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Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An example of this would be the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membrane during cellular respiration or photosynthesis. Hydrogen ions, or protons, will diffuse from an area of high proton concentration to an area of lower proton concentration, and an electrochemical concentration gradient of protons across a membrane can be harnessed to make ATP. This process is related 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 free energy difference to phosphorylate adenosine diphosphate (ADP), making ATP. The generation of ATP by chemiosmosis occurs in mitochondria and chloroplasts, as well as in most bacteria and archaea, an electron transport chain pumps H+ ions (protons) in the thylakoid spaces through thylakoid membranes to stroma (fluid). The energy from the electron movement through electron transport chains cross through ATP synthase which allows the proton to pass through them and use this free energy difference to photophosphorylate ADP making ATP.

The chemiosmotic theory

Proton-motive force

Equations

The proton-motive force is derived from the Gibbs free energy. Let N denote the inside of a cell, and let P denote the outside. Then :$\Delta\!G = zF \Delta\!\psi + RT \ln\frac$ where * $\Delta\!G$ is the Gibbs free energy change per unit amount of cations transferred from P to N; * $z$ is the charge number of the cation $\mathrm^$; * $\Delta\psi$ is the electric potential of N relative to P; * $mathrm^$ and $mathrm^$ are the cation concentrations at P and N, respectively; * $F$ is the Faraday constant; * $R$ is the gas constant; and * $T$ is the temperature. The molar Gibbs free energy change $\Delta\!G$ is frequently interpreted as a molar electrochemical ion potential $\Delta\!\mu _ = \Delta\!G$. For an electrochemical proton gradient $z=1$ and as a consequence: :$\Delta\!\mu _ = F \Delta\!\psi + RT \ln \frac = F \Delta\!\psi - \left(\ln 10\right)RT \Delta \mathrm$ where :$\Delta\!\mathrm = \mathrm_ - \mathrm_$. Mitchell defined the proton-motive force (PMF) as :$\Delta\!p = -\frac$. For example, $\Delta\!\mu_=1\,\mathrm\,\mathrm^$ implies $\Delta\!p = 10.4\,\mathrm$. At $298\,\mathrm$ this equation takes the form: $\Delta\!p = -\Delta\!\psi + \left\left(59.1\,\mathrm\right\right)\Delta\!\mathrm$. Note that for spontaneous proton import from the P side (relatively more positive and acidic) to the N side (relatively more negative and alkaline), $\Delta\!\mu _$ is negative (similar to $\Delta\!G$) whereas PMF is positive (similar to redox cell potential $\Delta E$). It is worth noting that, as with any transmembrane transport process, the PMF is directional. The sign of the transmembrane electric potential difference $\Delta\!\psi$ is chosen to represent the change in potential energy per unit charge flowing into the cell as above. Furthermore, due to redox-driven proton pumping by coupling sites, the proton gradient is always inside-alkaline. For both of these reasons, protons flow in spontaneously, from the P side to the N side; the available free energy is used to synthesize ATP (see below). For this reason, PMF is defined for proton import, which is spontaneous. PMF for proton export, i.e., proton pumping as catalyzed by the coupling sites, is simply the negative of PMF(import). The spontaneity of proton import (from the P to the N side) is universal in all bioenergetic membranes. This fact was not recognized before the 1990s, because the chloroplast thylakoid lumen was interpreted as an interior phase, but in fact it is topologically equivalent to the exterior of the chloroplast. Azzone et al. stressed that the inside phase (N side of the membrane) is the bacterial cytoplasm, mitochondrial matrix, or chloroplast stroma; the outside (P) side is the bacterial periplasmic space, mitochondrial intermembrane space, or chloroplast lumen. Furthermore, 3D tomography of the mitochondrial inner membrane shows its extensive invaginations to be stacked, similar to thylakoid disks; hence the mitochondrial intermembrane space is topologically quite similar to the chloroplast lumen.: 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: * the concentration gradient (via $\Delta\!\mathrm$) and * electric potential gradient $\Delta\!\psi$. When a system reaches equilibrium, $\Delta\!\rho = 0$; nevertheless, the concentrations on either side of the membrane need not be equal. Spontaneous movement across the potential membrane is determined by both concentration and electric potential gradients. The molar Gibbs free energy $\Delta\!G_$ of ATP synthesis :$\mathrm^ + \mathrm^ + \mathrm_3^ \rightarrow \mathrm^ + \mathrm$ is also called phosphorylation potential. The equilibrium concentration ratio $mathrm^+mathrm/math> can be calculated by comparing\Delta\!pand\Delta\!G_, for example in case of the mammalian mitochondrion:H+/ ATP = \Delta G$p / (Δp / 10.4 kJ·mol−1/mV) = 40.2 kJ·mol−1 / (173.5 mV / 10.4 kJ·mol−1/mV) = 40.2 / 16.7 = 2.4. The actual ratio of the proton-binding c-subunit to the ATP-synthesizing beta-subunit copy numbers is 8/3 = 2.67, showing that under these conditions, the mitochondrion functions at 90% (2.4/2.67) efficiency. In fact, the thermodynamic efficiency is mostly lower in eukaryotic cells because ATP must be exported from the matrix to the cytoplasm, and ADP and phosphate must be imported from the cytoplasm. This "costs" one "extra" proton import per ATP, hence the actual efficiency is only 65% (= 2.4/3.67).

In mitochondria

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 are generated by the 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 it uses energy released by the oxidation of NADH and FADH2 to phosporylate ADP into ATP.

In plants

The light reactions of photosynthesis generate ATP by the action of chemiosmosis. The photons in sunlight are received by the antenna complex of Photosystem II, which excites electrons to a higher energy level. These electrons travel down an electron transport chain, causing protons to be actively pumped across the thylakoid membrane into the thylakoid lumen. These protons then flow down their electrochemical potential gradient through an enzyme called ATP-synthase, creating ATP by the phosphorylation of ADP to ATP. The electrons from the initial light reaction reach Photosystem I, then are raised to a higher energy level by light energy and then received by an electron acceptor and reduce NADP+ to NADPH. The electrons lost from Photosystem II get replaced by the oxidation of water, which is "split" into protons and oxygen by the oxygen-evolving complex (OEC, also known as WOC, or the water-oxidizing complex). To generate one molecule of diatomic oxygen, 10 photons must be absorbed by photosystems I and II, four electrons must move through the two photosystems, and 2 NADPH are generated (later used for carbon dioxide fixation in the Calvin Cycle).

In prokaryotes

Bacteria and archaea also can use chemiosmosis to generate ATP. Cyanobacteria, green sulfur bacteria, and purple bacteria synthesize ATP 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 the product of endosymbiosis and trace back to incorporated prokaryotes. This process is described in the endosymbiotic theory. The origin of the mitochondrion triggered the origin of eukaryotes, and the origin of the plastid the origin of the Archaeplastida, one of the major eukaryotic supergroups. Chemiosmotic phosphorylation is the third pathway that produces ATP from inorganic phosphate and an ADP molecule. This process is part of oxidative phosphorylation.

*Bacteriorhodopsin *Cellular respiration *Citric acid cycle *Electrochemical gradient *Glycolysis *Oxidative phosphorylation

References