But their permeation kinetics does not follow the formula mentioned above, it resembles more an enzyme - substrate turnover kinetic. Such a kinetic is known as facilitated diffusion . The permeability is selective and dependent on the presence of specific membrane-bound transport molecules with an affinity to a selected group of chemically related substances. The substrates do not pass the membrane by themselves but are transported as a substrate-carrier complex. In contrast to enzymes do they not convert their substrates but transport them through the membrane instead.
Several substrates can compete for the same carrier.
Their binding properties concerning the carrier may differ and as a consequence differ their transport kinetics, too. The loaded carrier crosses the membrane with an other velocity than the unloaded carrier. The maximal transport velocity is dependent on the number of the carrier molecules in the membrane.