(a) Explain why an athlete in a race would be given a glucose drink rather than a piece of bread. (b) Outline the test for starch in a given food substance....

(a) Why an athlete is given a glucose drink rather than a piece of bread

Glucose is a simple (reducing) sugar that is already in its final absorbable form. It does not need to be digested, so it is absorbed directly and quickly from the alimentary canal into the blood and is at once available to the muscles for respiration to release energy rapidly. Bread contains starch, a complex carbohydrate, which must first be digested (broken down) into glucose by enzymes over a long time before it can be absorbed and used. During a race the athlete needs energy immediately, so glucose supplies it faster than bread.

(b) Test for starch in a food substance

• Put the food substance in a test tube or on a white tile.
• Add a few drops of iodine solution (iodine in potassium iodide).
• Observation: if the yellow/brown iodine turns blue-black, starch is present. If the colour remains yellow/brown, starch is absent.

(c) Five reasons why water is important in the human diet

• It is the medium (solvent) in which digestion and other chemical reactions of the body take place.
• It is needed for the transport of digested food, oxygen, hormones and waste materials in the blood and lymph.
• It helps in the regulation of body temperature through sweating (evaporation of sweat cools the body).
• It is needed for the removal (excretion) of waste products such as urea and excess salts in urine and sweat.
• It keeps body tissues and cells moist and turgid, and replaces water lost through urine, sweat and breathing; it also aids the passage of food and prevents constipation.