Structure and Functions in Living Organisms
Biological Molecules and Food Tests
Pearson Edexcel International GCSE Biology
Chemical elements
Carbohydrates and lipids contain carbon, hydrogen and oxygen only; proteins also contain nitrogen.
Large molecules
| Large molecule | Made from |
|---|---|
| starch and glycogen | glucose |
| proteins | amino acids |
| lipids | fatty acids and glycerol |
Food tests
Method: food tests
- Grind the food with water to make an extract.
- Starch: add iodine solution.
- Glucose: add Benedict's solution, then heat in a water bath.
- Protein: add biuret solution.
- Fat: grind the food with ethanol, shake, then add water.
| Food | Reagent | Positive result |
|---|---|---|
| starch | iodine solution | orange to blue-black |
| glucose | Benedict's solution, heated | blue to green, yellow, orange or red |
| protein | biuret solution | blue to lilac |
| fat | ethanol, then water | a white emulsion |
- No starch present: the iodine solution stays orange.
- Benedict's colour shows how much glucose: red in a high concentration of glucose; green, yellow or orange in a low concentration of glucose.
- Comparing glucose concentrations, keep constant the volume of sample, the volume of Benedict's and the time heated.
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