What statement best differentiates channels and carriers in membrane transport?

What statement best differentiates channels and carriers in membrane transport?

• A. Channels form pores to allow passive diffusion of ions; carriers undergo conformational changes to transport substrates.
• B. Channels require ATP to operate; carriers do not.
• C. Both channels and carriers can only transport substrates against their gradients.
• D. Carriers form pores for passive diffusion of ions.

Answer: (B)

The main idea is how the substance crosses the membrane. Channels form a continuous pore that allows ions or water to move through the membrane by passive diffusion when the channel is open. This flow happens down the electrochemical gradient and doesn’t require the carrier to change shape to move the substrate. In contrast, transporters or carriers do not form a pore. They bind the substrate on one side, undergo a conformational change that shuttles the substrate across, and then release it on the other side. This conformational cycling is the defining feature of carrier-mediated transport, and it underlies why carriers can be slower than channels and can exhibit saturation. Carriers can mediate facilitated diffusion (downhill, no energy) or active transport (against the gradient when powered by energy sources), but the essential distinction remains: channels provide a pore for passive flow, while carriers move substrates via shape changes.

The main idea is how the substance crosses the membrane. Channels form a continuous pore that allows ions or water to move through the membrane by passive diffusion when the channel is open. This flow happens down the electrochemical gradient and doesn’t require the carrier to change shape to move the substrate. In contrast, transporters or carriers do not form a pore. They bind the substrate on one side, undergo a conformational change that shuttles the substrate across, and then release it on the other side. This conformational cycling is the defining feature of carrier-mediated transport, and it underlies why carriers can be slower than channels and can exhibit saturation. Carriers can mediate facilitated diffusion (downhill, no energy) or active transport (against the gradient when powered by energy sources), but the essential distinction remains: channels provide a pore for passive flow, while carriers move substrates via shape changes.