Which molecules are usually prevented from moving across the selectively permeable membrane of a cell?

Consider the properties of each molecule: Oxygen and carbon dioxide are small, nonpolar molecules, which typically pass through the membrane easily. Water, although polar, can also move across the membrane through specialized channels called aquaporins.

Focus on ions: Ions are charged particles, and the lipid bilayer of the cell membrane is hydrophobic, making it difficult for charged particles to pass through without assistance.