Carrier-mediated transport is best described as:

• A. It uses specific carrier proteins to move molecules across membranes
• B. It uses energy only
• C. It occurs only in intestinal absorption
• D. It does not involve proteins

Answer: (A)

Carrier-mediated transport relies on specific carrier proteins that bind the substance and undergo conformational changes to shuttle it across the lipid bilayer. This is the defining feature: the involvement of dedicated proteins that recognize and move the solute, making the process saturable and highly selective. It can operate with direct energy input (active transport) or without direct energy input (facilitated diffusion), but the key point is the carrier protein requirement. That’s why this option is the best description. The other statements miss the essential role of carrier proteins, either by implying energy is the sole driver, restricting the process to intestinal absorption, or denying protein involvement altogether.