Question

which of the following statements accurately describes the facilitated diffusion of materials across a membrane?
a energy is required to move charged substances across the membrane.
b small, polar molecules require transport proteins to cross the membrane.
c oxygen molecules move directly across the membrane.
d ions move down their concentration gradient across the membrane.

Explanation:

Facilitated diffusion is a type of passive transport. In passive transport, no energy (like ATP) is required. Charged substances (ions) and polar molecules (even if small) cannot pass directly through the non - polar lipid bilayer of the membrane. They need transport proteins (channel or carrier proteins). Small non - polar molecules like oxygen can move directly across the membrane by simple diffusion. In facilitated diffusion, substances move down their concentration gradient (from high to low concentration), and ions moving down their concentration gradient across the membrane is a characteristic of passive transport (including facilitated diffusion).

Answer:

B. Small, polar molecules require transport proteins to cross the membrane.