Diferenzas entre revisións de «Quimiosmose»

Esta diferenza de carga ten como resultado un [[gradiente electroquímico]]. Este gradiente componse dun gradiente de pH e un gradiente eléctrico. O gradiente de pH corresponde á diferenza na concentración do ión H<sup>+</sup>. Xuntos, o gradiente electroquímico de protóns corresponde á concentración como á diferencia de carga que se poida xerar, e denomínase forza protón motriz (FPM).
Na mitocondria, a FPM créase maioritariamente polo compoñente eléctrico, mentres que nos cloroplastos, a FPM é practicamente creada maiormente polo gradiente de pH, xa que a carga dos protóns é neutralizada polo movemento de Clasup>-</sup> e outros anións. En cualquera dos casos, a FPM necesita ser de aproximadamente 50 kJ/mol para que a [[ATP sintase]] quede capacitada para producir ATP.
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>
&Delta;G is the Gibbs free energy change during transfer of 1&nbsp;mol of [[cation]]s X<sup>m+</sup> from the phase A to B down the electrical potential, &Delta;&psi; 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 &Delta;&mu;<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>
<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>
&Delta;&mu;<sub>H+</sub> = 1 kJ&middot;mol corresponds to &Delta;p = 10.4 mV. At 25&nbsp;°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 &Delta;pH
*electrical gradient &Delta;&psi;
When a system reaches equilibrium, &Delta;G (&Delta;&mu;<sub>m+</sub>, &Delta;p) = 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 !! &Delta;&psi;<br>(mV) !! &Delta;pH !! &Delta;p<br>(mV) !! &Delta;G<sub>p</sub><br>(kJ&middot;mol<sup>−1</sup>) !! H<sup>+</sup> / ATP
| [[Mitochondrion|mitochondrial]], inner (liver) || align = right | 170 || align = right | &le;0.5 || align = right | &le;200 || align = right | 66 || align = right | &ge;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 | &le;0.5 || align = right | &le;170 || align = right |40 || align = right | &ne;
&Delta;G<sub>p</sub> is the Gibbs free energy of ATP synthesis,
ADP + Pi &rarr; ATP
also called phosphorylation potential. The H<sup>+</sup> / ATP ratio values in the table above can be calculated by comparison of &Delta;p and &Delta;G<sub>p</sub>, for example:
H<sup>+</sup> / ATP = 66 kJ&middot;mol<sup>−1</sup> / (200 mV / 10.4 kJ&middot;mol<sup>−1</sup>/mV) = 66 / 19.2 = 3.4 (mitochondrion)
For mitochondria, &Delta;G<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 &ne;).
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}}
== Diferentes Mecanismos de quimiosmose ==