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A nutrient is a substance which is needed by the body for growth, repair and metabolism. The three main nutrients are:

• carbohydrates
• proteins
• lipids (fats and oils)

When these nutrients enter the body, they are large and insoluble molecules. In order for the body to be able to absorb and transport these nutrients in the bloodstream to all of the cells which need them, these molecules must be digested.

Biological molecules

Carbohydrates

Starch is a long chain carbohydrate. It is insoluble. During digestion the enzyme carbohydrase breaks it down into single molecules of glucose which are small and soluble enough to be carried in the blood to the cells for respiration, or to be stored as glycogen in the liver.

Proteins

Proteins are long chains of amino acids. During digestion the enzyme protease breaks them down into single amino acids which are small and soluble and can be used by cells to build new proteins.

Lipids

Lipids are large molecules. During digestion the enzyme lipase breaks them down into fatty acids and glycerol. These can be used by the body to provide energy.

Food tests

We can use different chemicals or reagents in the laboratory to detect if a food type is present or not.

Tests for carbohydrates

Starch – the test for starch is a chemical called iodine. Iodine solution is a brown coloured liquid. If it comes into contact with starch it turns blue/black.

Glucose – The test for glucose requires a chemical called Benedict’s reagent. It is blue in colour and needs to be heated vigorously with the sample to show a colour change. If glucose is present, a range of colours may be seen which determines the relative concentration of glucose in the sample.

Protein – The test for protein requires a chemical called biuret solution. It is blue in colour but turns violet in the presence of protein.

Summary table