CIE A Level Biology (9700) 2019-2021

Revision Notes

12.1.7 Investigating RQs

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

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 RQ

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
  • 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 = positive value, down = negative value)
  • The two measurements x and y can be used to calculate the RQ

RQ = CO2 / O2

RQ = (x + y)  / x

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
    • Eg. 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 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

Exam Tip

There are several ways you can manage variables and increase the reliability of results in respirometer experiments:

  • Use a controlled water bath to keep the temperature constant
  • Have a control tube with an equal volume of inert material to the volume of the organisms to compensate for changes in atmospheric pressure
  • Repeat the experiment multiple times and use an average

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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