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 CO_2?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

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