The three food molecules
Most of what you eat is made of three groups of large molecules, each built from smaller units.
| Molecule | Built from | Main job | Example foods |
|---|---|---|---|
| Carbohydrates | Simple sugars (like glucose) | Fast energy supply | Bread, rice, pasta, potatoes |
| Lipids (fats and oils) | Fatty acids and glycerol | Energy store, insulation | Butter, oil, nuts |
| Proteins | Amino acids | Growth and repair, enzymes | Meat, fish, eggs, beans |
- Polymer
- A large molecule made by joining many small repeating units together. Starch is a polymer of glucose; a protein is a polymer of amino acids.
Food tests
Each molecule has a chemical test that gives a colour change. Learn the reagent, the method and the positive result for each.
| Test for | Reagent | Method | Positive result |
|---|---|---|---|
| Reducing sugar (glucose) | Benedict's solution | Add and heat in a warm water bath | Blue turns green, then yellow, then brick-red |
| Starch | Iodine solution | Add a few drops at room temperature | Orange-brown turns blue-black |
| Lipid | Ethanol (emulsion test) | Mix with ethanol, then add water | A cloudy white emulsion forms |
| Protein | Biuret solution | Add at room temperature | Blue turns purple (lilac) |
Safety and accuracy
Benedict's test needs a water bath, not a naked flame, and ethanol is flammable, so keep it well away from heat. Always run a control with water so you can see what a negative result looks like.
Enzymes as biological catalysts
Chemical reactions in your body would be far too slow to keep you alive. Enzymes speed them up without being used up.
- Enzyme
- A protein that acts as a biological catalyst, speeding up a specific reaction without being changed or used up itself.
Each enzyme has a specially shaped dent called the active site. Only one kind of molecule, the substrate, fits into it, rather like a key fitting one lock. This is the lock-and-key model. When the substrate slots in, the enzyme holds it in just the right way to react, then releases the products and is free to do it again.
Explain why the enzyme amylase breaks down starch but not protein.
Effect of temperature and pH
Enzymes only work well in a narrow range of conditions. Picture a graph of reaction rate against temperature: the line climbs to a peak, then drops sharply.
As temperature rises, particles move faster and collide more often, so the rate climbs to a maximum at the optimum temperature (around 37°C in humans). Heat any further and the active site changes shape permanently. The substrate no longer fits and the rate crashes. This is denaturation, and it cannot be undone.
pH works the same way: the rate peaks at an optimum pH and falls off on either side. Stomach protease likes acidic conditions (about pH 2), while amylase in the mouth prefers a neutral pH of about 7. Too far from the optimum and the enzyme denatures again.
- Denaturation
- A permanent change to the shape of an enzyme's active site, caused by high temperature or the wrong pH, so the substrate no longer fits and the enzyme stops working.
Where this is assessed
Describing molecules and how enzymes work is Criterion A (knowing and understanding). Designing an enzyme experiment with a clear variable is Criterion B (inquiring and designing), and drawing the rate against temperature graph and explaining the peak is Criterion C (processing and evaluating).
Check yourself
1. Which test would you use for starch, and what is the positive result? +
Add iodine solution. If starch is present it turns from orange-brown to blue-black.
2. Why does an enzyme stop working at high temperature? +
Above the optimum, the heat changes the shape of the active site permanently. The substrate no longer fits, so the reaction stops. The enzyme has been denatured.
3. What does "an enzyme is specific" mean? +
It means each enzyme only speeds up one particular reaction, because only a substrate with the matching shape fits its active site (the lock-and-key model).
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