4.1 Biological molecules
- Syllabus
- 0610–2026–2027
- Topic
- 4.1
- Level
- —
Learning objectives
4.1.1Chemical elements that make up• List the chemical elements that make up: carbohydrates, fats and proteins4.1.2Large molecules are made from smaller• State that large molecules are made from smaller molecules, limited to: (a) starch, glycogen and cellulose from glucose (b) proteins from amino acids (c) fats and oils from fatty acids and glycerol4.1.3Food tests• Describe the use of: (a) iodine solution test for starch (b) Benedict’s solution test for reducing sugars (c) biuret test for proteins (d) ethanol emulsion test for fats and oils (e) DCPIP test for vitamin C4.1.4Structure of a DNA molecule: (a) two• Describe the structure of a DNA molecule: (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) the bases always pair up in the same way: A with T, and C with G (full names are not required)
Match biological molecules to their elements
Carbohydrates, fats and proteins contain characteristic sets of chemical elements.
| Molecule group | Elements present in all examples |
|---|---|
| carbohydrates | carbon, hydrogen and oxygen |
| fats and oils | carbon, hydrogen and oxygen |
| proteins | carbon, hydrogen, oxygen and nitrogen |
All three groups contain carbon, hydrogen and oxygen. Nitrogen distinguishes the required protein list from the carbohydrate and fat lists.
The element symbols are C, H, O and N. An exam asking for names requires carbon, hydrogen, oxygen and nitrogen rather than only the symbols.
Do not list nitrogen for every carbohydrate or fat. For this syllabus objective, protein contains C, H, O and N; carbohydrate and fat contain C, H and O.
Build large molecules from smaller units
Large biological molecules are made by joining many smaller molecules in specific combinations.
| Large molecule | Smaller molecule or molecules |
|---|---|
| starch | glucose |
| glycogen | glucose |
| cellulose | glucose |
| protein | amino acids |
| fat or oil | fatty acids and glycerol |
Starch, glycogen and cellulose are different large molecules but share glucose as their smaller unit. Proteins use amino acids, while fats and oils require two kinds of smaller molecule.
The mapping works in reverse for breakdown questions: starch, glycogen and cellulose yield glucose units; proteins yield amino acids; fats and oils yield fatty acids and glycerol.
Glycerol alone does not make a fat, and glucose does not make protein. Keep each large molecule paired with exactly the smaller units listed in the syllabus.
Carry out and interpret five food tests
Each food test uses a named reagent and a characteristic positive result to identify a biological substance.
| Substance | Method | Positive result |
|---|---|---|
| starch | add iodine solution | orange-brown → blue-black |
| reducing sugar | add Benedict’s solution and heat in a hot-water bath | blue → green, yellow, orange or brick-red precipitate |
| protein | add biuret solution; do not heat | blue → lilac or purple |
| fat or oil | add ethanol and shake, then add water | white or milky emulsion |
| vitamin C | add sample to blue DCPIP | DCPIP is decolourised |
Use separate clean samples and droppers to avoid contamination. Include a known positive and a negative control when evaluating an unfamiliar sample.
Record both the reagent and observed result. A final colour is meaningful only when compared with the reagent's starting colour and the correct method, including heat only for Benedict’s test.
Use a water bath for Benedict’s test. Ethanol is flammable, so keep the emulsion test away from flames and add water only after shaking with ethanol.
Benedict’s blue-to-red result does not test protein, and biuret does not require heating. DCPIP decolourisation indicates vitamin C, not reducing sugar.
Describe DNA structure and base pairing
A DNA molecule consists of two strands coiled together to form a double helix.
Each strand contains chemicals called bases. Bonds between pairs of bases hold the two strands together.
| Base on one strand | Base on the other strand |
|---|---|
| A | T |
| T | A |
| C | G |
| G | C |
To complete a complementary strand, work one position at a time and replace each base with its fixed partner. For example, A–C–G–T pairs with T–G–C–A.
Bases pair across the two strands, not along one strand. A never pairs with C or G in this model: A pairs with T, and C pairs with G.