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Biology 11–12 · Year 11

Oxygen uptake by germinating seeds in a simple respirometer

Module 1: Cells as the Basis of Life (Cell Function)

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Aerobic respiration consumes oxygen; when the carbon dioxide produced is absorbed, the gas volume falls at a rate that measures respiration.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

  • Potassium hydroxide 2.7 mol/L is corrosive
  • Seeds must not touch the alkali
  • Glass capillary can break

Controls

  • Teacher or technician dispenses the alkali; wear goggles; wash splashes at once and flood eyes with water
  • Keep the platform above the liquid
  • Handle the manometer gently

Note

Potassium hydroxide: NSW Department of Education Chemical Safety in Schools package (https://education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/1._Section_1_-_General_information_for_all_staff.pdf) and RiskAssess (https://www.riskassess.com.au/).

What you need

  • Germinating mung beans or peas, about 5 cm3 (the source's quantity of living material), soaked 24 hours; an equal volume of glass beads in the second tube as the thermobarometer (control)
  • Two boiling tubes with bungs; a wire basket or gauze platform; 1 mL syringe; capillary U-tube manometer with coloured oil on a scale (the source apparatus)
  • Potassium hydroxide solution 15 percent (2.7 mol/L), 5 mL per tube, in the base of each tube below the platform
  • Water bath at 20 degrees Celsius; thermometer; stop clock

How to do it

  1. Place potassium hydroxide in the base of both tubes, the seeds on the platform of one and glass beads in the other, and connect the tubes through the manometer.
  2. Stand both tubes in the water bath for 10 minutes with the tubes open to equilibrate, then close them.
  3. Record the manometer fluid position every 4 minutes until four consecutive readings lie on a straight line (the source criterion).
  4. Use the syringe to return the fluid to the start and repeat, three runs in all.
  5. Calculate oxygen uptake per minute from the fluid displacement and the capillary bore, volume per millimetre = pi x (bore radius)^2, adding the volume returned with the syringe each time the fluid was reset.
  6. Repeat at 30 degrees Celsius if time allows.

What you should see

The manometer fluid moves steadily toward the seed tube, showing that the gas volume on that side is falling as oxygen is consumed and carbon dioxide is absorbed; the glass-bead tube on the other arm acts as a thermobarometer, so room temperature and pressure changes act on both sides and cancel, as the source explains. With a 1.0 mm bore, each millimetre of travel is 0.785 microlitres (computed); germinating seeds move the fluid quickly enough that the syringe is used to return it to the scale during a run, and its readings are added to the total, as the source method records. The rate is higher at 30 degrees Celsius than at 20. The learner knows it worked when four consecutive readings lie on one straight line (the source's criterion) and the fluid returns to its start mark after each reset.

What changes

What you change
temperature (degrees Celsius) or seed type
What you measure
volume of oxygen taken up per minute
What you keep the same
  • mass of seeds
  • volume of potassium hydroxide
  • apparatus
  • equilibration time

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Seeds and plants do not respire; germinating seeds respire rapidly.
  • The fluid moves because the seeds warm the air; the water bath and control tube rule this out.
  • Respiration and breathing are the same thing; respiration is the cellular release of energy.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO11-8BIO11/12-3
  • Biology 11–12 Syllabus (2025), Year 11 focus area Cells as the basis of life; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-01
  • Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-03BI-11WS-05
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/biology-stage-6-2017
  2. www.nsw.gov.au/sites/default/files/noindex/2025-03/biology-stage-6-syllabus-2017.docx
  3. curriculum.nsw.edu.au/learning-areas/science/biology-11-12-2025/outcomes
  4. curriculum.nsw.edu.au/learning-areas/science/biology-11-12-2025/content/year-11/fa0edb304c
  5. practicalbiology.org/energy/gas-balance-in-respiration-and-photosynthesis/measuring-the-rate-of-metabolism.html
  6. practicalbiology.org/energy/gas-balance-in-respiration-and-photosynthesis/measuring-respiratory-quotient.html

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