Passive transport of substances across cell membranes.

• A. Facilitated Diffusion
• B. Active Transport
• C. Osmosis
• D. Endocytosis

Answer: (A)

Passive transport moves substances across membranes without energy input, following their concentration gradient. Facilitated diffusion fits this idea because it is a form of passive transport in which membrane proteins (channels or carriers) help polar or large molecules—things that cannot cross the lipid bilayer easily—to diffuse down their gradient. It doesn’t require ATP, just a gradient and a protein pathway.

In contrast, active transport requires energy to move substances against their gradient; endocytosis involves vesicles and energy use; and osmosis is specifically the diffusion of water across a semipermeable membrane (a type of passive transport but focused on water rather than general solutes). Thus, facilitated diffusion best matches passive transport for substances that need help crossing the membrane.