Which mechanism uses membrane proteins to move molecules across the membrane, functioning as channels or carriers?

• A. Facilitated diffusion
• B. Active transport
• C. Endocytosis
• D. Exocytosis

Answer: (A)

Membrane proteins that serve as channels or carriers move substances across the lipid bilayer by facilitating diffusion. They provide either a pore for ions and water or a binding site that undergoes a shape change to shuttle specific molecules, and this happens down the substance’s concentration (or electrochemical) gradient without using cellular energy. Channels enable rapid passage through a pore, while carriers bind the solute, change shape, and release it on the other side. Because no ATP or other energy source is required, this is passive transport. Endocytosis and exocytosis, in contrast, involve membrane vesicles and trafficking rather than simple passage through a protein pore or transporter, and active transport uses energy to move substances against their gradient.