Cell membranes allow some molecules to pass through . But not other . If a molecules can pass through a cell membrane . It will diffusion from an area of higher concentration on one side of the membrane to an area of lower concentration on the other side . Diffusion across a membrane is also called simple diffusion . And only allows certain molecules to pass through the membrane The simple diffusion of molecules across a cell membrane depends on the size ang type of molecules and on the chemical nature of the membrane . A membrane can be made , in part , of a phospholipid bilayer , and certain proteins can form pores in the membrane. Molecules that can dissolve in lipids may pass directly through the membrane by diffusion . For example , because of their nonpolar nature , both carbon dioxide and oxygen dissolve in lipids . Molecules that are very small but not soluble in lipids may diffusion across the membrane by moving through the pores in the membrane Osmosis
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