External concentration of solute Figure 3.9 Transport versus diffusion. In transport, the uptake rate shows saturation at relatively low extemal concentratons Transport systems show several characteristic properties. First, in contrast with diffusion, transport systems show a saturation effect. If the concentration of substrate is high enough to saturate the transporter, which can occur at even the very low substrate concentrations found in nature, the rate of uptake becomes maximal and the addition of more substrate does not increase the rate (Figure 3-9). This characteristic feature of trans port proteins is essential for a system that must concentrate nutrients from an often very dilute environment. A second char- acteristic of carrier-mediated transport is the high specificity of the transport event. Many carrier proteins react only with a sin gle molecule, whereas a few show affinities for a closely related class of molecules, such as sugars or amino acids. This economy in uptake reduces the need for separate transport proteins for each different amino acid or sugar.