Investigating the Rate of Respiration (AQA A Level Biology): Revision Note

Required practical: investigating respiration rate

• It is possible to investigate factors that affect respiration in yeast using a redox indicator

  • A redox indicator is a substance that changes colour depending on whether it is reduced or oxidised

  • E.g. DCPIP and methylene blue

    • Blue = oxidised

    • Colourless = reduced

• Redox indicators change colour in the presence of respiring cells because:

  • many of the reactions of respiration involve oxidation and reduction, e.g.:

    • during glycolysis triose phosphate is oxidised and NAD is reduced

    • during the Krebs cycle NAD and FAD are reduced

  • when a redox indicator is present it can become reduced instead of NAD or FAD

• When a redox indicator is added to a suspension of living cells, the rate at which colour change occurs can be used to represent respiration rate

  • The faster the rate of respiration, the faster the rate of hydrogen release and the faster the dyes get reduced and change colour

Investigating the effect of temperature on respiration rate in yeast

Apparatus

• Yeast

• Glucose solution

• Buffer solution

• Test tubes

• Water baths

• Stopwatch

• DCPIP

Method

1. Place a set volume and concentration of glucose solution into a series of test tubes

2. Add a set volume of buffer solution to each tube

   • This will maintain a constant pH

3. Place three test tubes into each water bath at range of different temperatures, e.g. 10 °C, 20 °C, 30 °C, 40 °C, 50 °C

4. Leave the tubes in their water baths for at least ten minutes

   • This allows the temperature of the tube contents to reach the desired temperature before testing

5. Add a set volume of yeast suspension to the first test tube and stir

6. Add a set volume of DCPIP to the first test tube and start the stopwatch immediately

7. Stop the stopwatch when the solution loses its blue colour

   • This is subjective and therefore the same person should be assigned this task for all repeat experiments

   • A control tube containing only yeast and glucose should be set up for the purpose of colour comparison

8. Record the time taken for a colour change to occur

9. Repeat steps 5-8 for the other two tubes at the same temperature

   • This ensures that there are three repeats at each temperature and allows anomalies to be identified

10. Repeat steps 5-9 at all of the other temperatures

Results

• A graph should be plotted of temperature against time taken for colour change

• As the temperature increases, the rate of respiration should also increase; we would expect the time taken for the solution to become colourless to reduce

Alternative investigations

• Alternatives to the investigation described above could study the effect of another variable, e.g.:

  • substrate concentration

  • substrate type, e.g. different types of sugar

  • pH

• A colorimeter could be used to measure to colour intensity of the redox indicator in a less subjective and more quantitative way

• An alternative method might be to measure the volume of carbon dioxide gas produced by a yeast suspension, and use this to determine respiration rate

  • Yeast cells can respire anaerobically, so it is possible to add a layer of oil on top of the yeast suspension and measure anaerobic respiration rate in this way