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.

Digestion is the breakdown of large, insoluble molecules into small, soluble molecules, so that they can be absorbed into the bloodstream. It is carried out by the digestive system.

Biological molecules


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.

Diagram showing starch molecules being broken down by the enzyme carbohydrase from a long string of attached molecules to individual glucose molecules.


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.

Diagram showing proteins being broken down by the enzyme protease from a long chain of amino acids to single amino acids.


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.

Diagram showing large lipids being broken down by the enzyme  lipase into glycerol and three fatty acids.

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.

Two test tubes. One contains iodine and is brown in colour. The other contains both iodine and starch and is black in colour.

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.

Five test tubes containing coloured solutions. 1 is blue, labelled none, 2 is green, labelled very low,  3 is yellow, labelled low, 4 is orange, labelled medium and 5 is red, labelled high.

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

Two test tubes. One contains biuret reagent and is blue in colour. The other contains both biuret reagent and protein and is purple in colour.

Biuret solution contains very dilute copper sulphate and sodium hydroxide, this makes it an irritant. Complete this risk assessment for biuret solution (the first part of the risk assessment has been completed for you).

HazardRiskControl measure
Biuret solution is an irritant.
HazardRiskControl measure
Biuret solution is an irritant.It may spill onto the skin when pouring. It may be transferred from the fingers to the eyes.Wear surgical gloves. Wash hands immediately after use. Wear eye protection.

Summary table

A table of results for the presence of starch, proteins and  lipids. Columns are labelled Name of enzyme that digests it, Digested molecule, Food test, and Positive result.