Practical: Investigating the Rate of Respiration (Edexcel A (SNAB) A Level Biology)

• 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

Apparatus

• Respirometer
• Glass beads
• Germinating seeds
  • These will be actively respiring and consuming oxygen
• Test tubes
• Soda-lime pellets (or potassium hydroxide)
  • To absorb the carbon dioxide produced
•  Stopwatch

A respirometer set up to measure the rate of respiration

Method

• Measure oxygen consumption: set up the respirometer and run the experiment with both tubes for a set amount of time (e.g. 30 minutes)
• As the seeds consume oxygen, the volume of air in the test tube will decrease (CO₂ produced during respiration is absorbed by soda lime or KOH) 
• This reduces the pressure in the capillary tube and manometer fluid will move towards the test tube containing the seeds
• Measure the distance moved by the liquid in a given time
• Use this measurement to calculate the change in gas volume within a given time, x cm³ min^-1
• Reset the apparatus: Allow air to re-enter the tubes via the screw cap and reset the manometer fluid using the syringe
• Repeat experiment several times and calculate the average volume of oxygen consumed

Calculations

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

 πr²h