Calcium as second messenger.
<P>
   Protein pumps maintain a Ca^2+ gradient across the plasma membrane and
   the membranes of the ER and mitochondrion.
<P>
   An  increase  in  Ca^2+  concentration  in the cytosol often acts as a
   second  message  in  processes  such  as  muscle  contraction and cell
   division.
<P>
   Muscle contraction:  <A ClAsS=MID Href="../../../med/Vid/Johnson6e/ch28/ATP-muscle_contraction.swf" oNClick="popurlt('../../../../med/Vid/Johnson6e/ch28/ATP-muscle_contraction.swf');txtcolor('calcium1','gray');return false;" Id=calcium1><iMg sRc="/images/shockwave.gif" aLt='' boRDer=0></A>