Stomach.
<P>
   Protein digestion starts in the stomach.
<P>
   Three types of cells secrete gastric juices.
<P>
<BR><LI> Mucus cells secrete mucus, which lubricates and protects the cells
lining the stomach.
<BR><LI> Chief  cells  secrete  pepsinogen,  an  inactive  enzyme  that  is
converted to pepsin to digest protein.
<BR><LI> Parietal cells secrete hydrochloric acid (HCl), lowering the pH of
the juice.
<P>
   These gastric juices convert food to acid chyme.