04) Biological molecules

Question 1

1.1. List the chemical elements that make up:
carbohydrates, fats and proteins

Answer 1

Carbohydrates: carbon, oxygen, hydrogen

Fats: carbon, oxygen, hydrogen, sulphur, nitrogen, phosphorous

Proteins: carbon, oxygen, hydrogen

Question 2

1.2. What are large molecules made from? Name examples from the syllabus

Answer 2

Smaller molecules
(a) starch, glycogen and cellulose from glucose
(b) proteins from amino acids
(c) fats and oils from fatty acids and glycerol

Question 3

1.3. Describe the food tests

Answer 3

(a) iodine solution test for starch
Using a dropper, place a few drops of iodine solution. Record any change in the colour
positive: blue-black
negative: yellow-brown (no change)

(b) Benedict’s solution test for reducing sugars
Add 1 cm3 of water and an equal volume of Benedict’s solution into boiling tube and mix, heat at 80°c in water bath for 5 minutes, take test tube out and observe colour
Clear blue to brick-red precipitate

(c) biuret test for proteins
Add 1 cm3 water and an equal volume of potassium hydroxide solution to the tube and stir. Add two drops of copper sulfate solution and stir for two minutes
positive: purple
negative: blue

(d) ethanol emulsion test for fats and oils
Add 2 cm3 of ethanol and an equal volume of distilled water is added, then shake the tube
positive: milky-white emulsion forms
negative: no change

(e) DCPIP test for vitamin C
Add 1cm3 of DCPIP solution to a test tube, then add a small amount of food sample (as a solution) a bit at a time
positive: blue colour of the dye disappearing
negative: no change

Question 4

1.4. Describe the structure of a DNA molecule

Answer 4

(a) Two strands coiled together to form a double helix

(b) Each strand contains chemicals called bases

(c) Bonds between pairs of bases hold the
strands together

(d) Bases always pair up in the same way:
A with T, and C with G (apples on trees and cars on the ground)