Which statement best describes how channel proteins differ from carrier proteins?

• A. They both function identically; no difference.
• B. Channel proteins bind solutes and gate them.
• C. Channel proteins form pores that allow passage; carrier proteins undergo conformational changes after solute binding.
• D. Carrier proteins form pores; channel proteins do not.

Answer: (C)

The main idea is how channel proteins differ from carrier proteins in moving substances across membranes. Channel proteins create hydrophilic pores that form a passage through the membrane, allowing ions or small polar molecules to diffuse through when the channel is open or gated. They typically do not bind the solute with high affinity and passages occur mainly by diffusion, which can be very fast. Carrier proteins, in contrast, have binding sites for the solute; after binding, the protein undergoes a conformational change that flips and releases the solute on the opposite side, a process that is often slower and can be saturated because it depends on binding and release steps. This distinction is why the statement that best describes their difference is that channel proteins form pores that allow passage, while carrier proteins undergo conformational changes after solute binding. Examples like aquaporins illustrate channels letting water through, whereas glucose transporters exemplify carriers that change shape to move glucose.