In sim ple diffusion, flux rate is lim ited nly by the Copyright o The McGraw-Hill Companies, Inc. Permission required for reproduction or display concentration gradient Diffusion In carrier. mediated transport, the num ber Maximal flt LL of available carriers places an Mediated transport upper limit on the flux rate Extracellular solute concentrationIn simple diffusion, flux rate is limited only by the concentration gradient. In carrier￾mediated transport, the number of available carriers places an upper limit on the flux rate