Human digestive system
Chewing, in which food is mixed with saliva, begins the mechanical process of digestion. This produces a bolus which can be swallowed down the esophagus to enter the stomach. Here it is mixed with gastric acid until it passes into the duodenum where it is mixed with a number of enzymes produced by the pancreas. Saliva also contains a catalytic enzyme called amylase which starts to act on food in the mouth. Another digestive enzyme called lingual lipase is secreted by some of the lingual papillae on the tongue and also from serous glands in the main salivary glands. Digestion is helped by the chewing of food carried out by the muscles of mastication, by the teeth, and also by the contractions of peristalsis, and segmentation. Gastric acid, and the production of mucus in the stomach, are essential for the continuation of digestion.
Exoenzyme
Amylase breaks down carbohydrates into mono- and disaccharides, so a starch agar must be used for this assay. Once the bacteria is streaked on the agar, the plate is flooded with iodine. Since iodine binds to starch but not its digested byproducts, a clear area will appear where the amylase reaction has occurred. Bacillus subtilis is a bacterium that results in a positive assay as shown in the picture.