12.1.5 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 seeds or invertebrates

The typical set-up of a respirometer

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

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
• 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
  • y may be a positive or negative value depending on the direction that the manometer fluid moves (up = negative value, down = positive value)
  • 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

Worked example: Calculating RQ from a respirometer experiment

x = 2.9 cm³ min^-1

y = -0.8 cm³ min^-1

(x + y) / x = RQ

(2.9 - 0.8) / 2.9 = 0.724

When equal volumes of oxygen are consumed and carbon dioxide produced (as seen with glucose) the manometer fluid will not move and y will be 0, making the RQ 1.

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
  • 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