Theory question 102
Food Substances and Enzymes(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.
(c) State five reasons why water is important in human diet.
Show the model answer
(a) Glucose is a simple sugar that needs no digestion; it is absorbed quickly into the blood and oxidised in respiration, giving energy almost at once. Bread contains starch, a complex carbohydrate that must first be digested (mouth to small intestine) before it can be absorbed, so its energy is released slowly.
(b) Crush the food and add a little water to make a suspension (or use a piece of the food); place some on a white tile or in a test tube; add a few drops of iodine solution; a blue-black colour shows starch is present (it stays brown if absent).
(c) Any five: main part of protoplasm, blood and tissue fluid; solvent for chemical reactions; transports digested food, hormones and gases; carries away excretory wastes (urine, sweat); helps regulate body temperature (sweating); lubricates joints (synovial fluid) and moistens food for swallowing; aids movement of food through the gut and prevents constipation; protects organs (e.g. cerebrospinal and amniotic fluid).
In (a) contrast 'no digestion needed' with 'must be digested first'.