Which statement best describes simple diffusion?

• A. Movement directly across the lipid bilayer without energy or proteins
• B. Movement through protein channels with energy
• C. Movement via vesicles
• D. Movement against gradient with ATP

Answer: (A)

Simple diffusion is a form of passive transport driven solely by a molecule’s concentration gradient, moving from higher to lower concentration directly through the lipid bilayer without the help of proteins and without using energy. This means small, nonpolar or lipid-soluble molecules—like oxygen and carbon dioxide—can pass through the membrane on their own. Water can diffuse across membranes too, but it often moves through channels called aquaporins rather than by simple diffusion in its pure form. Because no proteins are involved and no ATP is used, simple diffusion contrasts with facilitated diffusion (which requires transport proteins) and active transport (which uses energy to move substances against their gradient). Moving via vesicles or through protein channels with energy would not be simple diffusion.