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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.
Most of the bilayer is hydrophobic; therefore water or water-soluble molecules do not pass through easily. A cell controls what moves through the membrane by means of membrane proteins embedded in the phospholipid bilayer.t is said to be called the  gatekeeper. Plasma membrane---separates the cell from its surrounding enviro â¦ nment. Molecules of the cell membrane are arranged in a sheet. The mosaic of molecules is fluid; that is, the molecules are able to float around slowly (like islands in the sea).