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 . Saliva provides also water to the digestion process. It moistens and softens the food so that the tongue can mould it into a bolus suitable for swallowing. In addition, the mucus that is present in saliva lubricates the food so that it can slide easily down the next part of the alimentary canal . At this moment the breathing slows down, because the rear entry to the nose closes and the epiglottis shuts off the opening of the trachea so that food cannot enter.