Final answer:
Saturation of carriers limits the rate of transport across selectively permeable biological membranes when all carrier proteins are occupied, preventing an increase in transport rate despite higher concentration gradients.
Step-by-step explanation:
Saturation of carriers is a critical rate-limiting factor in the transport of selected substances across biological membranes that are selectively permeable. This transport may involve different types of transporter proteins such as uniporters, symporters, and transport proteins that facilitate the movement of specific molecules across the membrane. When all of the carrier proteins are occupied, or bound to their ligands, they are considered saturated, meaning the transport rate has reached its maximum capacity. Even if the concentration gradient increases further, the rate of transport will not increase because the carriers are unable to bind additional molecules.
Facilitated diffusion is one such transport mechanism where carrier proteins help molecules that cannot directly pass through the lipid bilayer diffuse down their concentration gradient. However, this process is limited by the number of available transport proteins and becomes inefficient when all carriers are saturated. To summarize, carrier saturation limits the efficiency of substance transport across cellular membranes during facilitated diffusion.