The Significance of Lipids for the Absorption and Release of Oxygen in Biological Organisms.

Abstract

A critically important step for the uptake and transport of oxygen (O2) in living organisms is the crossing of the phase boundary between gas (or water) and lipid/proteins in the cell. Classically, this transport across the phase boundary is explained as a transport by proteins or protein-based structures. In our contribution here, we want to show the significance of passive transport of O2 also (and in some cases probably predominantly) through lipids in many if not all aerobic organisms. In plants, the significance of lipids for gas exchange (absorption of CO2 and release of O2) is well recognized. The leaves of plants have a cuticle layer as the last film on both sides formed by polyesters and lipids. In animals, the skin has sebum as its last layer consisting of a mixture of neutral fatty esters, cholesterol and waxes which are also at the border between the cells of the body and the air. The last cellular layers of skin are not vascularized therefore their metabolism totally depends on this extravasal O2 absorption, which cannot be replenished by the bloodstream. The human body absorbs about 0.5% of O2 through the skin. In the brain, myelin, surrounding nerve cell axons and being formed by oligodendrocytes, is most probably also responsible for enabling O2 transport from the extracellular space to the cells (neurons). Myelin, being not vascularized and consisting of water, lipids and proteins, seems to absorb O2 in order to transport it to the nerve cell axon as well as to perform extramitochondrial oxidative phosphorylation inside the myelin structure around the axons (i.e., myelin synthesizes ATP) - similarly to the metabolic process occurring in concentric multilamellar structures of cyanobacteria. Another example is the gas transport in the lung where lipids play a crucial role in the surfactant ensuring incorporation of O2 in the alveoli where there are lamellar body and tubular myelin which form multilayered surface films at the air-membrane border of the alveolus. According to our view, the role played by lipids in the physical absorption of gases appears to be crucial to the existence of many, if not all, of the living aerobic species.