Characteristics of carriermediated transport

- apply to facilitated diffusion and primary and secondary active transport:

1. Stereospecificity. For example, D-glucose (the natural isomer) is transported by facilitated diffusion, but the L-isomer is not. Simple diffusion, however, would not distinguish between the two isomers because it does not involve a carrier.

2. Saturation. The transport rate increases as the concentration of the solute increases, until the carriers are saturated. The transport maximum (Tm) is analogous to the maximum velocity (Vmax) in enzyme kinetics.

3. Competition. Structurally related solutes compete for transport sites on carrier molecules. For example, galactose is a competitive inhibitor of glucose transport in the small intestine.

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