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5.2.8Energy Transfers In and Between Organisms (A Level only)

Required Practical- Investigating Respiration Rate — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 11 of 30 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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5.2.8 Required Practical- Investigating Respiration Rate — AQA A Level Biology 7402 notes: Plan and evaluate an investigation into the rate of respiration using a respirometer or redox indicators, and process the results.Respiration rate can be measured by the uptake of oxygen, t…

  • production of carbon dioxide
  • release of heat
  • activity of dehydrogenase enzymes
  • methylene blue
  • volume of gas decreases
  • water bath

Section 1 of 9Ways of measuring respiration

Respiration rate can be measured by the uptake of oxygen, the production of carbon dioxide, the release of heat or the activity of dehydrogenase enzymes. For AQA you must be able to use a respirometer and also redox indicators such as methylene blue or DCPIP (or tetrazolium) with yeast or tissue.

Section 2 of 9Respirometer method

Soda lime
Place the organisms (germinating seeds, small invertebrates, or yeast suspension in glucose) in a tube. A separate small tube or a pad holds soda lime or potassium hydroxide solution, which absorbs CO2 produced.
Volume of gas decreases
The tube is connected to a capillary tube containing a drop of coloured fluid (a manometer). As oxygen is used and the CO2 is absorbed, the volume of gas decreases and the fluid moves towards the organisms.
Water bath
The apparatus is kept in a water bath at constant temperature. Allow it to equilibrate, then record the position of the drop at time intervals.
Calculate the volume of oxygen taken in per unit time and per unit mass of organism: rate = distance moved × πr2 / (time × mass).
Control tube
A control tube with glass beads of equal volume (or dead organisms) allows for changes in atmospheric pressure and temperature.

To test temperature, repeat at different temperatures, using a water bath. To test substrate, vary the type or concentration of the respiratory substrate.

Section 3 of 9Redox indicator method

Methylene blue turns colourless when it accepts hydrogen from the reactions of respiration (the dehydrogenase enzymes).

1Yeast in glucose solution
A suspension of yeast in glucose solution with a few drops of the dye is kept at a set temperature, and the time taken for the colour to disappear is recorded.
2
The faster the colour loss, the faster the rate of respiration.
3
Rate can be given as 1/time.
4
Make sure the volumes and the concentrations are identical.

This works because hydrogen released by dehydrogenases normally goes to NAD; the dye competes as a hydrogen acceptor.

Section 4 of 9Variables and controls

Independent
Independent for example, temperature, or substrate concentration.
Dependent
Dependent volume of oxygen taken in per minute, or time for the dye to change.
Controlled
Controlled mass and type of organism, pH, volume of the solutions, apparatus, time to equilibrate.

Rates at different temperatures normally increase with temperature as enzymes have more kinetic energy, up to an optimum, then fall as enzymes denature. The temperature coefficient Q10 = rate at (T + 10) / rate at T is often about 2.

Section 5 of 9Evaluation and safety

Airtight
Apparatus must be airtight, otherwise results are unreliable.
Organisms should not be touched by the soda lime, which is corrosive; wear eye protection.
Living organisms (animals) must be treated humanely, with no harm and a short time in the apparatus.
Repeat and calculate means; consider the possibility of anaerobic respiration affecting the results.
The colour end-point is subjective, so use a colorimeter or a standard colour card.

Section 6 of 9Further methods and data handling

Other methods of measuring the rate of respiration include a data logger with an oxygen or carbon dioxide probe, a colorimeter to follow the colour change of a redox dye, and measuring the temperature rise of germinating seeds in a vacuum flask. Sensors give more precise and objective readings than counting or visual estimates, and can record continuously, so the rate can be found from the gradient of the graph.

You should be able to find the rate from a graph.

Further methods and data handling
Draw a tangent to the curve at the required time, and calculate the gradient as the change in the y-axis divided by the change in the x-axis, including units.
For percentage changes, use (final - initial) / initial × 100. You may also be asked to use the Q10 value: for example, a rate of 4 units at 20 degrees Celsius and 9 units at 30 degrees gives a Q10 of 2.25.

Choose the right graph: a scatter graph or line graph with the independent variable on the x-axis. If the repeats vary a lot, draw error bars. Where the results show a peak, you can identify the optimum temperature, but only to the nearest temperature tested, so more temperatures between would give a better estimate.

Ethics

  • yeast, seeds and invertebrates are used.
  • Animals such as woodlice or maggots should be handled gently, kept for only a short period and returned to their habitat.
  • Consider whether the results from yeast can be applied to humans: the metabolic pathways are similar, but there are differences in detail, so a conclusion should be stated carefully.

Section 7 of 9Exam-style questions

  1. In a respirometer, explain why soda lime is placed in the tube and why the coloured drop moves towards the organisms. [3]3 marks

  2. The drop moved 24 mm in 6 minutes. The capillary tube has a radius of 0.5 mm and the mass of the seeds was 2.0 g. Calculate the rate of oxygen uptake in mm3 g-1 min-1. [3]3 marks

  3. Suggest why a control tube containing glass beads is used. [2]2 marks

Section 8 of 9Key terms

TermMeaning
RespirometerApparatus for measuring the uptake of oxygen by living organisms.
Soda limeA chemical that absorbs carbon dioxide.
Methylene blueA redox indicator that turns colourless when reduced.
DehydrogenaseAn enzyme that removes hydrogen from a substrate.
ControlA set-up without the factor, to allow for other changes.
EquilibrateTo allow the apparatus to reach a steady temperature and pressure.
Q10The factor by which rate increases for a 10 degree rise in temperature.

Section 9 of 9Exam technique

1
Forgetting to use soda lime, so that the CO2 produced makes up for the oxygen used and there is no change.
2
Not keeping temperature constant with a water bath.
3
Failing to equilibrate the apparatus before taking readings.
4
Giving the rate without dividing by the mass of the organisms or the time.

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