The origin of photosynthesis
The first cells fed on organic substances in the primordial soup as its concentration gradually diminished. It is likely that the scarcity of energy resources imposed a selection. Some cells acquired the ability to feed on others, while other cells developed the ability to synthesize new organic substances by using energy from oxidation. Even today there still are prokaryotes (cells that don’t have an actual nucleus but a nuclear “equivalent”) that draw energy in this way to live, they are the so called chemosynthetic bacteria. Other cells instead could exploit energy from light to transform water and obtain the hydrogen necessary for photosynthesis (a reaction that can transform simple inorganic substances into organic ones such as carbohydrates thanks to the energy given by light). Living organisms that are able to perform photosynthesis however released simple oxygen which, thanks to its high affinity with organic substances, must have killed most primitive cell forms. Only those cells that could handle the increasing oxygen concentration survived. Subsequently some prokaryotes learned to use free oxygen as an oxidant for energy production. This is how respiration appeared bringing with it the great advantage of being a more efficient energy stocking method than fermentation and guaranteeing survival in the oxygen rich atmosphere.
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