What phenomenon occurs when the transport of substance B levels off despite an increase in outside concentration?

The phenomenon where the transport of substance B levels off despite an increase in outside concentration is known as saturation. This occurs when all available transporters or channels for substance B are occupied or fully utilized. As the concentration of substance B outside the cell increases, transport continues to rise until it reaches a maximum rate called the transport maximum. After this point, even if more substance B is available, the transport cannot increase further, resulting in a plateau in the transport rate. This reflects a limiting factor in the system, such as the number of transport proteins or the rate at which they can operate, leading to saturation.

In contrast, competition refers to interactions between different substances that share the same transport mechanism, and specificity pertains to the ability of transporters to preferentially bind to particular substances. Affinity describes the strength of binding between transporters and substrates, but does not directly relate to the leveling off of transport with increasing concentrations.