2. Carbohydrates

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Carbohydrates

• • Contain carbon, hydrogen and oxygen elements.

  • Hydrogen and oxygen atoms are present in the ratio of 2:1.

  • Represented by the chemical formulae Cm(H2O)n.

  • The 3 main types of carbohydrates are:

(a) Monosaccharides

(b) Disaccharides

(c) Polysaccharides

Functions of carbohydrates

1. Provides the cheapest and most readily available source of energy. 1 g of carbohydrate can provide 16 kJ of energy.

2. Forms supporting structures e.g. cellulose cell walls in plants.

3. To be converted to other organic compounds (e.g. amino acids and fats).

4. Formation of nucleic acids.

5. Synthesise lubricants e.g. mucus.

6. Produce nectar in insect-pollinated flowers.

Food Tests

1. Test for starch using iodine solution.

Step 1:  Add drops of iodine solution to unknown food sample.  If starch is present, iodine solution turns blue-black.  If starch is absent, iodine solution remains yellow.

Figure 1 Positive (left) and negative (right) results of the Iodine Test.

1. Benedict’s Test for reducing sugars. 

Step 1:  To 2 cm3 of liquid food sample, add 2 cm3 of Benedict’s solution. NB: For solid food sample, add 2 cm3 of distilled water to dissolve reducing sugars before addition of 2 cm3 Benedict’s solution.

Step 2:  Shake well.

Step 3:  Place the test tube into a boiling water-bath for 5 minutes.  Note:  Do not exceed 5 minutes.  A non-reducing sugar, eg. Sucrose cannot be broken down by excessive boiling to form reducing sugars à False positive result.

Result Interpretation

Positive result:  Brick-red precipitate is formed if reducing sugar is present.

Negative result:  Mixture remained blue if reducing sugar is absent.

Figure 2 Positive (left) and negative (right) results of the Benedict's Test.

            Examples of reducing sugars

            Glucose, Galactose, Lactose, Maltose, Fructose

            Examples of non-reducing sugars

            Sucrose