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The plasma membrane is selectively permeable; hydrophobic molecules and small polar molecules can diffuse through the lipid layer, but ions and large polar molecules cannot. Integral membrane proteins enable ions and large polar molecules to pass through the membrane by passive or active transport.
The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion
