AP BIOLOGY

Detailed explanation-1: -Gases, hydrophobic molecules, and small polar uncharged molecules can diffuse through phospholipid bilayers.

Detailed explanation-2: -4.1 Passive Diffusion For example, small nonpolar gases such as O2, CO2, and N2, and small polar molecules such as ethanol cross lipid membranes rapidly.

Detailed explanation-3: -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-4: -Only the smallest molecules like water, carbon dioxide, and oxygen can freely diffuse across cell membranes. Larger molecules or charged molecules often require an input of energy to be transported into the cell.