DIFFUSION vs. ACTIVE TRANSPORT
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    Diffusion vs. Active Transport: molecules move across cell membranes by two major processes diffusion or active transport.  Diffusion is the movement from a high concentration of molecules to a low concentration of molecules.  Molecules can diffuse across membranes through the phospholipid bilayer or using a special protein.  Either kind of diffusion does not need energy from the cell.  Some special proteins move certain molecules across cell membranes only with the help of cell energy (either directly or indirectly).  Moving molecules with cell energy is called active transport.  The cell energy is the ATP made in the mitochondria.
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MORE ON PHOSPHOLIPIDS...


    The heads of the PHOSPHOLIPIDS are composed of glycerol and a phosphate group and like to dissolve in water.  Water is a polar molecule (it acts like it has two poles) and other molecules that are polar easily dissolve in water.  These molecules that easily dissolve in water are called water-loving (or hydrophilic) molecules.

    The tails of the PHOSPHOLIPIDS are mostly fatty acids made up of long carbon and hydrogen chains.  Carbon and hydrogen chains are not polar and do not like to dissolve in water.  Molecules that do not easily dissolve in water are called water-hating (or hydrophobic) molecules.

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This page was created by Patrick Ireland 
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It was last updated08/22/00