Dioxygen in biological reactions
photolysis (light-driven oxidation and splitting) of water during photosynthesis in cyanobacteria, green algae, and plants. During oxidative phosphorylation in cellular respiration, oxygen is reduced to water, thus closing the biological water-oxygen redox cycle.
Photosynthesis
In nature, free oxygen is produced by the light-driven
splitting of water during oxygenic photosynthesis. Green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on earth.[1][need quotation to verify] The remainder is produced by terrestrial plants, although, for example, almost all oxygen produced in tropical forests is consumed by organisms living there.[2]
A simplified overall formula for photosynthesis is:[3]
- 6CO
2 + 6H
2O + photons → C
6H
12O
6 + 6O
2
- 6CO
(or simply carbon dioxide + water + sunlight → glucose + oxygen)
Photolytic oxygen evolution during photosynthesis occurs via the light-dependent oxidation of water to molecular oxygen and can be written as the following simplified chemical reaction: 2H2O → 4e− + 4H+ + O2
The reaction occurs in the
redox reactions onto plastoquinone.[4] Photosystem II therefore has also been referred to as water-plastoquinone oxido-reductase.[5]
The protons split off from the water molecules 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 couples the absorption of light energy and photolysis of water to the creation of chemical energy during photosynthesis.[4]
The O2 remaining after oxidation of the water molecule is released into the atmosphere.
Water oxidation is catalyzed by a
oxygen evolving complex (OEC) or water-splitting complex found associated with the lumenal side of thylakoid membranes. Manganese is an important cofactor, and calcium and chloride are also required for the reaction to occur.[4]
Oxygen uptake and transport
In
vertebrates
, oxygen uptake is carried out by the following processes:
Following inhalation into the lungs, oxygen
molluscs and some arthropods) or hemerythrin (spiders and lobsters).[9][10][11] A liter of blood can dissolve 200 cc of oxygen gas, which is much more than water can dissolve.[9]
After being carried in blood to a body tissue in need of oxygen, O2 is handed off from the heme group to
Blood circulates back to the lungs and the process repeats.[12]
Aerobic respiration
Molecular oxygen, O2, is essential for
mitochondria to generate chemical energy in the form of adenosine triphosphate (ATP) during oxidative phosphorylation. The reaction for the aerobic respiration is essentially the reverse of photosynthesis, except that now there is a large release of chemical energy which is stored in ATP molecules (up to 38 ATP molecules are formed from one molecule of glucose
and 6 O2 molecules). The simplified version of this reaction is:
- C
6H
12O
6 + 6O
2 → 6CO
2 + 6H
2O + 2880 kJ/mol
- C
Reactive oxygen species
free radicals such as the hydroxyl radical (HO·), superoxide anion radical (O2-), hydrogen peroxide (H2O2), hydroperoxyl radical, nitric oxide (NO) and singlet oxygen.[13][9] The body uses superoxide dismutase to reduce superoxide radicals to hydrogen peroxide. Glutathione peroxidase and similar enzymes then convert the H2O2 to water and dioxygen.[9]
Parts of the
dihydrogen trioxide, also known as trioxidane, (HOOOH), which is an antibody-catalyzed product of singlet oxygen and water. This compound, in turn, disproportionates to ozone and peroxide, providing two powerful antibacterials. The body's range of defense against all of these active oxidizing agents is hardly surprising, then, given their "deliberate" employment as antimicrobial agents in the immune response.[14] Reactive oxygen species also play an important role in the hypersensitive response of plants against pathogen attack.[4]
See also
- Oxygen cycle
- Oxygen-haemoglobin dissociation curve
- Oxygen
- Apparent oxygen utilisation
- CO-oximeter
- Oxygen catastrophe
- Oxygen toxicity
- Reactive oxygen species
References
- ISBN 1-4289-2397-7.
- ^ Broeker, W.S. (2006). "Breathing easy, Et tu, O2". Columbia University. Retrieved 2007-10-21.
- ISBN 0-13-048450-4, p. 958
- ^ ISBN 0-7167-1007-2.
- ^
Raval M, Biswal B, Biswal U (2005). "The mystery of oxygen evolution: analysis of structure and function of photosystem II, the water-plastoquinone oxido-reductase". Photosynthesis Research. 85 (3): 267–93. S2CID 12893308.
- ISBN 0-12-352651-5.
- ^ CO2 is released from another part of the hemoglobin molecule, as its acid, which causes CO2 to be released from bicarbonate, its major reservoir in blood plasma (see Bohr effect)
- ^ Stwertka 1998, p. 48.
- ^ a b c d e Emsley 2001, p. 298.
- ^ Cook & Lauer 1968, p. 500.
- ^ Figures given are for values up to 50 miles above the surface
- ^ Emsley 2001, p. 303.
- PMID 35213291.
- ^
Hoffmann, Roald (2004). "The Story of O". American Scientist. 92 (1): 23. doi:10.1511/2004.1.23. Archived from the originalon 2007-02-22. Retrieved 2007-03-03.
- Emsley, John (2001). "Oxygen". Nature's Building Blocks: An A-Z Guide to the Elements. Oxford, England, UK: Oxford University Press. pp. 297–304. ISBN 0-19-850340-7.
- Cook, Gerhard A.; Lauer, Carol M. (1968). "Oxygen". In Clifford A. Hampel (ed.). The Encyclopedia of the Chemical Elements. New York: Reinhold Book Corporation. pp. 499–512. LCCN 68-29938.
- Stwertka, Albert (1998). Guide to the Elements (Revised ed.). Oxford University Press. ISBN 0-19-508083-1.