Every living thing is built from a small set of molecules made mainly from carbon, hydrogen and oxygen. Understanding carbohydrates, fats (lipids) and proteins – and the simple chemical tests that reveal them – is one of the most reliable sources of marks in your IGCSE Biology exam.
Carbohydrates, fats and proteins
The three main classes of large biological molecule are each built from smaller repeating units called monomers.
- Carbohydrates contain carbon, hydrogen and oxygen. Simple sugars such as glucose are the monomers; many joined together form complex carbohydrates such as starch and glycogen (storage) and cellulose (cell walls).
- Fats and oils (lipids) also contain carbon, hydrogen and oxygen but with far less oxygen. They are made from fatty acids and glycerol.
- Proteins contain carbon, hydrogen, oxygen and nitrogen (sometimes sulfur). Their monomers are amino acids joined into long chains that fold into a precise shape.
Why these molecules matter
Each class of molecule has clear jobs in the body, and exam questions often link structure to function.
- Carbohydrates are the main energy source; starch and glycogen store energy; cellulose gives plant cells strength.
- Fats store energy (more per gram than carbohydrate), insulate the body and protect organs.
- Proteins build new cells and tissues, and form enzymes, antibodies and some hormones.
- The sequence of amino acids decides a protein’s shape, and the shape decides its function – vital for understanding enzymes later.
Food tests
Learn each test as a method, the reagent and the colour change. Always note the starting and final colours.
- Starch: add iodine solution – orange/brown turns blue-black.
- Reducing sugars (e.g. glucose): add Benedict’s solution and heat – blue turns green → yellow → orange → brick-red depending on amount.
- Protein: add biuret reagent – blue turns purple/violet (no heating).
- Fats: the ethanol emulsion test – a cloudy white emulsion forms when ethanol mixed with the sample is added to water.
Using food tests
Food tests can be quantitative as well as qualitative.
- Benedict’s test is semi-quantitative: the colour shows roughly how much reducing sugar is present, from green (a little) to brick-red (a lot).
- Use a control (e.g. distilled water) so you can compare colour changes fairly.
- To test an unknown food, grind it, mix with water, then run each test on separate samples.
- Wear eye protection – Benedict’s and biuret contain irritant chemicals and ethanol is flammable.