Practical: Investigating Respiration (SQA National 5 Biology): Revision Note

Demonstrating the production of carbon dioxide

• The process of how living organisms respire can be measured using chemical indicators or sensors

• During aerobic respiration, organisms take in oxygen and release carbon dioxide. These gases can be detected using:

  • Hydrogen carbonate indicator, which changes colour with changing CO₂ levels

  • Resazurin, which changes colour as oxygen is used up (it becomes reduced)

  • Gas sensors and data loggers, which measure changes in O₂ and CO₂ concentration directly

• By comparing results under different temperatures, substrate concentrations, or organism types, we can demonstrate how respiration rate depends on enzyme-controlled reactions within living cells

Apparatus

• Hydrogen carbonate indicator

• Small living organisms (e.g. germinating seeds or woodlice)

• Boiled seeds / glass beads (control)

• Airtight containers or boiling tubes with rubber bungs

• Gauze/mesh platform (to keep organisms off the liquid)

• Optional: water bath for temperature control

• Optional: a lamp (if also comparing light/dark with plant material)

Method

1. Add 2–3 cm³ hydrogen carbonate indicator to each tube. Fit a mesh platform above the liquid surface

2. Set up tubes:

   • A (respiring): living organisms (equal mass/number)

   • B (control): boiled seeds or glass beads (same volume)

   • Optional C (no organisms): indicator only (air control)

3. Seal with bungs, ensuring no direct contact between organisms and the indicator

4. Keep all tubes at the same temperature (bench or water bath)

5. Start the timer

6. Observe and record colour changes at set intervals (e.g. every 2–5 min) for 20–30 min

7. (If comparing temperature/light): repeat at different temperatures, or light vs dark

Expected results

• In this investigation, we would expect to note the following

  • Tube A should turn yellow as the seeds are respiring and producing carbon dioxide

  • Tube B should remain orange as the dead seeds produce no carbon dioxide

  • Tube C should turn purple as there is no living material in there

• The faster the change to yellow, the higher the rate of respiration / carbon dioxide production

Limitations & improvements

• Airtightness

  • Leaks dilute carbon dioxide changes

  • Improvement: Check bungs and joints; use grease/parafilm.

• Temperature drift

  • Room temperature can vary, affecting the rate of respiration in the organisms

  • Improvement: Use a water bath; pre-equilibrate tubes

• Unequal biomass / activity

  • Different numbers/masses of living organisms can skew results

  • Improvement: Standardise mass and species; repeat at least three times

• Subjective colour judgement

  • Relying on the human eye can be subjective; borderline colours can make results unclear

  • Improvement: Use a colorimeter (absorbance) or a printed colour chart for comparison to test colours

Investigating the rate of respiration

• Respirometers are used to measure and investigate the rate of oxygen consumption during aerobic respiration in organisms

• The experiments usually involve organisms such as seeds or invertebrates

• By adding the apparatus to a thermostatically controlled water bath, the effect of temperature on the rate of respiration can be investigated

Apparatus

• Respirometer

• Glass beads

• Germinating seeds

  • These will be actively respiring and consuming oxygen

• Temperature-controlled water bath

• Test tubes

• Soda-lime pellets

  • To absorb the carbon dioxide produced

•  Stopwatch

  

Method

1. Set up the respirometer with both tubes in a water bath at a controlled temperature

2. Allow the apparatus to equilibrate, then close the screw clip to begin the experiment

3. Use the manometer reading to measure the change in gas volume over a fixed time (e.g. x cm³ min⁻¹)

4. Reset the apparatus by opening the screw cap to allow air in and using the syringe to reset the manometer fluid

5. Change the temperature of the water bath and allow the tubes to acclimate.

6. Repeat the experiment at the new temperature and record the change in gas volume (e.g. y cm³ min⁻¹)

7. Continue for several different temperatures to compare oxygen consumption rates

Calculations

• The volume of oxygen consumed (cm³ min^-1) can be worked out using the radius of the capillary tube r (cm) and the distance moved by the manometer fluid h (cm) in a minute, using the formula:

 πr²h

Expected results

• The rate of oxygen consumption (cm³ min^-1) is often taken as the rate of respiration for organisms

• The different volumes of oxygen consumed obtained for the different temperatures can be presented in table or graph form to show the effects of temperature

Limitations

• The respirometer set up above is for measuring the rate of aerobic respiration. It cannot be used to measure the rate of anaerobic respiration as no oxygen is consumed during anaerobic respiration, as shown by the different equations for aerobic and anaerobic respiration.

  • Aerobic respiration: Glucose + Oxygen → Energy + Carbon Dioxide

  •  Anaerobic respiration (in mammals): Glucose → Energy + Lactic acid