Everything about Oxygen Evolution totally explained
Oxygen evolution is the process of generating molecular
oxygen through
chemical reaction. Mechanisms of oxygen evolution include the
photolysis of
water during oxygenic
photosynthesis,
electrolysis of water into oxygen and
hydrogen, and electrocatalytic oxygen evolution from
oxides and
oxoacids.
Oxygen evolution in nature
Photolytic oxygen evolution is the fundamental process by which breathable oxygen is generated in earth's
biosphere. The reaction is part of the
light-dependent reactions of photosynthesis in
cyanobacteria and the
chloroplasts of
green algae and
plants. It utilizes the energy of
light to split a water molecule into its
protons and
electrons for photosynthesis. Free oxygen is generated as a waste product of this reaction, and is released into the
atmosphere.
Biochemical reaction
Photolytic oxygen evolution occurs via the light-dependent oxidation of water to molecular oxygen and can be written as the following simplified chemical reaction:
2H
2O
4e
- + 4H
+ + O
2
The reaction requires the energy of four
photons. The electrons from the oxidized water molecules replace electrons in the P
680 component of
photosystem II which have been removed into an
electron transport chain via light-dependent
excitation and
resonance energy transfer onto
plastoquinone. Photosytem II therefore has also been referred to as water-plastoquinone oxido-reductase. The protons are released into the
thylakoid lumen, thus contributing to the generation of a proton gradient across the thylakoid membrane. This proton gradient is the driving force for
ATP synthesis via
photophosphorylation and coupling the absorption of light energy and photolysis of water to the creation of chemical energy during photosynthesis. Based on structural and
spectroscopic experiments, oxygen evolution involves a core three-plus-one cluster of three manganese
ions and one calcium ion, with one additional manganese, which are oxidized via intermediate states called
S-states. The O-O bond of molecular oxygen is formed between manganese-ligated oxygen atoms at the most oxidized, or S4, state.
Evolution of oxygen evolution
Oxygen production during
photosynthesis evolved on earth around 2.7 to 3.5 billion years ago. Oxygen wasn't only a waste product of this reaction, but was also toxic to many metabolic processes such as
nitrogen fixation. Consequently, it was released into the atmosphere as a means of
detoxification. This contributed to the conversion of earth's atmosphere from
anaerobic to its current
aerobic composition, triggering the
Oxygen Catastrophe and the evolution of
aerobic metabolism utilizing the oxygen that was released by photosynthetic organisms as part of the
oxygen cycle.
History of discovery
It wasn't until the end of the
18th century that
Joseph Priestley discovered by accident the ability of plants to "restore" air that had been "injured" by the burning of a candle. He followed up on the experiment by showing that air "restored" by
vegetation was
"not at all inconvenient to a mouse." He was later awarded a medal for his discoveries that:
"...no vegetable grows in vain... but cleanses and purifies our atmosphere." Priestley's experiments were followed up by
Jan Ingenhousz, a Dutch physician, who showed that "restoration" of air only worked in the presence of light and green plant parts.
Ingenhousz suggested in
1796 that CO
2 (
carbon dioxide) is split during photosynthesis to release oxygen, while the
carbon combined with water to form
carbohydrates. While this hypothesis was attractive and reasonable and thus widely accepted for a long time, it was later proven incorrect. Graduate student C.B. Van Niel at
Stanford University found that
purple sulfur bacteria reduce carbon to carbohydrates, but accumulate
sulfur instead of releasing oxygen. He boldly proposed that in analogy to the sulfur bacteria forming elemental sulfur from H
2S (
hydrogen sulfide), plants would form oxygen from H
2O (water). In
1937, this hypothesis was corroborated by the discovery that plants are capable of producing oxygen in the absence of CO
2. This discovery was made by
Robin Hill, and subsequently the light-driven release of oxygen in the absence of CO
2 was called the
Hill reaction. Our current knowledge of the mechanism of oxygen evolution during photosynthesis was further established in experiments tracing
isotopes of oxygen from water to oxygen gas.
Technological oxygen evolution
Oxygen evolution occurs as a byproduct of
hydrogen production via
electrolysis of water. While oxygen production isn't the main focus of industrial applications of water electrolysis, it becomes essential for
life support systems in situations that require the generation of oxygen for air revitalization. Human exploration of regions that lack breathable oxygen, such as the deep sea or outer space, requires means of reliably generating oxygen apart from earth's atmosphere.
Submarines and
spacecraft utilize either an
electrolytic mechanism (water or solid oxide electrolysis) or
chemical oxygen generators as part of their life support equipment.
Further Information
Get more info on 'Oxygen Evolution'.
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