Archive for the ‘Food and Digestion’ Category

The fibre in foods is made of a substance called cellulose. None of the digestive enzymes produced in the human gut can digest cellulose. This is why fibrous foods pass through the gut undigested. However, some animals, such as cattle and rabbits, feed on foods which contain a lot of cellulose. For example, grass contains a lot of cellulose. The digestive enzymes produced by cattle and rabbits cannot digest cellulose. These animals rely on the help of millions of microbes (bacteria and protozoa) which live in their gut. These microbes make an enzyme which digests cellulose. The soluble products of this digestion are released into the gut of the animal. The microbes feed on some of them. The rest of the soluble products of cellulose digestion pass through the gut wall of the cow or rabbit and into its blood.

The cow and rabbit both benefit from the microbes in their gut. Without these microbes, they would be unable to digest most of their food. The microbes also benefit. They have a safe warm home in the animal’s gut. When two different organisms live together so that both of them benefit, we call this symbiosis.

The cow is an interesting and important example of an animal which relies on microbes to help with cellulose digestion. The cow has a stomach made of four parts. The microbes live in the first chamber, which is called the rumen. All types of cattle are called ruminants because they have a rumen. Other examples of ruminants are goats, deer, antelopes, giraffes and sheep. Rabbits are not ruminants. They have a different way of making use of microbes for cellulose digestion, but that is another story.

Cows feed on grass. Grass contains a large amount of cellulose. Cows themselves cannot produce an enzyme which will digest cellulose. However, cellulose digestion takes place in the cow’s gut.

The mouth contains several salivary glands. These glands produce the liquid which we call saliva. There are three main pairs of salivary glands and some smaller ones. Try to find at least one pair of salivary glands in your mouth. Use your tongue to help search for the glands and to feel the saliva as it comes out. If you think of, or watch, someone sucking an orange it may make the saliva flow faster. This makes the glands easier to find.

The total amount of saliva produced in 24 hours by an adult is probably about 600 cm3. However, it is difficult to measure and varies from one person to another. Some estimates give a figure as large as 1500 cm3 of saliva each day. Being Produced in salivary glands, saliva is almost 99 percent water. Among other components of saliva are electrolytes, mucus, antibacterial substances, as well as several enzymes.

The main digestive function of saliva is moistening of food and creating a food bolus (for a person to be able to swallow it). Moreover, salivary glands produce the hormone Gustin that is believed to be very significant in development of taste buds.

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.

It was not easy to study the process of digestion. It was as late as the turn of the last century that the Russian scientist Ivan Petrovich Pavlov completed a detailed study of the main alimentary glands. They turned out to be numerous and, what is more, it was discovered that for each type of food they produce a special composition of digestive juices. Academician Pavlov was awarded the Nobel prize, the highest international award, for these investigations. Thus, the basic mystery surrounding the process of digestion seemed to have been unveiled. However, the discovery was not yet complete. Nobody could reproduce the entire process of digestion in the laboratory by pouring into a test tube the necessary digestive juices in the correct sequence, and thus imitating the process observed in living organisms under natural conditions. The food was also digested in the test tube, but the process was all too slow, much slower than in the alimentary tract.

Recently, scientists have succeeded in uncovering this mystery. An astonishing thing is that the food which comes into contact with the intestinal wall is digested much more quickly than that incorporated in the main mass of food. This is similar to what happens when food is fried in a pan: the food in immediate contact with the walls of the pan cooks much more quickly. This is quite under­standable, for the pan is much hotter than the food. But the intestinal wall is not at all hot, so why then does it accelerate digestion?

The first thing was to find out whether the intestinal wall ready accelerated digestion. With this in view, the following experiment was carried out. A piece of intestine from a freshly killed animal was placed in one of two test tubes containing equal amounts of a mixture of starch and an amylase (a starch-splitting enzyme). Splitting of the starch proceeded much more rapidly around the piece of intestine which proved that the intestinal wall did accelerate digestion. But how does this happen?

Even primitive people knew that the food eaten by man and animals is digested in their stomachs. When skinning their game, they were sure to peep into the stomachs. Even nowadays almost no housewife can resist the tempta­tion of learning what the pike had for dinner and whether the chicken’s stomachs contain anything of interest besides small stones and sand. When hunters cut up their prey, they found in the stomachs and intestine neither meat, nor grass or seeds, but a pasty mass, as though the food had been cooked there.

It took man a long time to find out what really occurs. The food is not changed under the influence of heat: the temperature in the stomachs of even the ‘hottest’ warm­blooded animals is no higher than 38-43°C and this is not sufficient to cook food. Digestion takes place with the aid of digestive juices containing special enzymes.

The alimentary tract of man and animals is a complex chemical laboratory. The food consumed is ground, mixed with various digestive juices and moves gradually from one part to another. In each part the food is held long enough for it to be digested, being saturated with special substances. These substances are absorbed during the digestive process, that is, during the breakdown of complex chemical substances into simple ones (proteins into amino acids, fats into glycerol and fatty acids, carbohydrates into monosaccharides). What cannot be digested and used by the organism is disposed of.