What distinguishes facilitated diffusion from simple diffusion?

What distinguishes facilitated diffusion from simple diffusion?

• A. Diffusion requires energy input.
• B. Facilitated diffusion uses transport proteins for larger or charged molecules.
• C. Facilitated diffusion moves against the gradient.
• D. Both require ATP.

Answer: (B)

The key idea is that diffusion can happen with or without help from membrane proteins. Simple diffusion moves small nonpolar molecules straight through the lipid bilayer without any proteins or energy, simply down their concentration gradient. Facilitated diffusion, while still passive, uses membrane transport proteins to help substances that are too large or charged cross the membrane. Channel proteins form pores for ions or water, and carrier proteins bind the molecule and change shape to shuttle it across. Both processes don't require ATP, but facilitated diffusion is needed for substances that can’t easily cross the lipid bilayer on their own.

The key idea is that diffusion can happen with or without help from membrane proteins. Simple diffusion moves small nonpolar molecules straight through the lipid bilayer without any proteins or energy, simply down their concentration gradient. Facilitated diffusion, while still passive, uses membrane transport proteins to help substances that are too large or charged cross the membrane. Channel proteins form pores for ions or water, and carrier proteins bind the molecule and change shape to shuttle it across. Both processes don't require ATP, but facilitated diffusion is needed for substances that can’t easily cross the lipid bilayer on their own.