Programme 21: Food and Digestion

Background Information

Food Tests

Modern food packaging contains a wealth of accurate quantitative information about the nutritional content of processed foods. Laboratory-based biochemical tests are a cheap and safe way of testing for the nutrients in foods.

The Benedict’s test for reducing sugars tests for a specific group of sugars which are able to reduce the reagent thus causing a colour change. Everyday table sugar is sucrose which is not a reducing sugar. It is possible to test for non-reducing sugars by boiling the sugar with dilute hydrochloric acid and neutralising the resulting mixture with sodium hydrogen carbonate. The non-reducing sugars have been hydrolysed by the acid. The Benedict’s test will now give a positive result.

The Digestive System

The digestive system or gut is in fact a muscular tube running from the mouth to the anus. Regions of the tube have developed to perform particular functions. Other organs associated with the gut act as glands, secreting digestive enzymes.

The gut performs three functions: digestion, absorption and egestion. Digestion is the breaking down of large nutrient molecules into smaller molecules which can pass into the blood. Complex carbohydrates, such as starch, protein and fat, are the groups of nutrients which need to be digested.

Absorption is the passage of digested food into the blood. Simple sugars, vitamins and minerals and water can be absorbed without digestion. Other substances, such as drugs, are also absorbed through the wall of the gut. The surface of the small intestine has a large surface area, thin wall and good blood supply in order to facilitate absorption. The products of digestion are absorbed into the blood through the epithelial cells of the finger like projections called villi. Most of the absorbed fat first passes through the lymphatic system before joining the blood system. Water and some mineral salts are absorbed in the large intestine.

Elimination, or egestion, is the removal of the indigestible component of food via the anus as faeces. Fibre, made up of the cellulose walls of plant cells, is the main component of faeces. This fibre makes the food solid enough for peristalsis to move it efficiently through the gut. Lack of fibre in the diet is associated with the increased incidence of a number of diseases including bowel cancer.

Digestive Enzymes

Enzymes are protein molecules which act as biological catalysts. Like all catalysts, they change the rate of a chemical reaction without being directly involved in the reaction. An active site on a large and complex enzyme molecule allows a substrate molecule to temporarily bind. Changes in shape of the enzyme and substrate facilitate a reaction, in effect lowering the activation energy required to begin a reaction.

The digestive enzymes all catalyse the breaking of bonds in large complex molecules using water. This is called hydrolysis. Each enzyme works on a specific substrate and is named after it, for example, proteases work on protein. Lipids are fats and oils and are digested by lipases. Starch is also known as amylos and is digested by amylases.

Because enzymes are proteins their effectiveness is governed by environmental conditions. Enzymes have an optimum temperature and pH at which their action will be most effective. Low temperatures slow down enzyme-catalysed reactions, whilst at very high temperature the shape of an enzyme molecule can be permanently altered. The enzyme has been denatured and will no longer work as a catalyst.

The pH can also change the shape of an enzyme, altering the active site. In the stomach hydrochloric acid destroys potentially harmful bacteria. Enzymes that work here, such as pepsin, must be able to work at low pH. In the small intestine bile salts neutralise the acid and enzymes work at neutral pH.

Enzymes are secreted by particular glands in a coordinated sequence as food passes through the gut. Starch is digested to maltose sugar in the mouth and in the small intestine. The maltose is again digested to glucose by enzymes in the cells lining the small intestine. The digestion of protein is also a complex process with a number of enzymes working in a coordinated way.