Final answer:
Channels allow passive molecule movement based on size and charge, whereas transporters are specific and transport molecules by binding them. Channels work much faster than transporters.
Step-by-step explanation:
The key difference between transporters (often referred to as carrier proteins) and channels lies in their mechanism of transporting substances across the cell membrane. Channels provide a path for the passive movement of molecules and discriminate mostly on the basis of size and electric charge; they are less selective and function more like a tunnel for ions and small molecules. In contrast, transporters are more specific and function by binding to the molecules they carry, much like an enzyme binds to its substrate, causing a conformational change to move the molecule across the membrane.
Answer choice B is correct: Channels discriminate between solutes mainly on the basis of size and electric charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate. Channels facilitate diffusion at extremely high rates, allowing tens of millions of molecules per second to pass through, while transporters, which undergo a conformational change upon binding their specific solute, work at a slower rate, moving a thousand to a million molecules per second.