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.
Extracellular digestion
The regulation of nutrient acquisition appears to be controlled by general phenomena. Only a small group of enzymes, mostly hydrolases, can be detected in the culture filtrate of well-fed fungi. This suggests that specific inducers control the manufacture and release of enzymes for degradation. The most common complex carbohydrate available in the environment is cellulose. In the absence of glucose, detection of cellulose, for instance, induces the expression of celluloses. As a consequence, fungi specifically target the breakdown of the cellulose in their environment, and do not waste energy on the unnecessary formation of enzymes for degradation of molecules that may not be present. Fungi have an efficient process to gain energy.