ABSTRACT
A tendency of particles to achieve a uniform spatial distribution is equivalent to the tendency of molecules in solution to achieve uniform concentrations that are independent of position throughout the solution. Local differences in concentrations are usually equilibrated by diffusion. The fluid mosaic model of lipid membranes describes the membrane as a fluidic phase in which lipids and embedded proteins undergo free lateral diffusion. Many membrane studies are performed on reconstituted membranes with a well-defined lipid composition that differs from the composition and complexity of natural cell membranes. The chemical potential is a driving force for chemical reactions. Flux of ions along a concentration gradient is an exergonic, spontaneous process. If the ion flux is controlled, the energy can be coupled to the endergonic synthesis of ATP. Instead of coupling the proton-motive force to ATP synthesis, the energy can also be converted into heat by uncoupled influx of protons.
