Which statement about diffusion across membranes is true?

Which statement about diffusion across membranes is true?

• A. It Requires Energy Input
• B. It Can Occur Passively Without Energy
• C. It Always Requires a Carrier Protein
• D. It Never Occurs for Drug Molecules

Answer: (B)

Diffusion across membranes can occur without input of energy, moving substances down their concentration or electrochemical gradient. This happens with simple diffusion for small, nonpolar molecules that pass directly through the lipid bilayer, and also with facilitated diffusion where a carrier or channel helps the molecule cross without using cellular energy. In pharmacology, many lipophilic drugs cross membranes by diffusion, whereas active transport and carrier-dependent uptake require energy and are separate processes.

Diffusion across membranes can occur without input of energy, moving substances down their concentration or electrochemical gradient. This happens with simple diffusion for small, nonpolar molecules that pass directly through the lipid bilayer, and also with facilitated diffusion where a carrier or channel helps the molecule cross without using cellular energy. In pharmacology, many lipophilic drugs cross membranes by diffusion, whereas active transport and carrier-dependent uptake require energy and are separate processes.