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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.
Thesodium-potassium pump carries Na to the outside of the cell and Kto the inside of the cell. Energy in the form of ATPmolecules is requiredfor active transport to occur. 4.5 Exocytosis and Endocytosis Larger substances can enter and exit a membrane by endocytosis andexocytosis. Exocytosis involves secretion.