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The plasma membrane serves as a barrier that only permits certain substances to pass. These include small uncharged molecules, such as water and carbon dioxide, and hydrophobic molecules, such as benzene. The hydrophobic middle of the lipid bilayer is a major reason the plasma membrane favors hydrophobic molecules.
Large uncharged molecules and ions cannot pass through the membrane by themselves. They often require protein channels or carriers to facilitate their entry into the cell. Endocytosis is also a method for cells to take up larger compounds.
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