From the Atmosphere to the Mitochondrion: The Oxygen Cascade

Oxygen is essential for energy metabolism whereby the energy trapped in chemical bonds in food stuffs is transferred in a series of stepwise chemical reactions to be available for all cellular functions in the form of high-energy phosphate compounds (adenosine triphosphate and creatine phosphate). Whereas there are energy-generating processes that do not require oxygen, their contribution to the total energy homeostasis is minimal and is of limited applicability. Food stuffs in the form of carbohydrates and fats are utilized by glycolysis, fatty acid oxidation and the citric acid (Krebs) cycle to generate initially the high energy phosphate compounds NADH and FADH in mitochondria. These are energy-rich because they contain a pair of electrons with high transfer potential. When these electrons are transferred to oxygen, energy is liberated. This energy is trapped with high efficiency in the form of ATP (adenosine triphosphate) by the process of oxidative phosphorylation. ATP is used in an enormous variety of biological processes comprising both maintenance functions at the basal level (e.g. maintenance of cellular ion channels and gradients) and in energy consuming processes involving external work (such as muscular exercise). Because of the diversity of the intensity of the processes involving external work of the organism, a wide range of energy requirements need to be met. Hence, the oxygen transport system needs to deliver oxygen to organs and cells at rates that accommodate the widely varying metabolic requirements. The requirements of the oxygen transport system from the atmosphere to all organs, tissues, cells and mitochondria include (Pennock and Attinger 1968):