We have a pretty good idea of what life would
be without oxygen because life started out
without oxygen and there are still organisms
that don’t need it. There are still many species
of bacteria, fungi, and other single-celled
organisms that don’t need oxygen, or oxygen is
even toxic to them. We need oxygen to make a lot
of energy. Oxygen doesn’t actually create
energy, since energy is conserved, but instead
makes a high-energy compounds that can be used
for functions later. Organisms, can still make
energy without oxygen, but they only make about
one tenth as much energy per sugar molecule.
The reason oxygen is so useful for making
energy is that it strongly attracts electrons.
Therefore, as electrons
move from sugars to oxygen, they create energy, in
a sense like how water going over a water wheel
makes electricity. Since sugars have multiple
carbons, the most “biologically broken up” they
can be is into CO2 since that has only
one carbon. Therefore, having oxygen allows an
organism to break up its
carbon containing sugars as much as possible to
extract as much energy as possible. So large
organisms, complex organisms generally require
more energy so they need to use oxygen. Without
oxygen, all organisms would likely be microscopic
and not very complex.
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