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.
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| Aspect of the Earth in the Archean, before the atmosphere had free oxygen. |
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.
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| Cyanobacteria, also known as blue-green algae. |
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.
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| 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.
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