Programme 21: Food and Digestion

Programme Outline

Food and digestion is about the food we eat and how it is digested. Laboratory experiments show how food can be tested for the nutrients it contains. The digestive system and the process of digestion are investigated using diagrams and graphic animations. The effect of pH on the action of enzymes and the role of bile salts are investigated in the laboratory.

This programme is divided into three parts:

21.1 Food Tests
21.2 The Digestive System
21.3 Digestive Enzymes

21.1 Food Tests

The foods available in a market are used to illustrate the range of nutrients that our bodies require. Bread, nuts and grapefruit juice are tested in a laboratory to find out which nutrients they contain.

A piece of filter paper is used to test for fat. Nuts leave a translucent stain when the paper is dry. Nuts contain fat.

Starch is a complex carbohydrate. When iodine solution combines with starch it makes a blue – black colour. Bread contains starch.

Sugars are simple carbohydrates. Some sugars belong to a group called reducing sugars. When Benedict’s solution is added to a reducing sugar and the mixture is boiled a colour change is seen. The mixture turns green then orange then red, depending on how much sugar is present. Grapefruit juice contains fructose, a reducing sugar.

To test for protein sodium hydroxide is added to the food. Dilute copper sulphate is added to form a layer on top of this mixture. This is known as the Biuret test. A mauve ring will appear after a few minutes if protein is present. Nuts and bread contain protein.

Which contains more protein? Nuts or bread?
Answer: Nuts because the mauve ring is deeper in colour.

21.2 The Digestive System

An egg sandwich contains starch in the bread, protein in the egg and fat in the margarine. These three nutrients consist of large molecules that are broken down by enzymes in the digestive system.

A diagram is used to illustrate the system. Teeth begin the process by breaking up the food. Saliva moistens the food and contains the enzyme amylase.

The food is moved down the oesophagus by a series of muscular contractions called peristalsis. This is illustrated using an x-ray of a person swallowing a barium meal.

In the stomach the food is churned by muscular contractions. Gastric pits secrete acid and the enzyme pepsin.

The pancreas secretes amylase, protease and lipase. These enzymes join food in the small intestine where digestion is completed.

The wall of the small intestine is folded and covered in tiny projections called villi. These structures increase the surface area through which food can be absorbed into blood capillaries.

Fibre makes up most of the indigestible component of food. It plays an important role in keeping our digestive system healthy.

Which part of the egg sandwich is indigestible?
Answer: Fibre in the wholemeal bread. This is made up of the skin of the wheat seed and its outer husk. It is not found in white flour.

21.3 Digestive Enzymes

Some nutrients are made up of large molecules which need to be broken down before they can pass into the blood system. Digestive enzymes speed up the breakdown of larger molecules. The most important enzymes are amylase which breaks down starch, proteases which break down protein, and lipase which breaks down fats.

Enzymes work best at a particular pH known as the optimum pH. In the laboratory the action of amylase and pepsin is measured at three different pH values. Amylase works best at pH 7 and pepsin at pH 3. Pepsin is secreted in the stomach where the pH is lowered by hydrochloric acid.

Lipase breaks down fat but it is assisted by bile salts secreted from the gall bladder. In the laboratory the action of lipase and bile salts is investigated using whole milk. When fat is digested fatty acids are made. The fall in pH causes an indicator to change colour from pink to colourless. This change occurs more quickly when lipase and bile work together. Bile causes larger fat droplets to break up into small pieces giving lipase a larger surface area to work on.