5.7.14 Practical: Respirometer

• Respirometers are used to measure and investigate the rate of oxygen consumption during aerobic respiration in organisms
• The experiments usually involve organisms such as seeds or invertebrates
• By adding the apparatus to a thermostatically controlled water bath the effect of temperature on the rate of respiration can be investigated

Investigating the effect of temperature on the rate of respiration

Apparatus

• Respirometer
• Glass beads
• Germinating seeds
  • These will be actively respiring and consuming oxygen

• Temperature-controlled water bath
• Test tubes
• Soda-lime pellets
  • To absorb the carbon dioxide produced

•  Stopwatch
  

Respirometer set up with temperature-controlled water bath

Method

• Measure oxygen consumption: set up the respirometer and run the experiment with both tubes in a controlled temperature water bath. Use the manometer reading to calculate the change in gas volume within a given time, x cm³ min^-1
• Reset the apparatus: Allow air to reenter the tubes via the screw cap and reset the manometer fluid using the syringe. Change the temperature of the water bath and allow the tubes to acclimate, then close the screw clip to begin the experiment
• Run the experiment again: use the manometer reading to calculate the change in gas volume in a given time, y cm³ min^-1
• Repeat experiment several times at different temperatures

Calculations

• The volume of oxygen consumed (cm³ min^-1) can be worked out using the diameter of the capillary tube r (cm) and the distance moved by the manometer fluid h (cm) in a minute using the formula:

 πr²h

Analysis

• The rate of oxygen consumption (cm³ min^-1) is often taken as the rate of respiration for organisms
• The different volumes of oxygen consumed obtained for the different temperatures can be presented in table or graph form to show the effects of temperature

Graph showing the effect of temperature on the rate of respiration

Limitations

• The respirometer set up above is for measuring the rate of aerobic respiration. It cannot be used to measure the rate of anaerobic respiration as no oxygen is consumed during anaerobic respiration, as shown by the different equations for aerobic and anaerobic respiration.
  • Aerobic respiration: Glucose + Oxygen → Energy + Carbon Dioxide
  •  Anaerobic respiration (in mammals): Glucose → Energy + Lactic acid