7.8 Core Practical 16: Respirometer to Calculate RQ

• Respirometers are used to measure and investigate the rate of oxygen consumption during respiration in organisms
• They can also be used to calculate respiratory quotients
• 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

1. Measure oxygen consumption: set up the respirometer and run the experiment with both tubes for a set amount of time (e.g. 30 minutes)
2. 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) 
3. This reduces the pressure in the capillary tube and manometer fluid will move towards the test tube containing the seeds
4. Measure the distance moved by the liquid in a given time
5. Use this measurement to calculate the change in gas volume within a given time, x cm³ min^-1
6. Reset the apparatus: allow air to re-enter the tubes via the screw cap and reset the manometer fluid using the syringe
7. Run the experiment again: remove the soda-lime from both tubes and use the manometer reading to calculate the change in gas volume in a given time, y cm³ min^-1

Equation for calculating change in gas volume

• 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

Calculations

• x tells us the volume of oxygen consumed by respiration within a given time
• y tells us the volume of oxygen consumed by respiration within a given time minus the volume of carbon dioxide produced within a given time
  • y may be a positive or negative value depending on the direction that the manometer fluid moves (up = positive value, down = negative value)
• The two measurements x and y can be used to calculate the RQ

Equation to calculate RQ values using a respirometer

x = 2.9 cm³min^-1

y = -0.8 cm³min^-1

Step 1: Write down equation

Step 2: Substitute values

Step 3: Calculate RQ

Interpretation of results

• Respirometers can be used in experiments to investigate how different factors affect the RQ of organisms over time
  • E.g. temperature – using a series of water baths
• When an RQ value changes it means the substrate being respired has changed
• Some cells may also be using a mixture of substrates in respiration e.g. An RQ value of 0.85 suggests both carbohydrates and lipids are being used
  • This is because the RQ of glucose is 1 and the RQ of lipids is 0.7
• Under normal cell conditions the order substrates are used in respiration: carbohydrates, lipids then proteins
• The RQ can also give an indication of under or overfeeding:
  • An RQ value of more than 1 suggests excessive carbohydrate/calorie intake
  • An RQ value of less than 0.7 suggests underfeeding