Which statement about diffusion through the lipid bilayer is true?

Which statement about diffusion through the lipid bilayer is true?

• A. Small nonpolar molecules diffuse rapidly through the lipid bilayer
• B. Ions diffuse rapidly through the lipid bilayer
• C. Large polar molecules diffuse rapidly
• D. Water diffuses rapidly through the lipid bilayer without assistance

Answer: (A)

Small nonpolar molecules diffuse rapidly across the lipid bilayer because they dissolve easily in the hydrophobic interior of the membrane and move down their concentration gradient without the need for energy or transport proteins. The bilayer’s core is hydrophobic, so nonpolar, low‑molecular‑weight molecules (like O2 and CO2) pass through quickly. Ions carry a charge and cannot cross the hydrophobic core readily, so they require channels or transporters. Large polar molecules are also poor at crossing the bilayer on their own due to both their size and polarity. Water can diffuse through the membrane, but it does so most efficiently through channels called aquaporins; diffusion through the bilayer itself is relatively slower. So the statement that small nonpolar molecules diffuse rapidly through the lipid bilayer best reflects how permeability works.

Small nonpolar molecules diffuse rapidly across the lipid bilayer because they dissolve easily in the hydrophobic interior of the membrane and move down their concentration gradient without the need for energy or transport proteins. The bilayer’s core is hydrophobic, so nonpolar, low‑molecular‑weight molecules (like O2 and CO2) pass through quickly. Ions carry a charge and cannot cross the hydrophobic core readily, so they require channels or transporters. Large polar molecules are also poor at crossing the bilayer on their own due to both their size and polarity. Water can diffuse through the membrane, but it does so most efficiently through channels called aquaporins; diffusion through the bilayer itself is relatively slower. So the statement that small nonpolar molecules diffuse rapidly through the lipid bilayer best reflects how permeability works.