Chemiosmosis
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'''Chemiosmosis''' is the diffusion of [[ions]] across a selectively-permeable membrane. More specifically, it relates to the generation of [[Adenosine triphosphate|ATP]] by the movement of [[hydrogen]] ions across a [[inner membrane|membrane]] during cellular respiration.
[[Image:Chemiosmosis1.png|thumb|right|400px|<center>An Ion gradient has [[potential energy]] and can be used to power chemical reactions when the ions pass through a [[Ion channel|channel]] (red).</center>]]
Hydrogen ions (protons) will [[diffusion|diffuse]] from an area of high proton concentration to an area of lower proton concentration. [[Peter D. Mitchell|Peter Mitchell]] proposed that an [[Electrochemical gradient|electrochemical concentration gradient]] of protons across a membrane could be harnessed to make [[adenosine triphosphate|ATP]]. He likened this process to [[osmosis]], the diffusion of water across a membrane, which is why it is called ''chemiosmosis''.
[[ATP synthase]] is the enzyme that makes ATP by chemiosmosis. It allows protons to pass through the membrane using the [[kinetic energy]] to [[phosphorylate]] ADP making ATP. The generation of [[adenosine triphosphate|ATP]] by chemiosmosis occurs in [[chloroplasts]] and [[mitochondria]] as well as in some [[bacteria]].
==The Chemiosmotic Theory==
[[Peter D. Mitchell]] proposed the '''chemiosmotic hypothesis''' in [[1961]].<ref>{{cite journal | author=Peter Mitchell | title=Coupling of phosphorylation to electron and hydrogen transfer by a chemi-osmotic type of mechanism | journal=Nature | year=1961 | volume=191 | issue= | pages= 144–148 | url= | doi=10.1038/191144a0 }}{{Entrez Pubmed|13771349}}</ref>
The theory suggests essentially that most [[Adenosine triphosphate|ATP]] synthesis in [[Cellular respiration|respiring]] cells comes from the [[electrochemical]] gradient across the inner membranes of [[mitochondrion|mitochondria]] by using the energy of [[NADH]] and [[Flavin|FADH<sub>2</sub>]] formed from the breaking down of energy rich molecules such as [[glucose]].
[[Image:Mitochondrial electron transport chain—Etc4.svg|thumb|left|450px|<center>Chemiosmosis in a mitochondrion</center>]]
Molecules such as glucose are metabolized to produce [[acetyl CoA]] as an energy-rich intermediate. The [[oxidation]] of acetyl CoA in the mitochondrial matrix is coupled to the reduction of a carrier molecule such as [[Nicotinamide adenine dinucleotide|NAD]] and [[FAD]].<ref>{{cite book | first=Bruce | last=Alberts | authorlink= | coauthors=Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts and Peter Walter | year=2002 | title=Molecular Biology of the Cell | edition= | publisher=Garland | location= | id=ISBN 0-8153-4072-9 | chapter=Proton Gradients Produce Most of the Cell's ATP | chapterurl=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=chemiosmotic+AND+mboc4%5Bbook%5D+AND+373681%5Buid%5D&rid=mboc4.section.2495#2519 }}</ref>
The carriers pass [[electron]]s to the [[electron transport chain]] (ETC) in the inner mitochondrial membrane, which in turn pass them to other proteins in the ETC. The energy available in the electrons is used to pump protons from the matrix across the inner mitochondrial membrane, storing energy in the form of a transmembrane [[electrochemical gradient]]. The protons move back across the inner membrane through the enzyme [[ATP synthase]]. The flow of protons back into the matrix of the mitochondrion via ATP synthase provides enough energy for [[Adenosine diphosphate|ADP]] to combine with inorganic [[phosphate]] to form ATP. The electrons and protons at the last pump in the ETC are taken up by [[oxygen]] to form [[water]].
This was a radical proposal at the time, and was not well accepted. The prevailing view was that the energy of electron transfer was stored as a stable high potential intermediate, a chemically more conservative concept.
The problem with the older paradigm is that no high energy intermediate was ever found, and the evidence for proton pumping by the complexes of the [[electron transfer chain]] grew too great to be ignored. Eventually the weight of evidence began to favor the chemiosmotic hypothesis, and in 1978, Peter Mitchell was awarded the [[Nobel Prize in Chemistry]].<ref>The [http://nobelprize.org/chemistry/laureates/1978/index.html Nobel Prize] in Chemistry 1978.</ref>
Chemiosmotic coupling is important for ATP production in [[chloroplast]]s<ref>{{cite book | first=Geoffrey M. | last=Cooper | authorlink= | coauthors= | year= | title=The Cell: A Molecular Approach | edition=2<sup>nd</sup> edition | publisher=Sinauer Associates, Inc. | location= | id=ISBN 0-87893-119-8 | chapter=Figure 10.22: Electron transport and ATP synthesis during photosynthesis | chapterurl=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=cooper.figgrp.1672 }}</ref>
and many [[bacteria]].<ref>{{cite book | first=Bruce | last=Alberts | authorlink= | coauthors=Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts and Peter Walter | year=2002 | title=Molecular Biology of the Cell | edition= | publisher=Garland | location= | id=ISBN 0-8153-4072-9 | chapter=Figure 14-32: The importance of H<sup>+</sup>-driven transport in bacteria | chapterurl=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.figgrp.2557 }}</ref>
==The proton-motive force==
In all cells, chemiosmosis involves the '''proton-motive force''' (PMF) in some step. This can be described as the storing of energy as a combination of a proton and voltage gradient across a membrane. The chemical potential energy refers to the difference in concentration of the protons and the electrical potential energy as a consequence of the charge separation (when the protons move without a counter-ion).
In most cases the proton motive force is generated by an electron transport chain which acts as both an electron and proton pump, pumping electrons in opposite directions, creating a separation of charge. In the mitochondria, free energy released from the electron transport chain is used to move protons from the mitochondrial matrix to the intermembrane space of the mitochondrion. Moving the protons to the outer parts of the mitochondrion creates a higher concentration of positively charged particles, resulting in a slightly positive, and slightly negative side (then electrical potential gradient is about -200 mV (inside negative). This charge difference results in an electrochemical gradient. This gradient is composed of both the pH gradient and the electrical gradient. The pH gradient is a result of the H+ ion concentration difference. Together the electrochemical gradient of protons is both a concentration and charge difference and is often called the proton motive force (PMF). In mitochondria the PMF is almost entirely made up of the electrical component but in chloroplasts the PMF is made up mostly of the pH gradient. In either case the PMF needs to be about 50 kJ/mol for the ATP synthase to be able to make ATP.
==In mitochondria==
[[Image:Electrontrans.gif|thumb|right|250px|A diagram of chemiosmotic phosphorylation]]
'''Chemiosmotic phosphorylation''' is the third pathway that produces [[Adenosine triphosphate|ATP]] from inorganic [[phosphate]] and an [[Adenosine diphosphate|ADP]] molecule. This process is part of [[oxidative phosphorylation]].
The complete breakdown of [[glucose]] in the presence of [[oxygen]] is called [[cellular respiration]]. The last steps of this process occur in mitochondria. The reduced molecules [[NADH]] and [[FADH2|FADH<sub>2]] are generated by the [[citric acid cycle|Krebs cycle]] and glycolysis. These molecules pass electrons to an [[electron transport chain]], which uses the energy released to create a proton gradient across the inner mitochondrial membrane. [[ATP synthase]] then uses the energy stored in this gradient to make ATP. This process is called oxidative phosphorylation because oxygen is the final electron acceptor and the energy released by reducing oxygen to water is used to phosphorylate ADP and generate ATP.
==In plants==
The [[Light-dependent reaction|Light reactions]] of [[photosynthesis]] generate energy by chemiosmosis. Chlorophyll loses an electron when energized by light. This electron travels down a [[Electron transport chain|photosynthetic electron transport chain]] ending on the high energy molecule [[NADPH]]. The [[electrochemical gradient]] generated across the [[thylakoid]] membrane drives the production of ATP by [[ATP Synthase]]. This process is known as [[photophosphorylation]].
==In prokaryotes==
[[Bacteria]] and [[archaea]] also can use chemiosmosis to generate ATP. [[Cyanobacteria]], [[green sulfur bacteria]], and [[purple bacteria]] create energy by a process called [[photophosphorylation]]. These bacteria use the energy of light to create a proton gradient using a photosynthetic [[electron transport chain]]. Non-photosynthetic bacteria such as ''E. coli'' also contain [[ATP synthase]].
In fact, mitochondria and chloroplasts are believed to have been formed when early eukaryotic cells ingested bacteria that could create energy using chemiosmosis. This is called the [[endosymbiotic theory]].
==See also==
*[[Mitochondrion]]
*[[Chloroplast]]s
*[[Electrochemical gradient]]
*[[Electron transfer chain]]
*[[Cytochrome]]
*[[Adenosine triphosphate|ATP]]
*[[Cellular respiration]]
*[[Citric acid cycle]]
*[[Glycolysis]]
*[[Oxidative phosphorylation]]
*[http://ats.doit.wisc.edu/Biology/cb/ch/t1.htm Chemiosmosis (University of Wisconsin) ]
==References cited==
<references/>
==Other references==
* ''biochemistry textbook reference, from the [[NCBI bookshelf]]'' — {{cite book | title=Biochemistry (5th edition) | chapter=18.4. A Proton Gradient Powers the Synthesis of ATP | editor=Jeremy M. Berg, John L. Tymoczko, Lubert Stryer | publisher=W. H. Freeman | url=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=stryer.section.2528 }}
* ''technical reference relating one set of experiments aiming to test some tenets of the chemiosmotic theory'' — {{cite journal | author=Seiji Ogawa and Tso Ming Lee | title=The Relation between the Internal Phosphorylation Potential and the Proton Motive Force in Mitochondria during ATP Synthesis and Hydrolysis | journal=Journal of Biological Chemistry | year=1984 | volume=259 | issue=16 | pages= 10004–10011 | url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=6469951&query_hl=1 }}
[[Category:Cell biology]]
[[Category:Cellular respiration]]
[[cs:Chemiosmotická teorie]]
[[de:Chemiosmotische Kopplung]]
[[es:Quimiosmosis]]
[[it:Chemiosmosi]]
[[pl:Chemiosmoza]]