Oxygen (O2) is approximately 21% of the Earth's atmosphere, but this has not always been the case: the primitive Earth's atmosphere contained only small amounts of this compound, probably as a result of the reaction of sunlight with steam water from volcanoes. One only could find oxygen in water molecules (H2O) or attached to the forming iron oxides, but never freely. The oxygen-rich atmosphere of today (eon up, eon below) is the result of one of the deadliest weather disasters in the history of our planet.
But then came on the scene cyanobacteria, bacteria capable of performing photosynthesis with the help of sunlight, water and carbon dioxide (CO2), they obtained energy from carbohydrates and, yes, oxygen as a waste product. While the first molecules of free oxygen are produced by photosynthesis of cyanobacteria, they were absorbed by the dissolved iron in the oceans, forming iron oxide (Fe2O3 and Fe3O4), which were deposited as rocks in the ocean, giving it a dark red colour. When there is no longer enough iron to react with oxygen to form oxides, oxygen began to accumulate in the atmosphere. The time interval between the appearance of cyanobacteria and the actual oxygenation of the atmosphere is known as the Great Oxidation.
The oxygen released into the atmosphere was toxic to anaerobic organisms and increasing concentrations could have killed most of the inhabitants of the Earth. They did not disappear but were relegated to low oxygen environments such as the ocean and stopped being the dominant life form. Cyanobacteria were therefore responsible for one of the most significant extinctions in Earth history.
Over time, organisms began to evolve and to consume oxygen, becoming aerobic and since then the free oxygen has been a major component of the atmosphere. Not only for microscopic life forms, but also for us because when we breathe, atmospheric oxygen enters our bodies.
When we breathe, the air passes through the larynx and trachea into the lungs, where there is a labyrinth of ever smaller bronchial tubes until it finally reaches the alveoli. These tiny air bags are crucial in the process, allowing oxygen to pass from the air to the blood through the membrane, filled with blood capillaries. At this point, oxygen molecules pass from gaseous state to be dissolved in the blood, where they are taken by red blood cells, the vehicle will transport it throughout the body.
Red blood cells have a protein that serves to bind oxygen, hemoglobin. Heme prefix comes from non-protein component present in its structure consisting of a ferrous ion (Fe2+) in the center of a porphyrin molecule. Each hemoglobin has four heme groups, which can carry four oxygen molecules.
Blood just oxygenated return to the heart through the pulmonary veins which get into the left atrium and then to the left ventricle. Now it is when the heart really squeezes: as the left ventricle contracts, oxygenated blood is pumped to the body's main artery, the aorta. This branches into smaller arteries, arterioles, ending in capillaries. These small blood vessels are surrounded by a thin membrane, allowing exchange substances between the blood and other substances, in this case oxygen for carbon dioxide as a waste product of cell metabolism.
|Gas exchange in an alveolus.|
Thus, oxygen enters every cells’ body, where it is used by mitochondria, their final destination, to generate the energy needed for proper cell function and so you may be reading this post, scratching your head or just breathing oxygen that once were produced by some bacteria.