Transport mechanisms that require carrier proteins (facilitated diffusion and active transport) are subject to competition and saturation.

• A. True
• B. False
• C. Depends on transporter
• D. Not necessarily

Answer: (A)

Carrier-mediated transport relies on a limited number of carrier proteins with specific binding sites. Because of this finite capacity, the transport rate cannot increase indefinitely and will plateau as all carriers become occupied—this is saturation. When more than one molecule uses the same transporter, they compete for the same binding site, so the presence of one substrate reduces the transport rate of the other—this is competition. This behavior occurs in both facilitated diffusion and active transport, since both depend on carrier proteins that can become saturated and can show competitive interactions between substrates. For example, glucose uptake via GLUT transporters shows saturation at higher concentrations, and multiple drugs that share a transporter can compete with each other.