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Killdeer
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A shorebird found in the Americas
For other uses, see Killdeer (disambiguation).
Killdeer
Conservation status

Least Concern (IUCN 3.1)[1]
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Charadriiformes
Family: Charadriidae
Genus: Charadrius
Species: C. vociferus
Binomial name
Charadrius vociferus
Linnaeus, 1758
Breeding range Resident range Non-breeding range
The killdeer (Charadrius vociferus) is a medium-sized plover. It is a noisy bird, whose English name comes from its distinctive kill deer cry.[2]
The adult has a brown back and wings, a white belly, and a white breast with two black bands. The rump is tawny orange. The face and cap are brown with a white forehead. The eyering is orange-red. The chicks are patterned almost identically with the adults, and are precocial (able to move around immediately after hatching). The killdeer often uses a "broken-wing act" to distract predators from the nest. It is named onomatopoeically after its call.[3]
Contents
1 Etymology and taxonomy
2 Description
3 Habitat and distribution
4 Behaviour
4.1 Breeding
4.2 Feeding
5 Predators
5.1 Responses to predators
6 Status
7 Gallery
8 References
9 External links
Etymology and taxonomy[edit]
The killdeer was first described as Charadrius vociferus by Carl Linnaeus in the 10th edition of his Systema Naturae. This was based on an account of it by Mark Catesby in his The Natural History of Carolina, Florida and the Bahama Islands,[4] where he gave it the name of the "chattering plover".[5]
The killdeer's binomial name is Charadrius vociferus.[6] The genus name Charadrius is a Late Latin word for a yellowish bird mentioned in the fourth-century Vulgate. It derives from Ancient Greek kharadrios a bird found in ravines and river valleys (kharadra, "ravine"). The specific vociferus is Latin and comes from vox, "cry" and ferre, "to bear".[7]
There are three subspecies of the killdeer, including the nominate:
C. v. vociferus Linnaeus, 1758 – The nominate subspecies, found in the US (including southeastern Alaska), southern Canada, Mexico, and very locally south to Panama. It winters to northwestern South America.[6]
C. v. ternominatus Bangs & Kennard, 1920 – This subspecies is found on the Bahamas, the Greater Antilles, and Virgin Islands.[6]
C. v. peruvianus (Chapman, 1920) – This South American subspecies is found in west Ecuador, Peru, and extreme northwest Chile.[6]
The killdeer's name comes from its frequently heard call.[8]
Description[edit]
The killdeer is a relatively large plover, with adults ranging in length from 20 to 28 centimetres (7.9 to 11.0 in) and having a wingspan between about 59 and 63 centimetres (23 and 25 in). It is usually between 72 and 121 grams (2.5 and 4.3 oz) in terms of weight.[6] It has a short, thick, and dark bill, flesh-coloured legs, and a red eye ring.[9] Its upperparts are mostly brown and fringed rufous,[6] with its cap, back, and wings being the former colour. It has a white forehead and a white stripe behind the eye, and its lores and the upper borders to the white forehead are black. The killdeer also has a white collar that is black on its upper border.[9] The female generally has a browner mask.[6] The rest of the face is brown. The breast and belly are white, with the exception of two black breast bands[9] (although the female's bands are usually more brown).[6] The rump is red, and the tail is mostly brown. The latter also has a black subterminal band, a white terminal band, and barred white feathers on the outer portion of the tail. In flight, a white wing stripe at the base of the flight feathers is visible. The juvenile is similar to the adult.[9] The subspecies Charadrius vociferus ternominatus is smaller and more pale and grey than the nominate, and the subspecies C. v. peruvianus differs on the basis of its smaller size and more extensive rufous feather fringes.[6]

C. v. ternominatus
Cuba
C. v. ternominatus
Cuba
Habitat and distribution[edit]
The nominate subspecies of the killdeer is found in the US (including southeastern Alaska), southern Canada, and Mexico in addition to breeding very locally to south Panama. It is resident in the southern half of its breeding range.[10] Migratory populations winter from the breeding range south to Central America, the West Indies, Colombia, Ecuador, and islands off Venezuela, leaving its breeding grounds after mid July,[6] with a peak in migration from August to September.[10] The subspecies Charadrius vociferus ternominatus is found in the Bahamas, Greater Antilles, and Virgin Islands, where it is thought to be resident. C. v. peruvianus is resident to west Ecuador, Peru, and extreme northwestern Chile.[6]
During the non-breeding season the killdeer uses coastal wetland and beach habitat, as well as coastal fields.[11] It prefers fields that have less recent precipitation.[12] It almost exclusively forages in fields, especially those with short vegetation and cattle combined with short vegetation or standing water.[13] When breeding, it has a home range of about 6 hectares (15 acres). Those with nests more than 50 metres (160 ft) away from water generally have larger home ranges.[14]
Behaviour[edit]
These birds will frequently use a distraction display ("broken-wing act") to distract predators from their nests. This involves the bird walking away from its nesting area holding its wings in a position that simulates an injury, flapping around on the ground emitting a distress call. The predators are then usually attracted to the seemingly easy prey. If the parent sees that a potential predator is not following them, it moves closer and gets louder until it gets the attention of the predator. This is repeated until the predator is far from the nest, and the killdeer suddenly "heals" and flies away.[15][16]
Breeding[edit]
Eggs in a nest on the ground
Play media
Parent protecting small chicks by performing a distraction display to draw attention to itself away from the nest
The killdeer likely forms pairs on breeding grounds.[17] Both males and females (although the former more often than the latter) advertise in flight with loud "killdeer" calls, and the former can advertise by standing in a high spot and making the same call. Ground chases occur when a killdeer has been approached multiple times by another killdeer.[18] Males also advertise to females by scraping out a fake nest.[17]
To nest, the killdeer uses open fields or other flat areas with short vegetation (usually below 1 centimetre (0.39 in) tall),[6] including agricultural fields, and meadows.[11] Nesting habitat is characterized as having enough nest materials to form a scrape but otherwise having little or no vegetation.[19] The killdeer frequently nests near the site it nested the previous year. It is likely that the male usually returns to nest in the same area regardless of whether or not it retains the same mate. This does not seem to be true of the female, who does not nest in the same territory if it does not have the same mate.[20] Nests are also sometimes located on rooftops.[6] The nest itself is merely a shallow depression or bowl in the ground, fringed by some stones and blades of grass.[15] It generally chooses to build with white nesting material over darker colours; the function of this is suspected to either help keep the nest cool or to help conceal the nest.[21] In a study of piping plovers, the former function was supported, as nests were 2 to 6 °C (36 to 43 °F) cooler than the surrounding ground. The latter function also had some support, as the plovers generally chose pebbles closer in colour to the eggs, but, nests with higher contrast with the ground suffered more predation.[22] When nesting on rooftops, the eggs may be placed on a flat roof, or in a nest of raised gravel, sometimes lined with white pebbles or pieces of seashells.[23]
The eggs of the killdeer are typically laid from mid March to early June in the southern portions of the range, and from mid April to mid July in the northern parts.[6] In both cases, the breeding season itself extends to about August.[24] In Puerto Rico, and possibility in other Caribbean islands, breeding occurs year round.[6]
The killdeer has a clutch of four to six eggs, buff to beige in colour, with brown markings and black speckles. It is about 38 by 27 millimetres (1.5 by 1.1 in) in size.[8] These eggs are laid at intervals of 24 to 48 hours.[6] When these eggs are being laid, the energy expenditure of both sexes is at its highest; the female needs to produce eggs, whereas the male needs to defend its territory.[25] Both the female and male are closer to the nest site during egg laying and incubation, with the male generally being closer to the nest during all stages of breeding. This latter fact is likely due to the male's increased investment in nest-site defense.[14] Up to five replacement clutches can be laid, and there are occasionally two broods.[6] The eggs are incubated for 22 to 28 days[8] by both the male and the female, with the former typically incubating during the night.[6] The time dedicated to incubation is related to temperature, with one study recording that killdeer incubated eggs 99% of the time when the temperature was about 13 °C (55 °F), 76% of the time when it was around 26 °C (79 °F), and 87% of the time during temperatures of about 35 °C (95 °F). When it is hot (above at least 25 °C (77 °F)), incubation does not take the form of warming the eggs, but instead occurs through the cooling of eggs, generally through shading.[26]
The day after the young hatch, they are led by both parents out of the nest. The family then generally moves to a feeding territory with dense vegetation which the chicks are able to hide under when a predator is near.[27] The chicks are raised, at least in single-brood pairs, by both parents, likely because of the high failure rate of nests and the need for both parents to be present to successfully raise the young.[25] In two parents broods, the young are usually attended by only one parent, generally the female, until about two weeks of age, after which the non-attentive parent occasionally tends the young. Otherwise, the non-attentive parent is generally at least 23 metres (75 ft) away from the chicks. Periods of attentiveness for each parent generally last about one to one hour thirty minutes. This time is mainly spent standing when the chicks are young, with the time dedicated to this decreasing as the chicks get older. When the young are below two weeks of age, the attending adult dedicates little time to foraging. The non-attentive adult defends the young most of the time when they chicks are less than a week old, but this task steadily shifts onto the attentive adult, until about three weeks of age, when the attending parent does almost all of the defense. One parent at a time generally broods the chicks, and does so frequently until they are two days old. They are also brooded, until about 15 days after hatching, during rain, and, until about 18 days after hatching, at night. The only time when the young are not in the presence of a parent is when the parents are responding to a predator or an aggressive conspecific, or when mating.[27]
There are sometimes one parent broods when a pair has two broods.[27] This brood is watched over by the male, who is able to hatch the chicks on its own, unlike the female.[25] In this case, the adult does not spend most of the time standing, and the time it did spend did not decrease as a function of age. Like attentive adults in two parent broods, the sole parent of a brood increases the time spent foraging as the chicks get older.[27]
The young fledge about 31 days after they hatch, and generally move to moister areas in valleys and on the banks of rivers. They may be cared for by their parents for up to 10 days after they fledge, and exceptionally 81 days after hatching.[6]
About 52% to 63% of nests fail to produce any fledged young,[6] and about 53% of eggs do not hatch.[28]
Feeding[edit]
The killdeer feeds primarily on insects (especially beetles and flies), in addition to millipedes, worms, snails, spiders, and some seeds. It opportunistically takes tree frogs and dead minnows.[6] It forages almost exclusively in fields (no matter the tide), especially those with cattle and short vegetation or standing water. Standing water alone does not usually have a significant effect on field choice, but when combined with cattle, it does.[13] Viable disseminules can be recovered from killdeer feces, indicating that it is important in the transport of various aquatic organisms.[29]
The killdeer uses visual cues to find food. An example of this is "foot-trembling",[30] where, when feeding in water, it stands on one foot and shakes the other in the water for about five seconds, and then pecks at any prey stirred up by it.[31] When feeding in fields, it may follow ploughs to take earthworms disturbed to the surface.[6] The female forages significantly more than male when it is breeding. Foraging by females peaks during pre-laying and laying itself, decreasing when incubation starts (as there is little time for foraging), and returning to high levels after.[24] During the non-breeding season, it forages during the night, depending on the lunar cycle. When the moon is full, it spends more time foraging during the night and more time roosting during the day. Foraging during the night has benefits that include increased insect abundance and reduced predation.[30]
Predators[edit]
Predators to the killdeer include herring gulls, common crows, raccoons, and striped skunks.[28]
Responses to predators[edit]
Various methods of distracting predators are employed during the breeding season by parents. During egg-laying, the most common response to predators is to quietly leave the nest. When incubation starts, and as it progresses, the intensity of predator responses increases, peaking just after hatching. This is likely because it is worth more to protect the young at that stage, as they are more likely to survive to fledge. After hatching, though, the intensity of responses decreases, until a normal response is calling at a stand. This is because as the young age, they get more independent.[28]
Status[edit]
The killdeer is considered by the International Union for Conservation of Nature to be a least-concern species. This is due to its large range of about 26.3 million kilometres (16.3 mi) and its large population, estimated by the IUCN to be about one million birds,[1] although another estimate, in the Handbook of the Birds of the World Alive, gives about two million birds.[6] Even though it has a declining population, it is not declining fast enough to be considered a vulnerable species.[1]
Gallery[edit]
Charadrius vociferus call
Bird call of the killdeer (Charadrius vociferus)
Problems playing this file? See media help.
Adult protecting its offspring in Minnesota
Female on nest in Pennsylvania
Juvenile in New Jersey
Chick shortly after hatching, Minnesota
Juveniles hiding under adult
References[edit]
^ Jump up to: a b c BirdLife International (2012). "Charadrius vociferus". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
Jump up ^ "Killdeer". All About Birds. Cornell Lab of Ornithology.
Jump up ^ Online Etymology Dictionary, Douglas Harper
Jump up ^ Linnaeus, Carl (1758). Systema Naturae per Regna Tria Naturae, Secundum Classes, Ordines, Genera, Species, cum Characteribus, Differentiis, Synonymis, Locis (in Latin). 1 (10th ed.). Stockholm, Sweden: (Laurentii Salvii). p. 150 – via The Internet Archive.
Jump up ^ Catesby, Mark (1731). The Natural History of Carolina, Florida and the Bahama Islands. 1. London. p. 71.
^ Jump up to: a b c d e f g h i j k l m n o p q r s t u v w Wiersma, P.; Kirwan, G. M.; Boesman, P. (2018). del Hoyo, Josep; Elliott, Andrew; Sargatal, Jordi; Christie, David A.; de Juana, Eduardo, eds. "Killdeer (Charadrius vociferus)". Handbook of the Birds of the World Alive. Barcelona, Spain: Lynx Edicions. Retrieved 12 June 2018. (Subscription required (help)).
Jump up ^ Jobling, James A (2010). The Helm Dictionary of Scientific Bird Names. London: Christopher Helm. pp. 99, 404. ISBN 978-1-4081-2501-4.
^ Jump up to: a b c Hauber, Mark E. (1 August 2014). The Book of Eggs: A Life-Size Guide to the Eggs of Six Hundred of the World's Bird Species. Chicago: University of Chicago Press. p. 140. ISBN 978-0-226-05781-1.
^ Jump up to: a b c d "Killdeer Charadrius vociferus". Patuxent Wildlife Research Center. USGS. Retrieved 1 August 2018.
^ Jump up to: a b Jonathan K. Alderfer; Paul Hess (2011). National Geographic Backyard Guide to the Birds of North America. National Geographic. p. 69. ISBN 978-1-4262-0720-4.
^ Jump up to: a b Johnsgard, P.A. (1981). The Plovers, Sandpipers and Snipes of the World. Lincoln, NE: University of Nebraska Press.
Jump up ^ Ogden, L. J. E.; S. Bittman; D. B. Lank & F. C. Stevenson (2008). "Factors influencing farmland habitat use by shorebirds wintering in the Fraser River Delta, Canada". Agriculture, Ecosystems & Environment: 252–258. doi:10.1016/j.agee.2007.10.004.
^ Jump up to: a b Long, Linda L.; Ralph, C. John (2001). "Dynamics of habitat use by shorebirds in estuarine and agricultural habitats in northwestern California". The Wilson Bulletin. 113 (1): 41–52. doi:10.1676/0043-5643(2001)113[0041:DOHUBS]2.0.CO;2. ISSN 0043-5643.
^ Jump up to: a b Plissner, Jonathan H.; Oring, Lewis W.; Haig, Susan M. (2000). "Space use of killdeer at a Great Basin breeding area". The Journal of Wildlife Management. 64 (2): 421. doi:10.2307/3803240. ISSN 0022-541X.
^ Jump up to: a b Hiller, Ilo (2008). "Killdeer". Texas Parks and Wildlife Department. Retrieved 2011-03-01.
Jump up ^ "Killdeer". Ohio Department of Natural Resources. Retrieved 2011-3-1.
^ Jump up to: a b Phillips, R.E. (1972). "Sexual and agonistic behaviour in the killdeer (Charadrius vociferus)". Animal Behaviour. 20 (1): 1–9. doi:10.1016/S0003-3472(72)80166-0. ISSN 0003-3472.
Jump up ^ Mundahl, John T. (1982). "Role specialization in the parental and territorial behavior of the killdeer". The Wilson Bulletin. 94 (4): 515–530. ISSN 0043-5643.
Jump up ^ DeGraaf, R.M.; Rapole, J.H. (1995). Neotropical Migratory Birds: Natural History, Distribution and Population Change. Ithaca, NY, US: Comstock Publishing Associates.
Jump up ^ Lenington, Sarah (1975). "Mate fidelity and nesting site tenacity in the killdeer". The Auk. 92 (1): 149–151. doi:10.2307/4084431. ISSN 0004-8038.
Jump up ^ Kull, Jr., Robert C. (1977). "Color selection of nesting material by killdeer". The Auk. 94 (3): 602–604. ISSN 0004-8038.
Jump up ^ Mayer, Paul M.; Smith, Levica M.; Ford, Robert G.; Watterson, Dustin C.; McCutchen, Marshall D.; Ryan, Mark R. (2009). "Nest construction by a ground-nesting bird represents a potential trade-off between egg crypticity and thermoregulation". Oecologia. 159 (4): 893–901. doi:10.1007/s00442-008-1266-9. ISSN 0029-8549.
Jump up ^ Fisk, Erma J. (1978). "The growing use of roofs by nesting birds". Bird-Banding. 49 (2): 134–141. doi:10.2307/4512343. ISSN 0006-3630.
^ Jump up to: a b Brunton, Dianne H. (1988). "Sexual differences in reproductive effort: time-activity budgets of monogamous killdeer, Charadrius vociferus". Animal Behaviour. 36 (3): 705–717. doi:10.1016/S0003-3472(88)80153-2. ISSN 0003-3472.
^ Jump up to: a b c Brunton, Dianne H. (1988). "Energy expenditure in reproductive effort of male and female killdeer (Charadrius vociferus)". The Auk. 105 (3): 553–564.
Jump up ^ Bergstrom, Peter W. (1989). "Incubation temperatures of Wilson's plovers and killdeers". The Condor. 91 (3): 634. doi:10.2307/1368114. ISSN 0010-5422.
^ Jump up to: a b c d Lenington, Sarah (1980). "Bi-parental care in killdeer: An adaptive hypothesis". The Wilson Bulletin. 92 (1): 8–20.
^ Jump up to: a b c Brunton, Dianne H. (1990). "The effects of nesting stage, sex, and type of predator on parental defense by killdeer (Charadrius vociferous): Testing models of avian parental defense". Behavioral Ecology and Sociobiology. 26 (3): 181–190. ISSN 0340-5443.
Jump up ^ Proctor, Vernon W.; Malone, Charles R.; DeVlaming, Victor L. (1967). "Dispersal of aquatic organisms: Viability of disseminules recovered from the intestinal tract of captive killdeer". Ecology. 48 (4): 672–676. doi:10.2307/1936517. ISSN 0012-9658.
^ Jump up to: a b Eberhart-Phillips, Luke J. (2016). "Dancing in the moonlight: evidence that killdeer foraging behaviour varies with the lunar cycle". Journal of Ornithology. 158 (1): 253–262. doi:10.1007/s10336-016-1389-4. ISSN 2193-7192.
Jump up ^ Smith, Susan M. (1970). ""Foot-trembling" feeding behavior by a killdeer". The Condor. 72 (2): 245.
External links[edit]
Wikimedia Commons has media related to Killdeer.
Wikispecies has information related to Charadrius vociferus
Killdeer at Enature.com - enature.com
"Killdeer media". Internet Bird Collection.
Killdeer - Charadrius vociferus - USGS Patuxent Bird Identification InfoCenter
Killdeer Fact Sheet Hinterland Who's Who - produced by Environment Canada and Canadian Wildlife Federation
Killdeer Bird Sound at Florida Museum of Natural History
Killdeer photo gallery at VIREO (Drexel University)
Taxon identifiers
Wikidata: Q755737
Wikispecies: Charadrius vociferus
ABA: killde
ADW: Charadrius_vociferus
ARKive: charadrius-vociferus
BirdLife: 22693777
BTO: KILLD
eBird: killde
EoL: 1049356
Fauna Europaea: 96873
Fossilworks: 129786
GBIF: 2480320
IBC: killdeer-charadrius-vociferus
iNaturalist: 4793
IRMNG: 10934850
ITIS: 176520
IUCN: 22693777
NBN: NBNSYS0000000324
NCBI: 50402
WoRMS: 159127
Retrieved from "https://en.wikipedia.org/w/index.php?title=Killdeer&oldid=856509935"
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text:
Electron transport chain
From Wikipedia, the free encyclopedia
Jump to: navigation, search
The electron transport chain in the mitochondrion is the site of oxidative phosphorylation in eukaryotes. The NADH and succinate generated in the citric acid cycle are oxidized, providing energy to power ATP synthase.
Photosynthetic electron transport chain of the thylakoid membrane.
An electron transport chain (ETC) is a series of complexes that transfer electrons from electron donors to electron acceptors via redox (both reduction and oxidation occurring simultaneously) reactions, and couples this electron transfer with the transfer of protons (H+ ions) across a membrane. This creates an electrochemical proton gradient that drives the synthesis of adenosine triphosphate (ATP), a molecule that stores energy chemically in the form of highly strained bonds. The molecules of the chain include peptides, enzymes (which are proteins or protein complexes), and others. The final acceptor of electrons in the electron transport chain during aerobic respiration is molecular oxygen although a variety of acceptors other than oxygen such as sulfate exist in anaerobic respiration.
Electron transport chains are used for extracting energy via redox reactions from sunlight in photosynthesis or, such as in the case of the oxidation of sugars, cellular respiration. In eukaryotes, an important electron transport chain is found in the inner mitochondrial membrane where it serves as the site of oxidative phosphorylation through the use of ATP synthase. It is also found in the thylakoid membrane of the chloroplast in photosynthetic eukaryotes. In bacteria, the electron transport chain is located in their cell membrane.
In chloroplasts, light drives the conversion of water to oxygen and NADP+ to NADPH with transfer of H+ ions across chloroplast membranes. In mitochondria, it is the conversion of oxygen to water, NADH to NAD+ and succinate to fumarate that are required to generate the proton gradient.
Electron transport chains are major sites of premature electron leakage to oxygen, generating superoxide and potentially resulting in increased oxidative stress.
Contents
[hide]
1 Background
2 In mitochondria
2.1 Mitochondrial redox carriers
2.1.1 Complex I
2.1.2 Complex II
2.1.3 Complex III
2.1.4 Complex IV
2.2 Coupling with oxidative phosphorylation
2.3 Summary
3 In bacteria
3.1 Electron donors
3.2 Dehydrogenases
3.3 Quinone carriers
3.4 Proton pumps
3.5 Cytochrome electron carriers
3.6 Terminal oxidases and reductases
3.7 Electron acceptors
3.8 Summary
4 Photosynthetic
5 Summary
6 See also
7 References
8 External links
Background[edit]
The electron transport chain consists of a spatially separated series of redox reactions in which electrons are transferred from a donor molecule to an acceptor molecule. The underlying force driving these reactions is the Gibbs free energy of the reactants and products. The Gibbs free energy is the energy available ("free") to do work. Any reaction that decreases the overall Gibbs free energy of a system is thermodynamically spontaneous.
The function of the electron transport chain is to produce a transmembrane proton electrochemical gradient as a result of the redox reactions.[1] If protons flow back through the membrane, they enable mechanical work, such as rotating bacterial flagella. ATP synthase, an enzyme highly conserved among all domains of life, converts this mechanical work into chemical energy by producing ATP,[2] which powers most cellular reactions. A small amount of ATP is available from substrate-level phosphorylation, for example, in glycolysis. In most organisms the majority of ATP is generated in electron transport chains, while only some obtain ATP by fermentation.[citation needed]
In mitochondria[edit]
Most eukaryotic cells have mitochondria, which produce ATP from products of the citric acid cycle, fatty acid oxidation, and amino acid oxidation. At the mitochondrial inner membrane, electrons from NADH and FADH2 pass through the electron transport chain to oxygen, which is reduced to water. The electron transport chain comprises an enzymatic series of electron donors and acceptors. Each electron donor will pass electrons to a more electronegative acceptor, which in turn donates these electrons to another acceptor, a process that continues down the series until electrons are passed to oxygen, the most electronegative and terminal electron acceptor in the chain. Passage of electrons between donor and acceptor releases energy, which is used to generate a proton gradient across the mitochondrial membrane by actively "pumping" protons into the intermembrane space, producing a thermodynamic state that has the potential to do work. The entire process is called oxidative phosphorylation, since ADP is phosphorylated to ATP using the energy of hydrogen oxidation in many steps.
A small percentage of electrons do not complete the whole series and instead directly leak to oxygen, resulting in the formation of the free-radical superoxide, a highly reactive molecule that contributes to oxidative stress and has been implicated in a number of diseases and aging.
Mitochondrial redox carriers[edit]
Energy obtained through the transfer of electrons down the ETC is used to pump protons from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient (ΔpH) across the inner mitochondrial membrane (IMM). This proton gradient is largely but not exclusively responsible for the mitochondrial membrane potential (ΔΨM). It allows ATP synthase to use the flow of H+ through the enzyme back into the matrix to generate ATP from adenosine diphosphate (ADP) and inorganic phosphate. Complex I (NADH coenzyme Q reductase; labeled I) accepts electrons from the Krebs cycle electron carrier nicotinamide adenine dinucleotide (NADH), and passes them to coenzyme Q (ubiquinone; labeled Q), which also receives electrons from complex II (succinate dehydrogenase; labeled II). Q passes electrons to complex III (cytochrome bc1 complex; labeled III), which passes them to cytochrome c (cyt c). Cyt c passes electrons to Complex IV (cytochrome c oxidase; labeled IV), which uses the electrons and hydrogen ions to reduce molecular oxygen to water.
Four membrane-bound complexes have been identified in mitochondria. Each is an extremely complex transmembrane structure that is embedded in the inner membrane. Three of them are proton pumps. The structures are electrically connected by lipid-soluble electron carriers and water-soluble electron carriers. The overall electron transport chain:
NADH+H+ → Complex I → Q → Complex III → cytochrome c → Complex IV → H2O ↑ Complex II ↑ Succinate

Complex I[edit]
In Complex I (NADH:ubiquinone oxidoreductase, NADH-CoQ reductase, or NADH dehydrogenase; EC 1.6.5.3), two electrons are removed from NADH and ultimately transferred to a lipid-soluble carrier, ubiquinone (UQ). The reduced product, ubiquinol (UQH2), freely diffuses within the membrane, and Complex I translocates four protons (H+) across the membrane, thus producing a proton gradient. Complex I is one of the main sites at which premature electron leakage to oxygen occurs, thus being one of the main sites of production of superoxide.[3]
The pathway of electrons is as follows:
NADH is oxidized to NAD+, by reducing Flavin mononucleotide to FMNH2 in one two-electron step. FMNH2 is then oxidized in two one-electron steps, through a semiquinone intermediate. Each electron thus transfers from the FMNH2 to an Fe-S cluster, from the Fe-S cluster to ubiquinone (Q). Transfer of the first electron results in the free-radical (semiquinone) form of Q, and transfer of the second electron reduces the semiquinone form to the ubiquinol form, QH2. During this process, four protons are translocated from the mitochondrial matrix to the intermembrane space. [4] As the electrons become continuously oxidized and reduced throughout the complex an electron current is produced along the 180 Angstrom width of the complex within the membrane. This current powers the active transport of four protons to the intermembrane space per two electrons from NADH.[5]
This complex is inhibited by Alkylguanides (Example : Guanethidine), Rotenone, Barbiturates, Chlorpromazine, Piericidin.
Complex II[edit]
In Complex II (succinate dehydrogenase or succinate-CoQ reductase; EC 1.3.5.1) additional electrons are delivered into the quinone pool (Q) originating from succinate and transferred (via flavin adenine dinucleotide (FAD)) to Q. Complex II consists of four protein subunits: succinate dehydrogenase, (SDHA); succinate dehydrogenase [ubiquinone] iron-sulfur subunit, mitochondrial, (SDHB); succinate dehydrogenase complex subunit C, (SDHC) and succinate dehydrogenase complex, subunit D, (SDHD). Other electron donors (e.g., fatty acids and glycerol 3-phosphate) also direct electrons into Q (via FAD). Complex 2 is a parallel electron transport pathway to complex 1, but unlike complex 1, no protons are transported to the intermembrane space in this pathway. Therefore, the pathway through complex 2 contributes less energy to the overall electron transport chain process.
This complex is inhibited by Carboxin.
Complex III[edit]
In Complex III (cytochrome bc1 complex or CoQH2-cytochrome c reductase; EC 1.10.2.2), the Q-cycle contributes to the proton gradient by an asymmetric absorption/release of protons. Two electrons are removed from QH2 at the QO site and sequentially transferred to two molecules of cytochrome c, a water-soluble electron carrier located within the intermembrane space. The two other electrons sequentially pass across the protein to the Qi site where the quinone part of ubiquinone is reduced to quinol. A proton gradient is formed by one quinol (2H+2e-) oxidations at the Qo site to form one quinone (2H+2e-) at the Qi site. (in total four protons are translocated: two protons reduce quinone to quinol and two protons are released from two ubiquinol molecules).
 QH2 + 2 cytochrome c (FeIII) + 2 H+in → Q + 2 cytochrome c (FeII) + 4 H+out

When electron transfer is reduced (by a high membrane potential or respiratory inhibitors such as antimycin A), Complex III may leak electrons to molecular oxygen, resulting in superoxide formation.
This complex is inhibited by British Antilewisite(BAL), Napthoquinone and Antimycin.
Complex IV[edit]
In Complex IV (cytochrome c oxidase; EC 1.9.3.1), sometimes called cytochrome AA3, four electrons are removed from four molecules of cytochrome c and transferred to molecular oxygen (O2), producing two molecules of water. At the same time, eight protons are removed from the mitochondrial matrix (although only four are translocated across the membrane), contributing to the proton gradient. The activity of cytochrome c oxidase is inhibited by cyanide, carbon monoxide, azide, hydrogen sulphide(H2S).
Coupling with oxidative phosphorylation[edit]
Depiction of ATP synthase, the site of oxidative phosphorylation to generate ATP.
According to the chemiosmotic coupling hypothesis, proposed by Nobel Prize in Chemistry winner Peter D. Mitchell, the electron transport chain and oxidative phosphorylation are coupled by a proton gradient across the inner mitochondrial membrane. The efflux of protons from the mitochondrial matrix creates an electrochemical gradient (proton gradient). This gradient is used by the FOF1 ATP synthase complex to make ATP via oxidative phosphorylation. ATP synthase is sometimes described as Complex V of the electron transport chain.[6] The FO component of ATP synthase acts as an ion channel that provides for a proton flux back into the mitochondrial matrix. It is composed of a, b and c subunits. Protons in the inter-membranous space of mitochondria first enters the ATP synthase complex through a subunit channel. Then protons move to the c subunits.[7] The number of c subunits it has determines how many protons it will require to make the FO turn one full revolution. For example, in humans, there are 8 c subunits, thus 8 protons are required.[8] After c subunits, protons finally enters matrix using a subunit channel that opens into the mitochondrial matrix.[7] This reflux releases free energy produced during the generation of the oxidized forms of the electron carriers (NAD+ and Q). The free energy is used to drive ATP synthesis, catalyzed by the F1 component of the complex.[9]
Coupling with oxidative phosphorylation is a key step for ATP production. However, in specific cases, uncoupling the two processes may be biologically useful. The uncoupling protein, thermogenin—present in the inner mitochondrial membrane of brown adipose tissue—provides for an alternative flow of protons back to the inner mitochondrial matrix. Thyroxine is also a natiral uncoupler. This alternative flow results in thermogenesis rather than ATP production.[10] Synthetic uncouplers (e.g., 2,4-dinitrophenol, 2,4-dinitrocresol, CCCP) also exist, and, at high doses, are lethal.[citation needed]
Summary[edit]
In the mitochondrial electron transport chain electrons move from an electron donor (NADH or QH2) to a terminal electron acceptor (O2) via a series of redox reactions. These reactions are coupled to the creation of a proton gradient across the mitochondrial inner membrane. There are three proton pumps: I, III, and IV. The resulting transmembrane proton gradient is used to make ATP via ATP synthase.
The reactions catalyzed by Complex I and Complex III work roughly at equilibrium. This means that these reactions are readily reversible, by increasing the concentration of the products relative to the concentration of the reactants (for example, by increasing the proton gradient). ATP synthase is also readily reversible. Thus ATP can be used to build a proton gradient, which in turn can be used to make NADH. This process of reverse electron transport is important in many prokaryotic electron transport chains.[11]
In bacteria[edit]
In eukaryotes, NADH is the most important electron donor. The associated electron transport chain is
NADH → Complex I → Q → Complex III → cytochrome c → Complex IV → O2 where Complexes I, III and IV are proton pumps, while Q and cytochrome c are mobile electron carriers. The electron acceptor is molecular oxygen.
In prokaryotes (bacteria and archaea) the situation is more complicated, because there are several different electron donors and several different electron acceptors. The generalized electron transport chain in bacteria is:
                     Donor Donor Donor ↓ ↓ ↓ dehydrogenase → quinone → bc1 → cytochrome ↓ ↓ oxidase(reductase) oxidase(reductase) ↓ ↓ Acceptor Acceptor

Note that electrons can enter the chain at three levels: at the level of a dehydrogenase, at the level of the quinone pool, or at the level of a mobile cytochrome electron carrier. These levels correspond to successively more positive redox potentials, or to successively decreased potential differences relative to the terminal electron acceptor. In other words, they correspond to successively smaller Gibbs free energy changes for the overall redox reaction Donor → Acceptor.
Individual bacteria use multiple electron transport chains, often simultaneously. Bacteria can use a number of different electron donors, a number of different dehydrogenases, a number of different oxidases and reductases, and a number of different electron acceptors. For example, E. coli (when growing aerobically using glucose as an energy source) uses two different NADH dehydrogenases and two different quinol oxidases, for a total of four different electron transport chains operating simultaneously.
A common feature of all electron transport chains is the presence of a proton pump to create a transmembrane proton gradient. Bacterial electron transport chains may contain as many as three proton pumps, like mitochondria, or they may contain only one or two. They always contain at least one proton pump.
Electron donors[edit]
In the present day biosphere, the most common electron donors are organic molecules. Organisms that use organic molecules as an energy source are called organotrophs. Organotrophs (animals, fungi, protists) and phototrophs (plants and algae) constitute the vast majority of all familiar life forms.
Some prokaryotes can use inorganic matter as an energy source. Such an organism is called a lithotroph ("rock-eater"). Inorganic electron donors include hydrogen, carbon monoxide, ammonia, nitrite, sulfur, sulfide, manganese oxide, and ferrous iron. Lithotrophs have been found growing in rock formations thousands of meters below the surface of Earth. Because of their volume of distribution, lithotrophs may actually outnumber organotrophs and phototrophs in our biosphere.
The use of inorganic electron donors as an energy source is of particular interest in the study of evolution. This type of metabolism must logically have preceded the use of organic molecules as an energy source.
Dehydrogenases[edit]
Bacteria can use a number of different electron donors. When organic matter is the energy source, the donor may be NADH or succinate, in which case electrons enter the electron transport chain via NADH dehydrogenase (similar to Complex I in mitochondria) or succinate dehydrogenase (similar to Complex II). Other dehydrogenases may be used to process different energy sources: formate dehydrogenase, lactate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, H2 dehydrogenase (hydrogenase), etc. Some dehydrogenases are also proton pumps; others funnel electrons into the quinone pool. Most dehydrogenases show induced expression in the bacterial cell in response to metabolic needs triggered by the environment in which the cells grow.[citation needed]
Quinone carriers[edit]
Quinones are mobile, lipid-soluble carriers that shuttle electrons (and protons) between large, relatively immobile macromolecular complexes embedded in the membrane. Bacteria use ubiquinone (the same quinone that mitochondria use) and related quinones such as menaquinone. Another name for ubiquinone is Coenzyme Q10.
Proton pumps[edit]
A proton pump is any process that creates a proton gradient across a membrane. Protons can be physically moved across a membrane; this is seen in mitochondrial Complexes I and IV. The same effect can be produced by moving electrons in the opposite direction. The result is the disappearance of a proton from the cytoplasm and the appearance of a proton in the periplasm. Mitochondrial Complex III uses this second type of proton pump, which is mediated by a quinone (the Q cycle).
Some dehydrogenases are proton pumps; others are not. Most oxidases and reductases are proton pumps, but some are not. Cytochrome bc1 is a proton pump found in many, but not all, bacteria (it is not found in E. coli). As the name implies, bacterial bc1 is similar to mitochondrial bc1 (Complex III).
Proton pumps are the heart of the electron transport process. They produce the transmembrane electrochemical gradient that enables ATP Synthase to synthesize ATP.
Cytochrome electron carriers[edit]
Cytochromes are pigments that contain iron. They are found in two very different environments.
Some cytochromes are water-soluble carriers that shuttle electrons to and from large, immobile macromolecular structures imbedded in the membrane. The mobile cytochrome electron carrier in mitochondria is cytochrome c. Bacteria use a number of different mobile cytochrome electron carriers.
Other cytochromes are found within macromolecules such as Complex III and Complex IV. They also function as electron carriers, but in a very different, intramolecular, solid-state environment.
Electrons may enter an electron transport chain at the level of a mobile cytochrome or quinone carrier. For example, electrons from inorganic electron donors (nitrite, ferrous iron, etc.) enter the electron transport chain at the cytochrome level. When electrons enter at a redox level greater than NADH, the electron transport chain must operate in reverse to produce this necessary, higher-energy molecule.
Terminal oxidases and reductases[edit]
When bacteria grow in aerobic environments, the terminal electron acceptor (O2) is reduced to water by an enzyme called an oxidase. When bacteria grow in anaerobic environments, the terminal electron acceptor is reduced by an enzyme called a reductase.
In mitochondria the terminal membrane complex (Complex IV) is cytochrome oxidase. Aerobic bacteria use a number of different terminal oxidases. For example, E. coli does not have a cytochrome oxidase or a bc1 complex. Under aerobic conditions, it uses two different terminal quinol oxidases (both proton pumps) to reduce oxygen to water.
Anaerobic bacteria, which do not use oxygen as a terminal electron acceptor, have terminal reductases individualized to their terminal acceptor. For example, E. coli can use fumarate reductase, nitrate reductase, nitrite reductase, DMSO reductase, or trimethylamine-N-oxide reductase, depending on the availability of these acceptors in the environment.
Most terminal oxidases and reductases are inducible. They are synthesized by the organism as needed, in response to specific environmental conditions.
Electron acceptors[edit]
Just as there are a number of different electron donors (organic matter in organotrophs, inorganic matter in lithotrophs), there are a number of different electron acceptors, both organic and inorganic. If oxygen is available, it is invariably used as the terminal electron acceptor, because it generates the greatest Gibbs free energy change and produces the most energy.[citation needed]
In anaerobic environments, different electron acceptors are used, including nitrate, nitrite, ferric iron, sulfate, carbon dioxide, and small organic molecules such as fumarate.
Since electron transport chains are redox processes, they can be described as the sum of two redox pairs. For example, the mitochondrial electron transport chain can be described as the sum of the NAD+/NADH redox pair and the O2/H2O redox pair. NADH is the electron donor and O2 is the electron acceptor.
Not every donor-acceptor combination is thermodynamically possible. The redox potential of the acceptor must be more positive than the redox potential of the donor. Furthermore, actual environmental conditions may be far different from standard conditions (1 molar concentrations, 1 atm partial pressures, pH = 7), which apply to standard redox potentials. For example, hydrogen-evolving bacteria grow at an ambient partial pressure of hydrogen gas of 10−4 atm. The associated redox reaction, which is thermodynamically favorable in nature, is thermodynamic impossible under "standard" conditions.[citation needed]
Summary[edit]
Bacterial electron transport pathways are, in general, inducible. Depending on their environment, bacteria can synthesize different transmembrane complexes and produce different electron transport chains in their cell membranes. Bacteria select their electron transport chains from a DNA library containing multiple possible dehydrogenases, terminal oxidases and terminal reductases. The situation is often summarized by saying that electron transport chains in bacteria are branched, modular, and inducible.
Photosynthetic[edit]
In oxidative phosphorylation, electrons are transferred from a low-energy electron donor (e.g., NADH) to an acceptor (e.g., O2) through an electron transport chain. In photophosphorylation, the energy of sunlight is used to create a high-energy electron donor and an electron acceptor. Electrons are then transferred from the donor to the acceptor through another electron transport chain.
Photosynthetic electron transport chains have many similarities to the oxidative chains discussed above. They use mobile, lipid-soluble carriers (quinones) and mobile, water-soluble carriers (cytochromes, etc.). They also contain a proton pump. It is remarkable that the proton pump in all photosynthetic chains resembles mitochondrial Complex III.
Photosynthetic electron transport chains are discussed in greater detail in the articles Photophosphorylation, Photosynthesis, Photosynthetic reaction center and Light-dependent reaction.
Summary[edit]
Electron transport chains are redox reactions that transfer electrons from an electron donor to an electron acceptor. The transfer of electrons is coupled to the translocation of protons across a membrane, producing a proton gradient. The proton gradient is used to produce useful work. About 30 work units are produced per electron transport.
See also[edit]
CoRR hypothesis
Electron equivalent
Hydrogen hypothesis
Respirasome
References[edit]
Jump up ^ Murray, Robert K.; Daryl K. Granner; Peter A. Mayes; Victor W. Rodwell (2003). Harper's Illustrated Biochemistry. New York, NY: Lange Medical Books/ MgGraw Hill. p. 96. ISBN 0-07-121766-5.
Jump up ^ Karp, Gerald (2008). Cell and Molecular Biology (5th ed.). Hoboken, NJ: John Wiley & Sons. p. 194. ISBN 978-0-470-04217-5.
Jump up ^ Lauren, Biochemistry, Johnson/Cole, 2010, pp 598-611
Jump up ^ Garrett & Grisham, Biochemistry, Brooks/Cole, 2010, pp 598-611
Jump up ^ Garret and Grisham (2016). biochemistry. University of Virginia. p. 687. ISBN 978-1-305-57720-6.
Jump up ^ Jonckheere, An I.; Smeitink, Jan A. M.; Rodenburg, Richard J. T. (2017-03-10). "Mitochondrial ATP synthase: architecture, function and pathology". Journal of Inherited Metabolic Disease. 35 (2): 211–225. doi:10.1007/s10545-011-9382-9. ISSN 0141-8955. PMC 3278611  . PMID 21874297.
^ Jump up to: a b Garrett, Reginald H.; Grisham, Charles M. (2012). Biochemistry (5th ed.). Cengage learning. p. 664. ISBN 1-133-10629-3.
Jump up ^ Fillingame, Robert H; Angevine, Christine M; Dmitriev, Oleg Y (2003-11-27). "Mechanics of coupling proton movements to c-ring rotation in ATP synthase". FEBS Letters. 555 (1): 29–34. doi:10.1016/S0014-5793(03)01101-3. ISSN 1873-3468.
Jump up ^ Berg, Jeremy M.; Tymoczko, John L.; Stryer, Lubert (2002-01-01). "A Proton Gradient Powers the Synthesis of ATP".
Jump up ^ Cannon, Barbara; Nedergaard, Jan (2004-01-01). "Brown Adipose Tissue: Function and Physiological Significance". Physiological Reviews. 84 (1): 277–359. doi:10.1152/physrev.00015.2003. ISSN 0031-9333. PMID 14715917.
Jump up ^ Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002-01-01). "Electron-Transport Chains and Their Proton Pumps".
Fenchel T; King GM; Blackburn TH (September 2006). Bacterial Biogeochemistry: The Ecophysiology of Mineral Cycling (2nd ed.). Elsevier. ISBN 978-0-12-103455-9.
Lengeler JW (January 1999). Drews G; Schlegel HG, eds. Biology of the Prokaryotes. Blackwell Science. ISBN 978-0-632-05357-5.
Nelson DL; Cox MM (April 2005). Lehninger Principles of Biochemistry (4th ed.). W. H. Freeman. ISBN 978-0-7167-4339-2.
Nicholls DG; Ferguson SJ (July 2002). Bioenergetics 3. Academic Press. ISBN 978-0-12-518121-1.
Stumm W; Morgan JJ (1996). Aquatic Chemistry (3rd ed.). John Wiley & Sons. ISBN 978-0-471-51185-4.
Thauer RK; Jungermann K; Decker K (March 1977). "Energy conservation in chemotrophic anaerobic bacteria". Bacteriol Rev. 41 (1): 100–80. PMC 413997  . PMID 860983.
White D. (September 1999). The Physiology and Biochemistry of Prokaryotes (2nd ed.). Oxford University Press. ISBN 978-0-19-512579-5.
Voet D; Voet JG (March 2004). Biochemistry (3rd ed.). John Wiley & Sons. ISBN 978-0-471-58651-7.
Kim HS.; Patel, K; Muldoon-Jacobs, K; Bisht, KS; Aykin-Burns, N; Pennington, JD; Van Der Meer, R; Nguyen, P; et al. (January 2010). "SIRT3 is a mitochondria-localized tumor suppressor required for maintenance of mitochondrial integrity and metabolism during stress". Cancer Cell. 17 (1): 41–52. doi:10.1016/j.ccr.2009.11.023. PMC 3711519  . PMID 20129246.
External links[edit]
Electron Transport Chain Complex Proteins at the US National Library of Medicine Medical Subject Headings (MeSH)
Khan Academy, video lecture
UMich Orientation of Proteins in Membranes families/superfamily-3 - Complexes with cytochrome b-like domains
UMich Orientation of Proteins in Membranes families/superfamily-4 - Bacterial and mitochondrial cytochrome c oxidases
UMich Orientation of Proteins in Membranes families/superfamily-2 - Photosynthetic reaction centers and photosystems
UMich Orientation of Proteins in Membranes families/superfamily-78 - Cytochrome PORN renodoxin reductase
UMich Orientation of Proteins in Membranes families/superfamily-130 - Electron transfer flavoproteins
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