Which statement best describes saturation and competition in transporter-mediated processes?

• A. They never occur
• B. They cause rate limitation when multiple substrates share a transporter
• C. They increase rate with higher substrate
• D. They only occur for active transport

Answer: (B)

Saturation and competition happen because transporter proteins have a limited capacity: there are only so many binding sites and a finite turnover rate. When substrate levels are low, transport increases with more substrate, but as soon as all transporter molecules are occupied, the rate plateaus at a maximum. If multiple substrates use the same transporter, they compete for those binding sites, so the presence of one substrate can reduce the transport rate of another. This is exactly why the statement that describes rate limitation when multiple substrates share a transporter is the best fit.

The other ideas aren’t accurate here: transport can and does saturate, so the rate doesn’t keep rising with higher substrate indefinitely; and these concepts aren’t limited to active transport—the same saturation and competition occur with facilitated transport as well.