offered by aerobic respiration (which uses oxygen) made this development possible.
Dioxygen (O₂), the molecular form of gaseous oxygen that we breathe, is essential to most living organisms because it plays a central role in the production of cellular energy. This process, called Aerobic Cellular Respiration, mainly occurs in the mitochondria of eukaryotic cells.
Without oxygen, our cells could not efficiently produce ATP (adenosine triphosphate), the universal energy molecule that powers all biological functions.
Oxygen is the final electron acceptor in the electron transport chain located in the inner membrane of the mitochondria. Oxygen therefore acts as the final electron acceptor in the respiratory chain, allowing the production of energy from Adenosine Triphosphate (ATP) molecules.
The production of ATP (C₁₀H₁₆N₅O₁₃P₃) from glucose (C₆H₁₂O₆) and dioxygen (O₂) is a complex process called aerobic cellular respiration (30-38 ATP produced by glycolysis). Without oxygen, energy yield drops significantly (only 2 ATP by glycolysis).
This transformation is the key to the energy metabolism of aerobic organisms!
Glucose (C₆H₁₂O₆) is a chemical structure rich in electrons (12 oxidizable C-H bonds). During their metabolic breakdown, the bonds of glucose are broken, easily releasing electrons for an acceptor like oxygen.
The electrons transferred to oxygen (O₂), via the mitochondrial respiratory chain, generate a proton gradient (H⁺). The flow of H⁺ powers the ATP synthase. O₂ ensures the removal of e⁻ and the maintenance of the gradient.
ATP (C₁₀H₁₆N₅O₁₃P₃)
A lack of oxygen (hypoxia) quickly leads to cellular dysfunction. After just a few minutes without oxygen, brain cells begin to die, which is why asphyxiation is so rapidly fatal.
Multicellular obligate anaerobes are extremely rare, as most multicellular beings depend on aerobic respiration for their high energy demand. However, a few notable exceptions exist, mainly in extreme environments with low oxygen, such as Henneguya salminicola (Salmon parasite), Loricifera (Anoxic ecosystems of the deep sea), Deep Sediment Nematodes, Anaerobic Fungi.
