Transport    across   plasma
   membrane.
<P>
   Passive  transport  moves  substances  through  the lipid bilayer down
   their concentration gradients:
<BR><LI> Diffusion  allows  hydrophobic  and small polar molecules to cross
the bilayer.
<BR><LI> Facilitated  diffusion enables hydrophilic substances to cross the
bilayer through embedded proteins.
<P>
   Active  transport  moves  substances  across  a membrane against their
   concentration gradients.
   Energy for this work is often supplied by ATP.