What distinguishes diffusion from facilitated diffusion?

• A. Facilitated diffusion requires energy input
• B. Diffusion occurs with or without membrane proteins
• C. Facilitated diffusion relies solely on active transport mechanisms
• D. Diffusion can only occur across a membrane with proteins

Answer: (B)

The distinction between diffusion and facilitated diffusion lies in the mechanisms through which substances move across cell membranes.

Diffusion is a passive process that occurs when molecules move from an area of higher concentration to an area of lower concentration. This process can occur directly through the lipid bilayer of a membrane for small nonpolar molecules or in some cases for small polar molecules, without the involvement of membrane proteins.

Facilitated diffusion, on the other hand, specifically refers to the transport of substances across a membrane via specific protein channels or carriers that aid this movement. This process is still passive, meaning it does not require energy, but it allows substances that cannot easily cross the lipid bilayer—such as ions or larger polar molecules—to permeate the membrane.

Thus, the correct statement highlights the fact that while diffusion can occur freely across a membrane, facilitated diffusion is reliant on specific membrane proteins that assist in transporting substances that wouldn't otherwise pass easily through the lipid bilayer.