Page 34 - Biology Form Two
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Biology for Secondary Schools
Questions (ii) they are insoluble in water.
1. What colour changes did you (When put in water they form a
observe? suspension);
2. What effect did hydrochloric acid (iii) they coagulate when boiled in
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have on sucrose? water; and
3. Why was the sodium hydroxide (iv) when mixed with iodine solution
solution or sodium bicarbonate starch changes colour to blue
solution added to the mixture? black.
Activity 1.6 (a): Test for the presence
Polysaccharides of starch in an Irish potato
Polysaccharides are formed by
condensation of several monosaccharides. Materials
They are commonly used as a storage form Iodine solution, Irish potato, a knife,
of food. Examples of polysaccharides mortar and pestle, test tubes, test tube
include starch, which is a plant storage holder, heat source, water, sieve, dropper,
compound; glycogen, which is an animal and a beaker
storage compound; cellulose which is Procedure
structural material in plant cell walls; 1. Peel the Irish potato and cut it into
and chitin, which is a component of the small pieces using a knife.
exoskeleton of arthropods and cell walls Safety precautions
of fungi. Do not taste any food in
the laboratory. Handle all
Starch: This is a carbohydrate made chemicals with care
by the condensation of many glucose
molecules. Starch occurs naturally in 2. Grind the pieces of potato and add
plant cells as small particles called water to make a mixture.
granules. It is found in abundance in 3. Decant and filter the mixture to
plants. Common starch-containing make a solution.
foods include potato, cassava, maize, 4. Put 2 ml of the solution into the
rice, and wheat. test tube.
5. Add 2-3 drops of iodine solution
Properties of starch and shake thoroughly.
Starch and other polysaccharides have
the following properties: Question
(i) they are tasteless, that is, they are What is your observation?
not sweet;
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