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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.
In general, a molecule's size and polarity decides whether it can pass through the bilayer... Lipid bilayer of the cell membrane is composed of phospholipids, which are amphiphilic (that is contains both the hydrophobic and hydrophilic ends). The hydrophilic ends points or heads outwards from the bilayer, whereas the hydrophobic tails point inwards (And towards each other).