IB DP Biology: SL复习笔记2.7.4 Skills: Respiration


Analysis of results from experiments involving measurement of respiration rates in germinating seeds or invertebrates using a respirometer

  • Respirometers are used to measure and investigate the rate of oxygen consumption during respiration in organisms
  • The experiments usually require live organisms such as seeds or invertebrates
    • Use of animals should be minimised when seeds can provide excellent data
  • There are many different designs of respirometers, though they all have certain features in common
    • A sealed container containing live organisms and air
    • An alkaline solution (eg. potassium hydroxide) to absorb CO2
    • A capillary tube connected to the container and set against a graduated scale (a manometer)
  • The organisms respire aerobically and absorb oxygen from the air
  • The CO2 they release is absorbed by the alkali
  • This reduces the air pressure inside the sealed chamber
  • The manometer fluid (shown in red below) moves towards the organisms because of the pressure drop inside the chamber
  • The respirometer must be kept in very temperature-controlled conditions because slight fluctuations in temperature can affect the air pressure
    • A thermostatically controlled water bath is the best way to maintain a constant temperature
  • Repeat readings should be carried out for each set of experimental conditions, in order to identify and eliminate anomalies
    • Repeat readings give a reliable mean


    • Respirometers can be used in experiments to investigate how different factors affect the rate of respiration of organisms over time
      • Eg. temperature – using a series of water baths

Use of technology to measure rate of respiration

  • Technological devices can automate and make the measurement of respiration rate easier
    • Not to be confused with breathing rate
  • Oxygen sensors and CO2 monitors can measure oxygen and CO2 concentration in real-time
    • Without the need to expose the subject to hazards such as strong alkalis
  • Dataloggers can record data over a period of time for analysis later


The typical set-up of a respirometer

The equation for calculating a change in gas volume

  • The volume of oxygen consumed (mm3 min-1) can be worked out using the radius of the lumen of the capillary tube r (mm) and the distance moved by the manometer fluid h (mm) in a minute using the formula:πr2h

Worked Example

A respirometer was set up with germinating mung beans in the experimental tube. After a  period of equilibration, the liquid in the capillary was measured to move by 2.3 cm in 25 minutes 30 seconds. The capillary tube had an internal diameter of 0.30 mm.Calculate the rate of respiration of the mung beans, measured as the rate oxygen uptake, in mm3 hr-1Use the value of pi (π) = 3.141 and state your final answer to 2 significant figures

Step 1: Calculate the cross-sectional area of the capillary tube

Diameter = 0.30mm, so radius = 0.30 ÷ 2 = 0.15 mm

Cross sectional area = πr2 = 3.141 ✕ 0.152 = 0.0707 mm2Step 2: Calculate the volume of oxygen that had been taken up

The liquid moved 2.3 cm, which is 23mm

Volume of liquid moved in 25 minutes 30 seconds =

πr2h , where h = 23 mm= 0.0707 ✕ 23 = 1.625 mm3

Step 3: Calculate the rate of oxygen consumption per hour

25 minutes 30 seconds = 25.5 minutes

Rate per hour = 1.625 ✕ (60 ÷ 25.5)3.824 mm3 hr-1

To 2 sf = 3.8 mm3 hr-1

NOS: Assessing the ethics of scientific research: the use of invertebrates in respirometer experiments has ethical implications

  • The use of live animals in experiments has raised ethical concerns
  • Should we be removing animals from their natural habitat?
    • Does human learning outweigh the suffering that may be caused?
  • Will the animals suffer or feel pain?
  • How can exposure to hazards be minimised for the animals eg. avoiding contact with the alkali
  • Animals must be returned to their natural habitat directly after the readings have been taken
  • Can an alternative method that uses other non-animal species be found that still provides learning eg. the use of germinating seeds?
  • There must be no laboratory work that causes pain or suffering to animals or humans

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 for reliability and calculate a mean