In the context of transmembrane transport, which statement best describes how active transport differs from simple diffusion across a lipid bilayer?

• A. Active transport is limited to small nonpolar molecules, whereas simple diffusion is used primarily for ions and large polar solutes.
• B. Active transport occurs only through the phospholipid bilayer without proteins, whereas simple diffusion requires a specific membrane transporter.
• C. Active transport always moves solutes down their concentration gradient, whereas simple diffusion moves solutes against the concentration gradient.
• D. Active transport requires an energy input (directly or indirectly, e.g., via ATP) and can move solutes against their electrochemical gradient, whereas simple diffusion is passive and occurs down the gradient.

Answer: (D)