In the second reaction, a second PQH 2 gets oxidized, adding an electron to another plastocyanin and PQ. The second electron is transferred to heme b L which then transfers it to heme b H which then transfers it to PQ. In the first reaction, PQH 2 binds to the complex on the lumen side and one electron is transferred to the iron-sulfur center which then transfers it to cytochrome f which then transfers it to plastocyanin. The process that occurs is similar to the Q-cycle in Complex III of the electron transport chain. The combined effect can be quantified as a gradient in the thermodynamic electrochemical potential: ∇ μ ¯ i = ∇ μ i ( r → ) + z i F ∇ φ ( r → ), Īfter being released from PSII, PQH 2 travels to the cytochrome b 6f complex which then transfers two electrons from PQH 2 to plastocyanin in two separate reactions. The combination of these two phenomena determines the thermodynamically-preferred direction for an ion's movement across the membrane. In the former effect, the concentrated charge attracts charges of the opposite sign in the latter, the concentrated species tends to diffuse across the membrane to an equalize concentrations. In mitochondria and chloroplasts, proton gradients generate a chemiosmotic potential used to synthesize ATP, and the sodium-potassium gradient helps neural synapses quickly transmit information.Īn electrochemical gradient has two components: a differential concentration of electric charge across a membrane and a differential concentration of chemical species across that same membrane. In biology, electrochemical gradients allow cells to control the direction ions move across membranes. It appears in electroanalytical chemistry and has industrial applications such as batteries and fuel cells. 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.Įlectrochemical gradients are essential to the operation of batteries and other electrochemical cells, photosynthesis and cellular respiration, and certain other biological processes.Įlectrochemical energy is one of the many interchangeable forms of potential energy through which energy may be conserved. Ions also carry an electric charge that forms an electric potential across a membrane. A concentration gradient is generated by diffusion between two regions where the concentration of a substance differs diffusion proceeds until the concentrations in the two regions become equal. 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. The concentration gradient of a solute is the change of concentration per unit distance in a solution. 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. Diagram of ion concentrations and charge across a semi-permeable cellular membrane.Īn electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane.
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