Biological molecules
🎯What you need to be able to do
- List the chemical elements in carbohydrates, fats and proteins.
- State which small molecules starch, glycogen, cellulose, proteins, fats and oils are made from.
- Describe the iodine, Benedict’s, biuret, ethanol emulsion and DCPIP tests.
- Describe the structure of DNA: double helix, bases, base pairing A–T and C–G EXTENDED.
📚The biology
Elements and building blocks
carbohydrates: carbon, hydrogen and oxygen
fats and oils: carbon, hydrogen and oxygen (much less oxygen than carbohydrates)
proteins: carbon, hydrogen, oxygen and nitrogen (some also sulfur)
Large molecules are made by joining many smaller ones:
Starch is the energy store in plants, glycogen the energy store in animals and fungi, and cellulose forms plant cell walls. There are about 20 different amino acids; their order makes each protein different.
Food tests
- Starch: add a few drops of iodine solution. Orange-brown stays → no starch; blue-black → starch.
- Reducing sugars (e.g. glucose): add Benedict’s solution and heat in a water bath at about 80 °C for a few minutes. Blue stays → none; green, yellow, orange then brick red as the amount increases.
- Protein: add biuret solution. Blue stays → none; purple (violet) → protein.
- Fats and oils: shake the food with ethanol, then pour the ethanol into water. A milky white emulsion → fat.
- Vitamin C: add the food solution drop by drop to DCPIP. The blue DCPIP turning colourless → vitamin C.
Solid food is first crushed with a little water to make a solution. Safety: wear eye protection, and use a water bath, not a flame, for Benedict’s.
DNA EXTENDED
- EXTENDED Two strands are coiled together to form a double helix.
- Each strand contains chemicals called bases (A, T, C and G).
- Bonds between pairs of bases hold the two strands together.
- The bases always pair the same way: A with T, and C with G.
✏️Worked example
Three unknown solutions are tested.
X: iodine stays orange-brown; Benedict’s turns brick red; biuret stays blue.
Y: iodine turns blue-black; Benedict’s stays blue; biuret stays blue.
Z: iodine stays orange-brown; Benedict’s stays blue; biuret turns purple.
(a) Identify the nutrient in each solution. [3]
(b) A student forgets to heat X with Benedict’s solution. Predict the result. [1]
(c) EXTENDED One strand of DNA reads ATGCGT. Write the sequence of the other strand. [1]
(a) X: a reducing sugar (e.g. glucose). Y: starch. Z: protein.
(b) It stays blue — a false negative, because Benedict’s only reacts when heated.
(c) TACGCA (A–T, T–A, G–C, C–G, G–C, T–A).
Check it. Each solution gave exactly one positive, so each contains one of the tested nutrients. Starch is not a reducing sugar, which is why Y
stayed blue with Benedict’s.
Colour words. Write “brick red”, “blue-black” and “purple”, and give the starting colour when asked for a
colour change. “It goes red” or “it changes colour” does not score.
📝Practise
In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.
1. (Multiple choice.) Which element is found in proteins but not in carbohydrates? A: carbon. B: hydrogen. C: nitrogen. D: oxygen.
C.
2. (Theory.) Name the smaller molecules that make up (a) glycogen, (b) a fat. [2]
(a) Glucose. (b) Fatty acids and glycerol.
3. (Practical.) Describe how to test a sample of milk for fat, and give the positive result. [3]
Shake the milk with ethanol, then pour the ethanol into a test-tube of water. A milky white emulsion shows that fat is present.
4. (Practical.) Four glucose solutions of different concentrations are tested with Benedict’s solution. They turn green, brick red, yellow and orange. Put them in order from lowest to highest concentration. [1]
green, yellow, orange, brick red.
5. (Practical.) Give one safety precaution for the Benedict’s test and one for the ethanol emulsion test. [2]
Benedict’s: wear eye protection / heat in a water bath rather than directly in a flame. Ethanol: keep it away from flames (it is flammable).
6. (Theory.) A student adds lemon juice drop by drop to DCPIP solution. State and explain the colour change. [2]
Blue to colourless, because the lemon juice contains vitamin C.
7. (Theory.) EXTENDED Describe how the two strands of a DNA molecule are held together. [2]
By bonds between pairs of bases on the two strands; A always pairs with T and C with G.
8. (Theory.) EXTENDED In a DNA sample, 30% of the bases are A. Calculate the percentage that are G, and show your reasoning. [2]
A pairs with T, so T = 30% and A + T = 60%. C + G = 40%, and C = G, so G = 20%.
🔗Go deeper — other people’s work
These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.
- Royal Society of Biology — food tests practical, with photographs of the colour changes
- yourgenome — what DNA is and how its structure was discovered