Everything Life Sciences Grade 10

but the difference in the properties of these substances can be attributed to the way in which

the glucose molecules join together to form different structures. Below are images of glyco-

gen and starch.

Figure 2.9: A comparison between starch and glycogen. Glycogen is more extensively branched than

starch.

Chemical tests to identify presence of carbohydrates

Substances containing starch turn a blue-black colour in the presence of iodine. An observ-

able colour change is therefore the basis of a chemical test for the compound.

Figure 2.10: Granules of wheat starch, stained with iodine and photographed through a light micro-

scope.

Investigation: Test for the presence of starch (Essential investigation-CAPS)

Aim:

To test for the presence of starch.

Apparatus:

• piece of potato or bread


• lettuce leaf


• petri dish


• iodine solution

32 2.4. Organic compounds