Skills: Respiration (DP IB Biology: SL)

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

Analysis

• • 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

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

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

Step 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 =

= 0.0707 ✕ 23 = 1.625 mm³

Step 3: Calculate the rate of oxygen consumption per hour

25 minutes 30 seconds = 25.5 minutes

3.824 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