Biological molecules
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
Chemical Elements in Biological Molecules
All biological molecules are built from a small set of chemical elements. You need to know which elements are found in each major group.
- Carbohydrates are made from Carbon (C), Hydrogen (H) and Oxygen (O)
- Fats and oils (lipids) are also made from Carbon (C), Hydrogen (H) and Oxygen (O) - the same elements as carbohydrates, but in different proportions (much less oxygen)
- Proteins are made from Carbon (C), Hydrogen (H), Oxygen (O) and Nitrogen (N) - and often Sulfur (S) too
Common Mistakes
- ❌ Forgetting that proteins are the only major group containing nitrogen
- ✓ carbohydrates and fats both contain only C, H and O. Proteins are uniquely identified by containing nitrogen (and often sulfur) - this is why nitrogen is an essential element in fertilisers, since plants need it to make proteins
Large Molecules from Small Molecules
Large biological molecules are built by joining many smaller molecules (monomers) together.
- Starch, glycogen and cellulose are all large molecules made from many glucose molecules joined together:
- Starch - energy storage in plants (e.g. in seeds, potato tubers)
- Glycogen - energy storage in animals and fungi (stored in the liver and muscles)
- Cellulose - structural; makes plant cell walls and provides support
- Proteins are made from many amino acids joined together - proteins have many roles, including enzymes, antibodies, haemoglobin, structural proteins (e.g. keratin, collagen) and hormones (e.g. insulin)
- Fats and oils are made from fatty acids and glycerol (one glycerol molecule joined to three fatty acid molecules) - used for energy storage, insulation, as a cell membrane component, and to protect organs
Common Mistakes
- ❌ Thinking starch, glycogen and cellulose must be unrelated since they behave so differently
- ✓ all three are polysaccharides made from the same monomer, glucose - but they have different structures and functions: starch (plants, energy store), glycogen (animals, energy store), cellulose (plants, structural). Cellulose cannot be digested by humans because we lack the enzyme cellulase
Food Tests
Specific chemical tests are used to identify the presence of biological molecules. You must know the reagent, method, and results (positive and negative) for all five tests.
| Test | Substance tested for | Reagent / method | Negative result | Positive result |
|---|---|---|---|---|
| Iodine solution test | Starch | Add iodine solution (orange-brown) directly to the sample | Remains orange-brown | Turns blue-black |
| Benedict's test | Reducing sugars (e.g. glucose, maltose) | Add Benedict's reagent (blue) to the sample; heat in a boiling water bath for 5 minutes | Remains blue | Turns brick-red / orange precipitate (colour deepens with increasing sugar concentration: green → yellow → orange → red) |
| Biuret test | Proteins | Add sodium hydroxide (NaOH) solution then a few drops of dilute copper sulfate solution (or use premixed biuret reagent) | Remains blue | Turns purple / violet |
| Ethanol emulsion test | Fats and oils (lipids) | Shake the sample with ethanol; pour the ethanol layer into water | Solution remains clear | Forms a cloudy white emulsion |
Benedict's test requires a hot water bath - do not heat the test tube directly in a flame, as it may spit. Iodine solution stains skin and clothing. NaOH in the biuret test is corrosive, so wear eye protection. Ethanol is flammable, so keep it away from flames.
Common Mistakes
- ❌ Treating Benedict's test as a simple yes/no result
- ✓ Benedict's test is semi-quantitative - the colour gives an indication of the amount of reducing sugar present: blue (none) → green (trace) → yellow (low) → orange (moderate) → brick-red (high)
- ❌ Expecting sucrose to give a positive Benedict's test
- ✓ sucrose is a non-reducing sugar and will not give a positive Benedict's test unless it is first hydrolysed with acid and then neutralised
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