CELL MEMBRANE
THE BASICS
     The cell membrane also known as the plasma membrane, is the structure that separates the cell from its external environment.  It gives shape and flexibility to the cell.  The cell membrane is a complex barrier that keeps out some molecules but allows others to permeate, or pass, into the inside of the cell.  The cell membrane is therfore called a selectively permeable membrane.  This membrane is about 7.5 to 10 nm thick.

THE STRUCTURE
    The cell membrane is composed of two layers of molecules.  Each layer is made up of a sheet of lipids.  A lipid is a fatty compound made up of carbon, hydrogen, and oxygen.  This molecule has a carboxyl group as it's "head" and a hydrocarbon chain as a "tail".  These lipid molecules are arranged so that their heads form the outside of the membrane and their tails form the inside of the membrane.  Another component of the membrane is globular glycoproteins that are embedded in the lipid layers.

THE FLUID MOSAIC MODEL
    The lipids that form the membrane are fluid.  Therefore they can move around like a liquid.  Some of the proteins are also free to move about, so that the mosaic, or pattern, of lipids and proteins changes.  Because of these certain characteristics, scientists have called their model the fluid mosaic model.

CYTOPLASM
    Cytoplasm is the jellylike material found inside the cell membrane.  It surrounds and protects the organelles.  This substance is always moving due to the movement of particles and organelles.  The movement is known as cytoplasmic streaming.
 
 
 
 

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