2. Digestion Of Sugars

Although sugars are perfectly soluble and easily absorbed, they must all be converted into monosaccharides or simple sugars (chiefly dextrose), before they can be utilized by the body. Hence we find a number of enzymes in the alimentary tract acting upon disaccharides. Two of these have already been indicated in discussing the digestion of starch, i. e., the maltases of the saliva and intestinal juice, which convert each molecule of maltose into two of dextrose. In the stomach, a slight hydrolysis of the disaccharides may occur through the influence of the hydrochloric acid. In the intestines, provision is made for their complete transformation. Both the pancreatic and intestinal juices contain enzymes of this type. In young animals, or older individuals on a milk diet, the pancreatic juice contains lactase, which converts lactose (milk sugar) into dextrose and galactose. The intestinal juice contains three such enzymes, sucrase, acting on sucrose (cane sugar) ; lactase, acting on lactose; and maltase, acting on maltose.

Summary

The chemical changes in sugar during the process of digestion are shown in the following table:

Mineral Matter and Water require no changes to prepare them for absorption.

Resume Of Digestion

The processes of digestion are both mechanical and chemical. By mechanical processes the foods are first softened and finely divided, so as to be non-irritating to the walls of the alimentary tract, and to expose as much surface as possible to the action of the digestive enzymes; secondly, they are moved along the alimentary tract from time to time, to facilitate digestion and absorption, and to expel indigestible residues. The movements of the mouth in mastication, of the oesophagus in swallowing, and of the intermediate and pyloric regions of the stomach in mixing the food with the gastric juice; the rhythmic and peristaltic movements of the small intestines, by which the food is alternately churned in a stationary position and then pushed along to a new region to repeat the process; and finally, the anti-peristaltic movements of the first part of the large intestines and the slow peristalsis throughout its entire length, form a series of events of tremendous importance to the welfare of the organism.

By chemical processes, complex food-stuffs are converted into simpler substances out of which the body can build the compounds essential to its persistence and activity. The digestive fluids and the enzymes contained therein, with the results of their activity, are most readily indicated by the following table:

The bile contains no important enzymes, but greatly facilitates the digestion of fats. The intestinal juice contains an enzyme which makes trypsin an active enzyme, and a substance which helps to stimulate the flow of pancreatic juice.

Inspection of the table on page 44 shows that there are five so-called digestive juices, viz.: saliva, gastric juice, pancreatic juice, bile, and intestinal juice. All are produced intermittently except the bile, which is secreted continuously into the gall-bladder and discharged at intervals into the intestine. All are alkaline except the gastric juice. All contain powerful enzymes except the bile. Enzymes which digest protein occur in the gastric juice (pepsin), the pancreatic juice (trypsin), and the intestinal juice (erepsin). Ferments which transform starch to sugar occur especially in the saliva and pancreatic juices, and are commonly known as diastatic enzymes. Ptyalin is an old name for salivary diastase, amy-lopsin for pancreatic diastase.

The principal fat-splitting enzyme occurs in the pancreatic juice, and is known as pancreatic lipase or steapsin. Numerous enzymes of minor importance occur in the gastric, pancreatic, and intestinal juices. Bile in itself is able to emulsify fat as a step toward digestion.