Which type of diffusion occurs without transport proteins?

• A. Facilitated diffusion
• B. Simple diffusion
• C. Active diffusion
• D. Osmosis

Answer: (B)

Simple diffusion is the movement of small, nonpolar molecules directly through the phospholipid bilayer from where they are more concentrated to where they are less concentrated, without any transport proteins or energy input. Because these molecules can dissolve in and pass through the lipid interior, they cross the membrane by this down-the-gradient flow. In contrast, facilitated diffusion always needs channel or carrier proteins to help substances cross, active transport requires energy to move substances against the gradient, and osmosis specifically describes water diffusion across a membrane (often via aquaporins), not general solute diffusion through the bilayer.