Some solutes enter cells by diffusion according to the concentration gradient without energy expenditure. Hydrophobic solutes diffuse through the lipid part of the membrane, and others diffuse through carrier proteins. The former is called simple diffusion and the latter facilitated diffusion. Facilitated diffusion shows different kinetics from that of simple diffusion. The initial diffusion rate is propor- tional to the concentration gradient in simple diffusion, while facili- tated diffusion shows a relationship between the diffusion rate and the concentration of the solute similar to the Michaelis—Menten kinetics known in enzyme catalysis. Solutes transported through facilitated diffusion do not passively leak into the cell to any signifi- cant extent, and the rate of transport is directly proportional to the fraction of carrier proteins associated with them. When the carrier protein is fully saturated with the solute, the rate of transport reaches a maximum, and the rate does not increase further with any further increase in solute concentration. By definition, charged solutes are not transported through diffusion, since the transport of charged solutes changes the membrane potential (Section 5.7.1).