|
|
| Line 1: |
Line 1: |
| ''' Chemiosmosis''' is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. 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.
| | My name is Xavier and I am studying Dramatic Literature and History and Athletics and Physical Education at Wyssachen / Switzerland.<br><br>Also visit my web page: [http://free-minecraft-project.hackservices.com free minecraft project] |
| | |
| [[Image:Chemiosmosis1.png|thumb|right|300px|An Ion gradient has [[potential energy]] and can be used to power chemical reactions when the ions pass through a [[Ion channel|channel]] (red).]]
| |
| | |
| 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 linked 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 and uses 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 most [[bacteria]] and [[archaea]].
| |
| | |
| ==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= 4784| pages= 144–148 | url= | doi=10.1038/191144a0 |pmid=13771349 | bibcode=1961Natur.191..144M}}</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 group|FADH<sub>2</sub>]] formed from the breaking down of energy-rich molecules such as [[glucose]].
| |
| [[Image:Mitochondrial electron transport chain—Etc4.svg|thumb|right|450px|Chemiosmosis in a [[mitochondrion]].]]
| |
| 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= | 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= 2000| title=The Cell: A Molecular Approach | edition=2nd | publisher=Sinauer Associates, Inc. | location= | 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]] and [[archaea]].<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= | 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==
| |
| [[File:Chemiosmotic coupling mitochondrion.gif|thumb|400px|Energy conversion by the inner mitochondrial membrane and chemiosmotic coupling between the chemical energy of redox reactions in the [[respiratory chain]] and the [[oxidative phosphorylation]] [[enzyme|catalysed]] by the [[ATP synthase]] {{r|Nicholls92}}<ref name=Stryer95>{{cite book |last=Stryer |first=Lubert |year=1995 |title=Biochemistry |publisher=W. H. Freeman and Company |location=New York - Basingstoke |edition=fourth |isbn=978-0716720096 }}</ref> (sometimes called as ''"mitochondrial mushrooms"'').]]
| |
| | |
| The movement of ions across the membrane depends on a combination of two factors:
| |
| # [[Diffusion]] force caused by concentration gradient - all particles including ions tend to diffuse from higher concentration to lower.
| |
| # Electrostatic force caused by electrical potential gradient - [[cations]] like protons H<sup>+</sup> tend to diffuse down the electrical potential, [[anions]] in the opposite direction.
| |
| These two gradients taken together can be expressed as an [[electrochemical gradient]].
| |
| | |
| [[Lipid bilayer]]s of [[biological membrane]]s however are barriers for ions. This is why energy can be stored as a combination of these two gradients across the membrane. Only special membrane proteins like for example [[ion channel]]s can sometimes allow ions to move across the membrane (see also: [[Membrane transport]]). In chemiosmotic theory transmembrane [[ATP synthase]]s are very important. They convert energy of spontaneous flow of protons through them into chemical energy of [[Adenosine triphosphate|ATP]] bonds .
| |
| | |
| Hence researchers created the term '''proton-motive force''' (PMF), derived from the [[electrochemical gradient]] mentioned earlier. It can be described as the measure of the potential energy stored as a combination of proton and voltage gradients across a membrane (differences in proton concentration and electrical potential). The electrical gradient is a consequence of the charge separation across the membrane (when the protons H<sup>+</sup> move without a [[counterion]], such as chloride Cl<sup>-</sup>).
| |
| | |
| In most cases the proton motive force is generated by an electron transport chain which acts as a proton pump, using the energy of electrons from an electron carrier ([[Gibbs free energy]] of [[redox]] reactions) to pump protons (hydrogen ions) out across the membrane, separating the charge across the membrane. In mitochondria, energy released by the electron transport chain is used to move protons from the mitochondrial matrix to the intermembrane space of the mitochondrion. Moving the protons out of the mitochondrion creates a lower concentration of positively charged protons inside it, resulting in a slight negative charge on the inside of the membrane. The electrical potential gradient is about -170 mV {{r|Nicholls92}}. These gradients - charge difference and the proton concentration difference both create a combined electrochemical gradient across the membrane, often expressed as 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 because the charge of protons H<sup>+</sup> is neutralized by the movement of Cl<sup>-</sup> and other anions. In either case, the PMF needs to be about 50 kJ/mol for the ATP synthase to be able to make ATP.
| |
| | |
| ===Equations===
| |
| | |
| The proton-motive force is derived from the [[Gibbs free energy]]: {{r|Nicholls92}}
| |
| | |
| <math>\Delta G(kJ\cdot mol^{-1}) = -mF \Delta \psi + 2.3RT \log_{10}\left ({[X^{m+}]_B\over [X^{m+}]_A}\right )</math>
| |
| | |
| ΔG is the Gibbs free energy change during transfer of 1 mol of [[cation]]s X<sup>m+</sup> from the phase A to B down the electrical potential, Δψ is the electrical potential difference (mV) between phases P and N (A and B), [X<sup>m+</sup>]<sub>A</sub> and [X<sup>m+</sup>]<sub>B</sub> are our cation concentrations on opposite sides of the membrane, F is the [[Faraday constant]], R [[gas constant]]. The Gibbs free energy change here is expressed frequently also as electrochemical ion gradient Δμ<sub>m+</sub>
| |
| | |
| <math>\Delta \mu _{Xm+} (kJ\cdot mol^{-1}) = \Delta G(kJ\cdot mol^{-1})</math>
| |
| | |
| In case of the '''electrochemical proton gradient''' the equation can be simplified to:
| |
| | |
| <math>\Delta \mu _{H+} = -F \Delta \psi + 2.3RT \Delta pH</math>
| |
| | |
| where
| |
| | |
| <math>\Delta pH = pH_A - pH_B</math>
| |
| | |
| (pH in phase P - pH in phase N)
| |
| | |
| Mitchell defined the '''proton-motive force''' (PMF) as
| |
| | |
| <math>\Delta p (mV) = -{\Delta \mu _{H+}\over F}</math>
| |
| | |
| Δμ<sub>H+</sub> = 1 kJ·mol corresponds to Δp = 10.4 mV. At 25°C (298K) this equation takes the form:
| |
| | |
| <math>\Delta p = \Delta \psi - 59 \Delta pH</math>
| |
| | |
| The energy expressed here as Gibbs free energy, electrochemical proton gradient, or proton-motive force (PMF), is a combination of two gradients across the membrane:
| |
| *concentration gradient expressed here as ΔpH
| |
| *electrical gradient Δψ
| |
| When a system reaches equilibrium, ΔG (Δμ<sub>m+</sub>, Δp) = 0, but it doesn't mean that concentrations are equal on both sides of the membrane. The ions' electrical gradient, in addition to the concentration difference, affects spontaneous movement across the membrane.
| |
| | |
| Sample values: {{r|Nicholls92}}
| |
| [[Image:Electrontrans.gif|thumb|right|250px|A diagram of chemiosmotic phosphorylation]]
| |
| | |
| {| class="wikitable"
| |
| |-
| |
| ! Membrane !! Δψ<br>(mV) !! ΔpH !! Δp<br>(mV) !! ΔG<sub>p</sub><br>(kJ·mol<sup>−1</sup>) !! H<sup>+</sup> / ATP
| |
| |-
| |
| | [[Mitochondrion|mitochondrial]], inner (liver) || align = right | 170 || align = right | ≤0.5 || align = right | ≤200 || align = right | 66 || align = right | ≥3.4
| |
| |-
| |
| | [[chloroplast]], [[thylakoid]] || align = right | 0 || align = right | 3.3 || align = right | 195 || align = right | 60 || align = right | 3.1
| |
| |-
| |
| | ''[[Escherichia coli|E. coli]]'' cells, pH 7.5 || align = right | 140 || align = right | ≤0.5 || align = right | ≤170 || align = right |40 || align = right | ≠
| |
| |}
| |
| | |
| ΔG<sub>p</sub> is the Gibbs free energy of ATP synthesis,
| |
| | |
| ADP + Pi → ATP
| |
| | |
| also called phosphorylation potential. The H<sup>+</sup> / ATP ratio values in the table above can be calculated by comparison of Δp and ΔG<sub>p</sub>, for example:
| |
| | |
| H<sup>+</sup> / ATP = 66 kJ·mol<sup>−1</sup> / (200 mV / 10.4 kJ·mol<sup>−1</sup>/mV) = 66 / 19.2 = 3.4 (mitochondrion)
| |
| | |
| For mitochondria, ΔG<sub>p</sub> takes here into account 1 H<sup>+</sup> consumed to transfer a phosphate molecule (Pi) across the inner membrane into the matrix by the [[SLC25A3|phosphate carrier]] (PiC). Otherwise it would be lower. In ''E. coli'' the H<sup>+</sup> / ATP ratio is difficult to determine (marked as ≠).
| |
| | |
| The energy of more than 3 H<sup>+</sup> is required to generate the chemical energy to convert a single [[Adenosine triphosphate|ATP]]. This value is slightly lower than the theoretical number of 4 H<sup>+</sup> involved in [[oxidative phosphorylation]] of one ADP molecule to ATP during [[cellular respiration#Efficiency of ATP production|cellular respiration]] (3 H<sup>+</sup> flowing through the [[ATP synthase]] / 1 ATP + 1 leaking from the cytoplasm through the phosphate carrier PiC). {{r|Nicholls92|Stryer95}}
| |
| | |
| ==In mitochondria==
| |
| [[Image:Chemiosmotic proton transfer.gif|thumb|400px|Directions of chemiosmotic proton transfer in the [[mitochondrion]], [[chloroplast]] and in [[gram-negative bacteria]]l cells ([[cellular respiration]] and [[photosynthesis]]). The bacterial [[cell wall]] is omitted, [[gram-positive bacteria]]l cells don't have outer membrane.<ref name=Nicholls92>{{cite book |title=Bioenergetics 2 |edition=2nd |authors=Nicholls D. G., Ferguson S, J. |year=1992 |publisher=Academic Press |location=San Diego |isbn=9780125181242 }}</ref>]]
| |
| | |
| 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]], glycolysis, and pyruvate processing. 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. Light energy (photons) are received by the antenna complex of Photosystem 2, which excites a pair of electrons to a higher energy level. These electrons travel down an electron transport chain, causing H+ to diffuse across the thylakoid membrane into the inter-thylakoid space. These H+ are then transported down their concentration gradient through an enzyme called ATP-synthase, creating ATP by phosphorylation of ADP to ATP. The electrons from the initial light reaction reach Photosystem 1, then are raised to a higher energy level by light energy and then received by an electron receptor and reduce NADP+ to NADPH+H. The electrons from Photosystem 2 get replaced by the splitting of water, called "photolysis." Two water molecules must be split in order to gain 4 electrons (as well as O2, the oxygen eudicots require for survival).
| |
| | |
| ==In prokaryotes==
| |
| [[File:Bacteriorhodopsin chemiosmosis.gif|thumb|220px|Chemiosmotic coupling between the sun energy, bacteriorhodopsin and [[phosphorylation]] (chemical energy) during [[photosynthesis]] in [[Halophile|halophilic]] bacteria ''[[Halobacterium salinarum]]'' (syn. ''H. halobium''). The bacterial [[cell wall]] is omitted. {{r|Nicholls92|Stryer95}}]]
| |
| | |
| [[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 transfer energy using chemiosmosis. This is called the [[endosymbiotic theory]].
| |
| | |
| '''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]].
| |
| | |
| ==See also==
| |
| *[[Electrochemical gradient]]
| |
| *[[Cellular respiration]]
| |
| *[[Bacteriorhodopsin]]
| |
| *[[Citric acid cycle]]
| |
| *[[Glycolysis]]
| |
| *[[Oxidative phosphorylation]]
| |
| | |
| ==References cited==
| |
| {{Reflist}}
| |
| | |
| ==Further reading==
| |
| * ''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 | pmid=6469951 }}
| |
| | |
| ==External links==
| |
| *[http://ats.doit.wisc.edu/Biology/cb/ch/t1.htm Chemiosmosis (University of Wisconsin) ]
| |
| | |
| [[Category:Cell biology]]
| |
| [[Category:Cellular respiration]]
| |