The mucus is more viscous in the upper part of the small intestine than in the large intestine and forms a protective physical barrier to the intestinal lining, reducing contact with irritating substances and bacteria . The alkalinity of the mucus contributes further to the protection of the intestinal lining by neutralizing acidic dietary and bacterial products. The rate of most chemical reactions can be speeded up by substances called catalysts , and the catalysts that are produced by living organisms are called enzymes . In industrial chemistry, catalysts work at high temperatures or pressure. An enzyme , on the other hand, promotes reactions at body temperature and ordinary atmospheric pressure.
Enzymes consist of large protein molecules whose surfaces are the site of intense chemical activity.
Each enzyme has a unique surface with a particular spot where the substrate molecule can work on. For this reason the enzyme -substrate complex is often compared to a lock and key, where the substrate is the lock and the enzyme the key. A special feature of enzymes in the alimentary canal , is that although they are made inside the cells they work outside. Digestion starts in the mouth with the partial break-down of starch by the enzyme ptyalin. The ptyalin is transported to the mouth in the watery saliva from the salivary glands .