Chapter 3: Biological Molecules

How do you test for reducing sugars?

Carry out the Benedict’s test

1. Add 2.0cm cube of food solution into a test tube.

2. Add 2.0cm cube of Benedict’s solution to the food sample.

3. Shake the mixture and place the test tube in a boiling water bath.

4. Record the colour of the solution.

Explain why a water bath is used in the Benedict’s test instead of heating the solution over a flame.

The water bath ensures a constant and uniform temperature is maintained during heating. Direct heating over a flame will decompose the sugar, creating a brown solution.

What are the positive results of the Benedict’s test

Solution turned from blue to green precipitate —> traces of reducing sugar

Solution turned from blue to yellow/orange precipitate —> moderate amount of reducing sugar

Solution turned from blue to brick-red precipitate —> large amount of reducing sugar

Why are starch and glycogen stores of glucose?

1. insoluble in water, so they cannot change the water potential of cells

2. large molecules, cannot diffuse through cell membranes

3. compact shapes, don’t take up much space

What are the functions of carbohydrates?

1. Immediate source of energy

2. Needed for the formation of nucleic acids

3. Needed as a substrate for respiration, to release energy for cell activities

Describe the digestion of starch

Amylase digests starch into maltose. Maltase digests maltose into glucose.

How do you test for the presence of starch?

Carry out the iodine test

1. Add a few drop of iodine solution to a food sample

2. The iodine solution turns from brown to blue-black (positive)

3. The iodine solution will remain brown (negative)

Describe the digestion of fats

Lipase digests fats into glycerol and three fatty acids.

What are the functions of fats?

1. A source and long-term storage of energy

2. An insulating material that prevents extensive heat loss

3. A solvent for fat-soluble vitamins and many other vital substances

How do you test for fats in liquid foods?

Carry out the ethanol emulsion test

1. Add 2cm cube of ethanol to 2cm cube of food sample in a test tube. Ethanol dissolves fat.

2. Shake the contents of the test tube thoroughly to dissolve fat.

3. Add 2cm cube of water to the test tube and shake thoroughly.

4. Record observations

5. A white emulsion is formed (positive)

6. The solution remains clear (negative)

How do you test for fats in solid foods?

Carry out the ethanol emulsion test.

1. Crush food into small pieces and place the pieces in a test tube

2. Add 2cm cube of ethanol to the test tube. Ethanol dissolves fat.

3. Shake the contents of the test tube thoroughly.

4. Allow the solid particles to settle. Decant the ethanol into another test tube containing 2cm cube of water. Shake the mixture

5. Record observations

6. A white emulsion is formed (positive)

7. The solution remains clear (negative)

Describe the digestion of proteins

Proteins are digested into polypeptides by protease and are further digested by protease into amino acids.

What are the functions of proteins?

1. Synthesise protoplasm for growth and repair of worn-out tissues.

2. Used in the formation of antibodies to combat diseases.

3. Synthesise antibodies and some hormones.

How do you test for the presence of proteins?

Carry out the Biuret test

1. Add an equal amount of Biuret solution to 2cm cube of food solution

2. Shake well and record observations

3. The solution turns from blue to violet (positive)

4. The solution stays blue (negative)

What is the difference between emulsion and precipitate?

Emulsion is a liquid-in-liquid mixture. Precipitate is a solid-in-liquid mixture.

How to identify if a molecule is a protein?

If it has nitrogen.