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Small molecules are free to move across a semi-permeable membrane as long as there is a concentration gradient on either side of the membrane. The greater the lipid solubility of the diffusing particle, the more permeable the membrane will be. Water, oxygen and carbon dioxide rapidly diffuse across the lipid bilayer of the cell membrane. Charged particles cannot diffuse across and require ion channels and specific transporter molecules to get ferried across, as do larger molecules.
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