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Fluid-mosaic model. 1  Plasma membrane consists of a phospholipid bilayer studded with proteins, polysaccharides, lipids. 2  The lipid bilayer is semipermeable Regulates passage of substances into and out of the cell. H2O and some small, uncharged, molecules (O2, CO2) can pass through.
As you learned in the chapter on cells, the cell membrane is primarily responsible for regulating what can cross the membrane and what stays on only one side. The cell membrane is a phospholipid bilayer, so only substances that can pass directly through the hydrophobic core can diffuse through unaided.