Final answer:
The rate of active transport is tied to the speed of individual carrier proteins, which utilize energy to transport substances against their concentration gradient. As these proteins can become saturated, their maximum rate limits the overall active transport rate. Examples include the sodium-potassium pump, with transport speeds ranging from a thousand to a million molecules per second.
Step-by-step explanation:
The rate of active transport depends on the speed of individual carrier proteins because these proteins operate by binding to the specific molecules they are designed to transport and then using energy, typically ATP, to move them across the cell membrane, against their concentration gradient. There are a limited number of carrier proteins in a cell membrane, which can become saturated, meaning they are working at full capacity. When carrier proteins are saturated, the rate of transport cannot increase even if the concentration gradient increases.
The sodium-potassium pump is a well-known example of a carrier protein that utilizes active transport to exchange sodium for potassium across the plasma membrane. This process requires energy in the form of ATP and results in the transport of ions against their concentration gradient. The speed at which individual carrier proteins can function, which ranges from a thousand to a million molecules per second, directly affects the overall rate of active transport. In contrast, passive transport processes, like facilitated diffusion through channel proteins, do not require energy and can transport substances more quickly, at a rate of tens of millions of molecules per second.