The Process of Digestion

Food is digested in two stages. The first stage involves the bolus (where the concentration of enzymes is low) as it moves along the alimentary tract. At this stage, the food is subjected to primary treatment, the boluses are first broken down into smaller ones and these in turn become separate molecules.

The main process of digestion (the breakdown of molecules) occurs at the second stage when digestion takes place in the intestine near the intestinal wall. This type of digestion, called parietal digestion, is very good for the organism. The first advantage, which has already been mentioned, is that it is possible to attain a very high rate of digestion with only small amounts of enzymes. The other advantage is that the digestive enzymes can be used sparingly. The enzymes that are adsorbed on the intestinal wall are preserved and continue to serve the organism for a long time, while those from the bolus are eliminated together with the remains of the undigested food and are thus lost. The third and final advantage is that the completely digested food, which is ready to be absorbed by the blood, appears to be just where absorption takes place, i. e. close to the intestinal wall. This greatly accelerates and improves absorption.

This discovery allowed another mystery to be solved. Physicians have long been aware that sometimes in some humans the alimentary glands almost stop function as a result of illness. The sick person does not notice this since it almost does not aftect his digestion. How the food was digested remained a puzzle. Now it has been discovered that the negligible amounts of enzymes secreted by a faulty gland are adsorbed by the intestinal wall, accumulated and retained, thus ensuring the normal digestion of food.

An experiment was carried out. A piece of intestine was placed for some time in a test tube containing a starch solution. The idea was that if the intestine contained digestion-accelerating substances, they would be secreted into the test tube. The intestine was then removed and some amylase added to the starch. Digestion proceeded slowly, just as in the original experiments.

Perhaps the piece of intestine did not have enough time to secrete the substance it was supposed to have. Yet another experiment was carried out. An extract was obtained from the intestine of a slaughtered animal. The extract should, no doubt, have contained the required substance.

However, when the extract was added to the test tube containing the starch and amylase, it did not accelerate the rate of digestion. This meant that the intestinal wall did not contain substances accelerating the process of digestion. What then triggered off the process?

The puzzle was solved unexpectedly. It was the very structure of the intestinal wall that facilitated the process of digestion. The surface of the epithelial cells lining the intestine carries ultra-microscopic shoots. Each cell carries as many as three thousand shoots, and this makes the surface area of the intestine very extensive, enabling it to adsorb, i. e. precipitate and retain, a great many enzymes. These enzymes act as catalysts accelerating chemical reactions. The enzymes interact chemically with the reagents, but as soon as the reaction is complete, they regain their previous chemical composition. This explains why even small amounts of catalysts markedly accelerate the rate of chemical reactions.

It is only natural that digestion is more energetic on the surface of the intestinal wall where the concentration of enzymes is much greater than within the mass of food. The total amount of enzymes may not be large; they can be used again and again. What is important is their extremely high concentration and this is why even moderate amounts of enzymes ensure a high rate of digestion.