Organisation of an Ecosystem (AQA GCSE Biology): Revision Notes
Investigating decay
What is decay investigation?
You can study how temperature affects the rate of decay by measuring changes in pH. This practical helps you understand how quickly things break down in different conditions.
Decay happens when bacteria break down organic materials like milk. This process produces acids, which lower the pH. In nature, milk decay takes several days. But scientists use an enzyme called lipase to speed up this process for experiments.
Why use lipase?
Lipase is an enzyme that breaks down milk much faster than natural bacteria. This makes the practical quicker and easier to observe. The lipase still produces acids, so the pH drops just like in natural decay.
Equipment needed
You will need these items for the practical:
- Test tubes
- Test tube rack
- 10cm³ graduated pipettes
- Pipette filler
- Thermometer
- Stop clock
- Water baths (or beakers with hot water)
- Fresh full-fat milk
- Lipase solution
- Sodium carbonate solution
- Cresol red indicator
Method
Worked Example: Step-by-Step Procedure
Step 1: Add 5cm³ of milk and 7cm³ of sodium carbonate solution to a test tube. Add a few drops of cresol red indicator. Put a thermometer in the tube.
Step 2: Add 5cm³ of lipase solution to another test tube. Put both tubes in a water bath and wait until they reach the same temperature.
Step 3: Add 1cm³ of lipase solution to the milk mixture. Start the stop clock immediately. Stir until the indicator changes colour and record the time.
Step 4: Repeat steps 1-3 using different temperatures.
Understanding the indicators
Sodium carbonate makes the mixture alkaline at the start. This means the pH is high.
Cresol red indicator shows pH changes by changing colour:
- Purple = alkaline conditions (high pH)
- Yellow = acidic conditions (low pH)
As the lipase breaks down the milk, acids are produced. The pH drops and the indicator changes from purple to yellow.
Recording results
Create a table showing:
- Temperature used
- Time taken for colour change
Then plot a graph with:
- Temperature on the horizontal axis
- Time taken on the vertical axis
What the results show
Higher temperatures usually make decay happen faster. This is because enzymes like lipase work more quickly when it's warmer.
Lower temperatures slow down decay. This is why we keep food in fridges - it stops bacteria and enzymes working so quickly.
Practical tips
Helpful Alternatives and Tips:
- You can use plastic syringes instead of graduated pipettes
- Beakers of hot water work if proper water baths aren't available
- You can try temperatures below room temperature using ice water baths
- Set up multiple test tubes to save time and do repeats
- Some teachers use phenolphthalein indicator instead of cresol red
Remember!
Key Points to Remember:
- Decay produces acids, which lower pH
- Lipase enzyme speeds up the natural decay process
- Cresol red is purple in alkaline conditions and yellow in acidic conditions
- Higher temperatures generally increase the rate of decay
- You measure decay rate by timing how long it takes for the indicator to change colour