The importance of enzymesSpec 1.12
Chemical reactions in living things would be far too slow to keep us alive without enzymes. Enzymes are biological catalysts that speed up reactions in cells. They are needed both to build large molecules and to break them down.
| Large molecule | Made (synthesised) from | Broken down into |
|---|---|---|
| Carbohydrates | Sugars | Sugars |
| Proteins | Amino acids | Amino acids |
| Lipids | Fatty acids and glycerol | Fatty acids and glycerol |
Different enzymes catalyse each of these reactions, because enzymes are specific. Breaking down large molecules into small ones is a vital job, and so is building the carbohydrates, proteins and lipids the body needs.
Core practical: food testsSpec 1.13BTriple only
You can use chemical reagents to identify the main biological molecules in a food. If the food is a solid, crush it with a little water and use the liquid; for the fat test, shake the food with ethanol instead. Always wear eye protection.
| Biological molecule | Reagent | Method | Positive result |
|---|---|---|---|
| Starch | Iodine solution | Add a few drops of iodine solution to the sample | Orange-brown to blue-black |
| Reducing sugars | Benedict's solution | Add Benedict's solution and heat in a hot water bath for a few minutes | Blue to green, yellow, orange or brick-red, depending on the amount of sugar |
| Protein | Biuret reagent | Add biuret reagent to the sample and mix | Blue to purple (lilac) |
| Fats | Ethanol | Shake the sample with ethanol, then pour the mixture into water | A cloudy white emulsion forms |
In a negative result, iodine stays orange-brown, Benedict's solution and biuret reagent stay blue, and the ethanol test stays clear. Ethanol is flammable, so keep it away from naked flames, and Benedict's solution is heated in a water bath, not directly over a flame.
Measuring the energy in foodSpec 1.14BTriple only
Food contains stored chemical energy. Energy can be measured using calorimetry: the food is burned and the energy it releases is used to heat a known mass of water.
- Measure a known volume of water into a boiling tube and record its starting temperature. 1 cm³ of water has a mass of 1 g.
- Weigh a small piece of food and hold it on a mounted needle.
- Set fire to the food and hold it under the boiling tube until the food stops burning.
- Record the highest temperature of the water and work out the temperature change.
Calculating the energy in food
Burning 0.5 g of a snack raises the temperature of 20 g of water by 12 °C. Calculate the energy released per gram of the snack.
- energy released = 20 × 4.2 × 12 = 1008 J
- energy per gram = 1008 ÷ 0.5 = 2016 J/g
Answer: 2016 J/g
The result is usually lower than the real value because heat is lost to the surroundings and the food may not burn completely. Surrounding the boiling tube with a draught shield, putting the flame close to the tube and stirring the water improve the results. Wear eye protection, take care with the burning food, and do not use foods such as nuts that people may be allergic to.
Quick check
What are proteins broken down into?
Show answer
Amino acids.
What does a catalyst do?
Show answer
It speeds up a reaction and is not used up.
Which reagent turns purple when protein is present?
Show answer
Biuret reagent (Triple only).
How can the energy in food be measured?
Show answer
By burning it to heat water and using the temperature change (calorimetry) (Triple only).