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Publisher Summary This chapter discusses the fundamental principles of basic processes of diffusion and membrane permeability, by which we can understand the fundamentals of membrane transport and the mechanisms for development of the resting electrical potential of cells. The chapter discusses Fick's law of diffusion, diffusion coefficient, diffusion across a membrane with partitioning, electrodiffusion, and Ussing flux ratio equation. Molecules of gases and liquids, and of all dissolved solutes, are in continuous motion. The velocities of individual molecules vary to a wide extent, as do their kinetic energies. Diffusion is the process whereby particles in a gas, liquid, or solid tend to intermingle because of their spontaneous motion caused by thermal agitation. The relevant equations that govern the diffusion of uncharged particles and charged ions across membranes with and without partitioning are described.. The relationship between the diffusion coefficient (D) and the permeability coefficient (P) and the factors that determine these coefficients are discussed in the chapter. The dependence of P on the mobility of an ion through the membrane under a voltage gradient has also been discussed. Electrochemical potential has been defined, and the inter conversion between flux and current has been developed. Also the Ussing flux ratio equation has been described along with examples. Relating to this, the concept of potential energy wells and barriers has been portrayed. The time course of diffusion is an exponential process and it may be noted that half-time for completion of diffusion does not depend on the concentration gradient, but merely on the diffusion coefficient, the area and the thickness of the membrane, and the volume of solution.The dependence of P on the mobility of an |
