Advanced Placement (AP) Biology Practice Exam

What is the process called where diffusion occurs through protein channels?

1. Active transport

2. Facilitated diffusion

3. Osmosis

4. Simple diffusion

The correct answer is: Facilitated diffusion

Facilitated diffusion is the correct term for the process in which molecules move across a cell membrane through protein channels. This process is passive, meaning it does not require energy, and instead relies on the concentration gradient of the molecules involved. During facilitated diffusion, specific proteins embedded in the cell membrane act as channels that allow certain substances to pass through. These channels are selective and typically designed for particular molecules, such as glucose or ions. The presence of these specialized protein channels enhances the rate of transport for molecules that cannot easily pass through the lipid bilayer of the membrane on their own due to their size, polarity, or charge. This is distinct from active transport, which involves the movement of substances against their concentration gradient and requires energy input, often in the form of ATP. Osmosis specifically refers to the movement of water through a selectively permeable membrane, driven by differences in solute concentration, while simple diffusion describes the direct movement of small or nonpolar molecules through the lipid bilayer without assistance from proteins.