EVERYDAY SCIENCE

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: -"Simple diffusion.” Simple diffusion is pretty much exactly what it sounds like – molecules move down their gradients through the membrane. Molecules that practice simple diffusion must be small and nonpolar*, in order to pass through the membrane.

Detailed explanation-3: -Polar molecules are less free to diffuse across membranes. In order for a polar molecule to cross a membrane, it must break its interactions with water as it enters the hydrophobic interior of the bilayer. Breaking these interactions is energetically unfavorable and requires an input of energy.