Transport of large molecules via carrier proteins.

• A. Endocytosis
• B. Active Transport
• C. Osmosis
• D. Carrier-Mediated Diffusion

Answer: (D)

Carrier-mediated diffusion is the movement of large solutes across the cell membrane using specific carrier proteins, down their concentration gradient and without the need for cellular energy. The carrier protein binds the solute on the outside, changes shape, and releases it on the inside, a process that allows large molecules to cross the membrane in a controlled, facilitated way. This method is saturable because there are a finite number of carriers, so transport cannot exceed a maximum rate even if more solute is present. It’s distinct from endocytosis, which uses vesicles to engulf material (energy-dependent and not purely diffusion), from osmosis, which is water movement, and from active transport, which moves substances against their gradient using energy.