AP BIOLOGY

Detailed explanation-1: -Charged molecules, such as ions, are unable to diffuse through a phospholipid bilayer regardless of size; even H+ ions cannot cross a lipid bilayer by free diffusion.

Detailed explanation-2: -The plasma membrane is virtually impermeable against larger, uncharged polar molecules and all charged molecules including ions.

Detailed explanation-3: -Facilitated diffusion therefore allows polar and charged molecules, such as carbohydrates, amino acids, nucleosides, and ions, to cross the plasma membrane. Two classes of proteins that mediate facilitated diffusion are generally distinguished: carrier proteins and channel proteins.

Detailed explanation-4: -Larger charged and polar molecules, like sugars and amino acids, also need help from proteins to efficiently cross the membrane.