Investigating RQs (Cambridge (CIE) A Level Biology): Revision Note

Investigating RQs

• 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 germinating seeds or invertebrates

Respirometer Set-up Diagram

The typical set-up of a respirometer

The equation for calculating a 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

Using a respirometer to determine the Respiratory Quotient

Method

• Measure oxygen consumption: set up the respirometer and run the experiment with soda-lime present in both tubes. Use the manometer reading to calculate the change in gas volume within a given time, x cm³ min^-1

  • Always read from the side of the U-tube manometer closest to the respiring organisms (the left side as depicted in this diagram)

• Reset the apparatus: allow air to re-enter the tubes via the screw cap and reset the manometer fluid using the syringe

• 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

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

  • (x - y) is therefore the volume of CO₂ given off by the organisms 

  • remembering to read the scale on the side of the U-tube manometer closest to the respiring organisms

• The two measurements x and y can be used to calculate the RQ

RQ Equation for Respirometer experiment

Analysis

• 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 at the same time

  • This is because the RQ of glucose is 1 and the RQ of lipids is 0.7

• Under normal cellular conditions, the order which substrates are used in respiration is: carbohydrates, lipids and 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