Which statement about diffusion across the cell membrane is true?

• A. It requires channels exclusively for all molecules
• B. It moves substances down their concentration gradient without energy
• C. It moves substances against their gradient
• D. It cannot occur for small molecules

Answer: (B)

Diffusion across the cell membrane is a passive process driven by a concentration gradient, so it moves substances from high to low concentration without the cell using energy. Small nonpolar molecules like oxygen and carbon dioxide can pass directly through the lipid bilayer by simple diffusion. For larger or polar molecules, diffusion can still occur but often needs membrane proteins—channels or carriers—to provide a pathway, and it still proceeds down the gradient without input of energy (facilitated diffusion). The key point is that no cellular energy is required and movement follows the gradient. Moving substances uphill, against the gradient, requires energy (active transport), and diffusion is possible for small molecules, so claiming it cannot occur for small molecules isn’t accurate.