Ronald Hyman, age 11, of Milton, Massachusetts, for his question:
Why must our food be digested?
The body is a working unit of many separate biological teams. And each team is a unit of specialized living cells. It is these cells, of course, that do the work and for this we need food and fuel. They are infinitesimally small, much too small to absorb the smallest bite on a menu. Besides, they need only certain items in the food we eat. So their diet must be sifted and mashed and served in liquid form.
The digestive system occupies a bulky part of the body but it has a large and complex job to perform. Think of all the food you consume those heaped up plates, those snacks and assorted drinks. Except for a very few items, none of the food in its menu form is any use at all to the body. The job of the digestive system is to process it into liquid formulas that the busy living cells can use. The food you eat must be chomped and chewed, pummeled and pounded into fine fragments. It must be doused and squirted with special digestive juices. sifted and sorted and properly blended into the suitable liquid formulas.
The useful chemicals on your dinner plate are fats, proteins and carbohydrates. These same three are just what the cells need to carry on their life processes. But they cannot accept them as served. The complicated job of digestion begins in the mouth. As you chew each bite, juicy squirts of saliva mix the fragments into a wad of pulp. The saliva contains an enzyme called ptyalin that starts the job of changing starches into sugars. With each swallow the mushy mix starts on its way down to the stomach.
Down here the meal is mixed with a large variety of digestive juices. Pepsin and hydrochloric acid begin to break up the protein molecules in such foods as meat, dairy products and certain vegetables. The muscular stomach walls crush and squeeze and mix the partly digested food with more liquid. More enzymes and digestive juices are added as the food passes on down through the tubular small intestine. Here the proteins are broken down into amino acids, the starches are changed completely into sugar and the food fats are converted into glycerol and fatty acids.
These are the basic food chemicals that the cells can use and there is a very wide assortment of them, And they must be thoroughly mixed into a thin, soupy liquid.
The walls of the small intestine have special fixtures to let the liquid diet pass through. Some of the vital mixture is piped into the blood stream to maintain a balance of chemicals in the plasma. Most of it reaches the lymph system, that soothing bath of chemicals surrounding the living cells of the whole body. The cells can use water in the form we drink it. Minerals and vitamins also are extracted and used in the form we eat them. All other foods must go through the amazingly complex digestive system to be converted into suitable formulas for the living cells.
Naturally, a great deal of the material we eat cannot be digested and many chemicals in our food are no use at all to the body. This excess baggage is sifted and sorted from the food during the process of digestion. It is sent on down to the large intestine. From there it is ordered to depart, along with all the other waste materials created by the chemical activity of the cells.