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Concept
Gradient électrochimique
Résumé
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.
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 gradients are essential to the operation of batteries and other electrochemical cells, photosynthesis and cellular respiration, and certain other biological processes.
Electrochemical energy is one of the many interchangeable forms of potential energy through which energy may be conserved. It appears in electroanalytical chemistry and has industrial applications such as batteries and fuel cells. In biology, electrochemical gradients allow cells to control the direction ions move across membranes. 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.
An 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 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. The combination of these two phenomena determines the thermodynamically-preferred direction for an ion's movement across the membrane.
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