Dioxygen in biological reactions

Last updated

Dioxygen (O
2) plays an important role in the energy metabolism of living organisms. Free oxygen is produced in the biosphere through 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.

Contents

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

(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 thylakoid membranes of cyanobacteria as well as algal and plant chloroplasts and requires the energy of four photons. The electrons extracted from the water molecules transfer to the electron-deficient high-energy state P680+ of the P680 pigment of Photosystem II, which have been removed into an electron transport chain after light-dependent excitation and a series of 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 manganese-containing enzyme complex known as the 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 all vertebrates, the heme group of hemoglobin binds most of the oxygen dissolved in the blood. Heme b.svg
In all vertebrates, the heme group of hemoglobin binds most of the oxygen dissolved in the blood.

In vertebrates, oxygen uptake is carried out by the following processes:

Following inhalation into the lungs, oxygen diffuses through alveoli into the serum of the blood, where some remains in direct relation to the partial pressure of gasses in the inhaled gas and the balance is bonded to red blood cells. They are bound to dioxygen complexes, which are coordination compounds that contain O2 as a ligand, [6] providing a higher oxygen-loading capacity. In blood, the heme group of hemoglobin binds oxygen when it is present, changing hemoglobin's color from bluish red to bright red. [7] [8] Vertebrate animals use hemoglobin in their blood to transport oxygen from their lungs to their tissues, but other animals use hemocyanin (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 monooxygenase, an enzyme that also has an active site with an atom of iron. [9] Monooxygenase uses oxygen for many oxidation reactions in the body. Oxygen that is suspended in the blood plasma equalizes into the tissue according to Henry's law. Carbon dioxide, a waste product, is released from the cells and into the blood, where it is converted to bicarbonate or binds to hemoglobin for transport to the lungs. Blood circulates back to the lungs and the process repeats. [12]

Aerobic respiration

Molecular oxygen, O2, is essential for cellular respiration in all aerobic organisms. Oxygen is used as an electron acceptor in 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

Reactive oxygen species

Reactive oxygen species are molecules containing at least one oxygen atom and one or more unpaired electrons. Reactive oxygen species (ROS) are by-products of cell aerobic respiration. Important examples include; oxygen 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 immune system of higher organisms, however, create peroxide, superoxide, and singlet oxygen to destroy invading microbes. Recently, singlet oxygen has been found to be a source of biologically-produced ozone: This reaction proceeds through an unusual compound 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

Related Research Articles

<span class="mw-page-title-main">Oxygen</span> Chemical element, symbol O and atomic number 8

Oxygen is a chemical element; it has symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds. Oxygen is the most abundant element in Earth's crust, and after hydrogen and helium, it is the third-most abundant element in the universe. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O
2. Diatomic oxygen gas currently constitutes 20.95% of the Earth's atmosphere, though this has changed considerably over long periods of time. Oxygen makes up almost half of the Earth's crust in the form of oxides.

<span class="mw-page-title-main">Photosynthesis</span> Biological process to convert light into chemical energy

Photosynthesis is a biological process used by many cellular organisms to convert light energy into chemical energy, which is stored in organic compounds that can later be metabolized through cellular respiration to fuel the organism's activities. The term usually refers to oxygenic photosynthesis, where oxygen is produced as a byproduct and some of the chemical energy produced is stored in carbohydrate molecules such as sugars, starch, glycogen and cellulose, which are synthesized from endergonic reaction of carbon dioxide with water. Most plants, algae and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the biological energy necessary for complex life on Earth.

In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula O−2. The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature. Molecular oxygen (dioxygen) is a diradical containing two unpaired electrons, and superoxide results from the addition of an electron which fills one of the two degenerate molecular orbitals, leaving a charged ionic species with a single unpaired electron and a net negative charge of −1. Both dioxygen and the superoxide anion are free radicals that exhibit paramagnetism. Superoxide was historically also known as "hyperoxide".

<span class="mw-page-title-main">Thylakoid</span> Membrane enclosed compartments in chloroplasts and cyanobacteria

Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as grana. Grana are connected by intergranal or stromal thylakoids, which join granum stacks together as a single functional compartment.

<span class="mw-page-title-main">Reactive oxygen species</span> Highly reactive molecules formed from diatomic oxygen (O₂)

In chemistry and biology, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (O2), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (O2H), superoxide (O2-), hydroxyl radical (OH.), and singlet oxygen. ROS are pervasive because they are readily produced from O2, which is abundant. ROS are important in many ways, both beneficial and otherwise. ROS function as signals, that turn on and off biological functions. They are intermediates in the redox behavior of O2, which is central to fuel cells. ROS are central to the photodegradation of organic pollutants in the atmosphere. Most often however, ROS are discussed in a biological context, ranging from their effects on aging and their role in causing dangerous genetic mutations.

<span class="mw-page-title-main">Plastoquinone</span> Molecule which moves electron in photosynthesis

Plastoquinone (PQ) is a terpenoid-quinone (meroterpenoid) molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. The most common form of plastoquinone, known as PQ-A or PQ-9, is a 2,3-dimethyl-1,4-benzoquinone molecule with a side chain of nine isoprenyl units. There are other forms of plastoquinone, such as ones with shorter side chains like PQ-3 as well as analogs such as PQ-B, PQ-C, and PQ-D, which differ in their side chains. The benzoquinone and isoprenyl units are both nonpolar, anchoring the molecule within the inner section of a lipid bilayer, where the hydrophobic tails are usually found.

<span class="mw-page-title-main">Photosystem</span> Structural units of protein involved in photosynthesis

Photosystems are functional and structural units of protein complexes involved in photosynthesis. Together they carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems are found in the thylakoid membranes of plants, algae, and cyanobacteria. These membranes are located inside the chloroplasts of plants and algae, and in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: PSI and PSII.

<span class="mw-page-title-main">Photosystem II</span> First protein complex in light-dependent reactions of oxygenic photosynthesis

Photosystem II is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen.

<span class="mw-page-title-main">Photophosphorylation</span> Biochemical process in photosynthesis

In the process of photosynthesis, the phosphorylation of ADP to form ATP using the energy of sunlight is called photophosphorylation. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions, driven by the main primary source of energy available to living organisms, which is sunlight. All organisms produce a phosphate compound, ATP, which is the universal energy currency of life. In photophosphorylation, light energy is used to pump protons across a biological membrane, mediated by flow of electrons through an electron transport chain. This stores energy in a proton gradient. As the protons flow back through an enzyme called ATP synthase, ATP is generated from ADP and inorganic phosphate. ATP is essential in the Calvin cycle to assist in the synthesis of carbohydrates from carbon dioxide and NADPH.

Cytochrome b<sub>6</sub>f complex Enzyme

The cytochrome b6f complex (plastoquinol/plastocyanin reductase or plastoquinol/plastocyanin oxidoreductase; EC 7.1.1.6) is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, that catalyzes the transfer of electrons from plastoquinol to plastocyanin:

Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule.

<span class="mw-page-title-main">Photosynthetic reaction centre</span> Molecular unit responsible for absorbing light in photosynthesis

A photosynthetic reaction center is a complex of several proteins, pigments, and other co-factors that together execute the primary energy conversion reactions of photosynthesis. Molecular excitations, either originating directly from sunlight or transferred as excitation energy via light-harvesting antenna systems, give rise to electron transfer reactions along the path of a series of protein-bound co-factors. These co-factors are light-absorbing molecules (also named chromophores or pigments) such as chlorophyll and pheophytin, as well as quinones. The energy of the photon is used to excite an electron of a pigment. The free energy created is then used, via a chain of nearby electron acceptors, for a transfer of hydrogen atoms (as protons and electrons) from H2O or hydrogen sulfide towards carbon dioxide, eventually producing glucose. These electron transfer steps ultimately result in the conversion of the energy of photons to chemical energy.

<span class="mw-page-title-main">Photoinhibition</span>

Photoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga, or cyanobacterium. Photosystem II (PSII) is more sensitive to light than the rest of the photosynthetic machinery, and most researchers define the term as light-induced damage to PSII. In living organisms, photoinhibited PSII centres are continuously repaired via degradation and synthesis of the D1 protein of the photosynthetic reaction center of PSII. Photoinhibition is also used in a wider sense, as dynamic photoinhibition, to describe all reactions that decrease the efficiency of photosynthesis when plants are exposed to light.

Oxygenevolution is the process of generating molecular oxygen (O2) by a chemical reaction, usually from water. Oxygen evolution from water is effected by oxygenic photosynthesis, electrolysis of water, and thermal decomposition of various oxides. The biological process supports aerobic life. When relatively pure oxygen is required industrially, it is isolated by distilling liquefied air.

Lithium superoxide is an unstable inorganic salt with formula LiO2. A radical compound, it can be produced at low temperature in matrix isolation experiments, or in certain nonpolar, non-protic solvents. Lithium superoxide is also a transient species during the reduction of oxygen in a lithium–air galvanic cell, and serves as a main constraint on possible solvents for such a battery. For this reason, it has been investigated thoroughly using a variety of methods, both theoretical and spectroscopic.

There are several known allotropes of oxygen. The most familiar is molecular oxygen, present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone. Others are:

<span class="mw-page-title-main">Radical (chemistry)</span> Atom, molecule, or ion that has an unpaired valence electron; typically highly reactive

In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.

Dioxygen complexes are coordination compounds that contain O2 as a ligand. The study of these compounds is inspired by oxygen-carrying proteins such as myoglobin, hemoglobin, hemerythrin, and hemocyanin. Several transition metals form complexes with O2, and many of these complexes form reversibly. The binding of O2 is the first step in many important phenomena, such as cellular respiration, corrosion, and industrial chemistry. The first synthetic oxygen complex was demonstrated in 1938 with cobalt(II) complex reversibly bound O2.

<span class="mw-page-title-main">Light-dependent reactions</span> Photosynthetic reactions

Light-dependent reactions refers to certain photochemical reactions that are involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and the second occurs at photosystem I (PSI).

All living cells produce reactive oxygen species (ROS) as a byproduct of metabolism. ROS are reduced oxygen intermediates that include the superoxide radical (O2−) and the hydroxyl radical (OH•), as well as the non-radical species hydrogen peroxide (H2O2). These ROS are important in the normal functioning of cells, playing a role in signal transduction and the expression of transcription factors. However, when present in excess, ROS can cause damage to proteins, lipids and DNA by reacting with these biomolecules to modify or destroy their intended function. As an example, the occurrence of ROS have been linked to the aging process in humans, as well as several other diseases including Alzheimer's, rheumatoid arthritis, Parkinson's, and some cancers. Their potential for damage also makes reactive oxygen species useful in direct protection from invading pathogens, as a defense response to physical injury, and as a mechanism for stopping the spread of bacteria and viruses by inducing programmed cell death.

References

  1. ↑ Fenical, William (September 1983). "Marine Plants: A Unique and Unexplored Resource". Plants: the potentials for extracting protein, medicines, and other useful chemicals (workshop proceedings). DIANE Publishing. p. 147. ISBN   1-4289-2397-7.
  2. ↑ Broeker, W.S. (2006). "Breathing easy, Et tu, O2". Columbia University. Retrieved 2007-10-21.
  3. ↑ Brown, LeMay, Burslen, Chemistry The Central Science, ISBN   0-13-048450-4, p. 958
  4. 1 2 3 4 Raven, Peter H.; Ray F. Evert; Susan E. Eichhorn (2005). Biology of Plants, 7th Edition. New York: W.H. Freeman and Company Publishers. pp. 115–127. ISBN   0-7167-1007-2.
  5. ↑ 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. doi:10.1007/s11120-005-8163-4. PMID   16170631. S2CID   12893308.
  6. ↑ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN   0-12-352651-5.
  7. ↑ 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)
  8. ↑ Stwertka 1998, p. 48.
  9. 1 2 3 4 5 Emsley 2001, p. 298.
  10. ↑ Cook & Lauer 1968, p. 500.
  11. ↑ Figures given are for values up to 50 miles above the surface
  12. ↑ Emsley 2001, p. 303.
  13. ↑ Zhang, Baoyi; et al. (December 2022). "Role of mitochondrial reactive oxygen species in homeostasis regulation". Redox Report: Communications in Free Radical Research. 27 (1): 45–52. doi:10.1080/13510002.2022.2046423. PMC   8890532 . PMID   35213291.
  14. ↑ Hoffmann, Roald (2004). "The Story of O". American Scientist. 92 (1): 23. doi:10.1511/2004.1.23. Archived from the original on 2007-02-22. Retrieved 2007-03-03.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.