Electrochemical gradient

An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and the electrical gradient, or difference in charge across a membrane. When there are unequal concentrations of an ion across a permeable membrane, the ion will move across the membrane from the area of higher concentration to the area of lower concentration through simple diffusion. Ions also carry an electric charge that forms an electric potential across a membrane. If there is an unequal distribution of charges across the membrane, then the difference in electric potential generates a force that drives ion diffusion until the charges are balanced on both sides of the membrane.(100-200)10 2.2-2.6 (100 - 116)Normally 7.35 to 7.45 in arterial blood An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and the electrical gradient, or difference in charge across a membrane. When there are unequal concentrations of an ion across a permeable membrane, the ion will move across the membrane from the area of higher concentration to the area of lower concentration through simple diffusion. Ions also carry an electric charge that forms an electric potential across a membrane. If there is an unequal distribution of charges across the membrane, then the difference in electric potential generates a force that drives ion diffusion until the charges are balanced on both sides of the membrane. Electrochemical potential is important in electroanalytical chemistry and industrial applications such as batteries and fuel cells. It represents one of the many interchangeable forms of potential energy through which energy may be conserved. In biological processes, the direction an ion moves by diffusion or active transport across a membrane is determined by the electrochemical gradient. In mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is also known as a proton motive force. This potential energy is used for the synthesis of ATP by oxidative phosphorylation or photophosphorylation, respectively. An electrochemical gradient has two components. First, the electrical component is caused by a charge difference across the lipid membrane. Second, a chemical component is caused by a differential concentration of ions across the membrane. The combination of these two factors determines the thermodynamically favourable direction for an ion's movement across a membrane. An electrochemical gradient is analogous to the water pressure across a hydroelectric dam. Membrane transport proteins such as the sodium-potassium pump within the membrane are equivalent to turbines that convert the water's potential energy to other forms of physical or chemical energy, and the ions that pass through the membrane are equivalent to water that ends up at the bottom of the dam. Also, energy can be used to pump water up into the lake above the dam. In similar manner, chemical energy in cells can be used to create electrochemical gradients. The term is typically applied in contexts wherein a chemical reaction is to take place, such as one involving the transfer of an electron at a battery electrode. In a battery, an electrochemical potential arising from the movement of ions balances the reaction energy of the electrodes. The maximum voltage that a battery reaction can produce is sometimes called the standard electrochemical potential of that reaction (see also Electrode potential and Table of standard electrode potentials). In instances pertaining specifically to the movement of electrically charged solutes, the potential is often expressed in units of volts. See: Concentration cell. The generation of a transmembrane electrical potential through ion movement across a cell membrane drives biological processes like nerve conduction, muscle contraction, hormone secretion, and sensory processes. By convention, a typical animal cell has a transmembrane electrical potential of -50 mV to -70 mV inside the cell relative to the outside. Electrochemical gradients also play a role in establishing proton gradients in oxidative phosphorylation in mitochondria. The final step of cellular respiration is the electron transport chain. Four complexes embedded in the inner membrane of the mitochondrion make up the electron transport chain. However, only complexes I, III, and IV pump protons from the matrix to the intermembrane space (IMS). In total, there are ten protons translocated from the matrix to the IMS which generates an electrochemical potential of more than 200mV. This drives the flux of protons back into the matrix through ATP synthase which produces ATP by adding an inorganic phosphate to ADP. Thus, generation of a proton electrochemical gradient is crucial for energy production in mitochondria. The total equation for the electron transport chain is: Similar to the electron transport chain, the light-dependent reactions of photosynthesis pump protons into the thylakoid lumen of chloroplasts to drive the synthesis of ATP by ATP synthase. The proton gradient can be generated through either noncyclic or cyclic photophosphorylation. Of the proteins that participate in noncyclic photophosphorylation, photosystem II (PSII), plastiquinone, and cytochrome b6f complex directly contribute to generating the proton gradient. For each four photons absorbed by PSII, eight protons are pumped into the lumen. The total equation for photophosphorylation is shown: