WebLab STEM

Biology · 8. Cellular respiration

Cellular respiration, gas exchange and respiratory quotient

Connect respiration equations to ATP, measure O₂ uptake with a respirometer and infer substrates from RQ within valid limits.

45 minutesContent version: 2.2
Start the interactive pathwayOpen the virtual experiment

Driving question

How can a small gas-volume change reveal which substrate cells are using?

Curriculum coverage

Vietnam Biology 10–11; Cambridge IGCSE 0610 and AS & A Level 9700: respiration, gas exchange and practical skills.

Model scope and limitations

RQ near 1 is commonly consistent with carbohydrate respiration and near 0.7 with lipid, while protein values vary; mixed substrates, anaerobic metabolism, growth or unstable conditions weaken the inference. A respirometer does not measure ATP directly.

Learning objectives

  • Distinguish ventilation, gas exchange and cellular respiration.
  • Explain the roles of KOH, a control and a thermostatic bath in a respirometer.
  • Calculate RQ = CO₂ produced/O₂ consumed and interpret it cautiously.

Prerequisite knowledge

  • Diffusion and concentration gradients.
  • Mitochondria, enzymes and chemical equations.

Core knowledge

Aerobic respiration

Glucose is oxidised through glycolysis, the link reaction, Krebs cycle and oxidative phosphorylation; O₂ is the final electron acceptor.

Respirometer principle

KOH absorbs CO₂, so the gas-volume or pressure decrease mainly reflects O₂ uptake after correction against a control.

Respiratory quotient

RQ = VCO₂/VO₂ when gases are measured under the same conditions. Carbohydrate has theoretical RQ 1.0; lipids are often near 0.7 because more O₂ is required per CO₂ produced.

Worked example

After control correction, a sample consumes 30 cm³ O₂ and produces 21 cm³ CO₂. RQ = 21/30 = 0.70, consistent with lipid as the main aerobic substrate; this is an inference, not an absolute identification.

Misconceptions to avoid

  • Cellular respiration is not breathing; breathing is ventilation, whereas respiration is a cellular reaction pathway that releases usable energy.
  • Germinating seeds both respire and grow; conditions and measurement interval must be stated when comparing rates.
  • KOH is hazardous and is used only to absorb CO₂; it must not contact the organisms.

Virtual experiment procedure: Respirometer and respiratory quotient

  1. Place equal masses of respiring organisms in the vessels and physically separate them from KOH.
  2. Place the system in a thermostatic bath, allow equilibration, then measure marker displacement over time with a non-respiring control.
  3. Correct with the control; compare CO₂-absorbing and suitable companion setups to infer O₂ and CO₂ before calculating RQ.

Safety and cautions

  • KOH is corrosive; real work requires eye protection, suitable gloves, a barrier and teacher-approved spill procedures.
  • Do not deprive vertebrates of oxygen; prefer germinating seeds and short measurement intervals.

Evidence to collect

  • A control-corrected time–displacement table with temperature, sample mass and units.
  • A rate or RQ calculation and a conclusion stating assumptions and limits of substrate inference.
Open the virtual experiment
📋 Embed this simulation in online lectures (For Teachers & LMS)

Copy the iframe code below to embed this interactive simulation into Canvas, Moodle, Google Sites or educational blogs:

<iframe src="https://visuallab.win/bio-lab/?experiment=respirometer" width="100%" height="560" frameborder="0" allow="fullscreen" loading="lazy"></iframe>

Diagnostic check

1. What is the role of O₂ in aerobic respiration?

  1. Final electron acceptor
  2. Carbon source for glucose
  3. Glycolysis enzyme
Show answer and explanation

Correct answer: Final electron acceptor

O₂ accepts electrons and protons at the end of the electron-transport chain to form water.

2. What is KOH used for in a respirometer?

  1. Absorb CO₂
  2. Produce O₂
  3. Supply glucose
Show answer and explanation

Correct answer: Absorb CO₂

Removing CO₂ allows gas decrease to reflect O₂ consumption.

3. Why is a thermostatic bath needed?

  1. Reduce gas-volume changes caused by temperature
  2. Make KOH harmless
  3. Remove the need for a control
Show answer and explanation

Correct answer: Reduce gas-volume changes caused by temperature

Temperature affects gas volume/pressure and enzyme-controlled rate.

Application practice

1. A sample produces 24 cm³ CO₂ and uses 24 cm³ O₂. What is its RQ?

  1. 1.00
  2. 0.50
  3. 24
Show answer and explanation

Correct answer: 1.00

RQ = 24/24 = 1.00, consistent mainly with carbohydrate under the stated conditions.

2. A vessel shows 12 cm³ O₂ uptake in 30 min using 4.0 g of seeds. What is the mass-specific rate?

  1. 0.10 cm³ g⁻¹ min⁻¹
  2. 1.0 cm³ g⁻¹ min⁻¹
  3. 90 cm³ g⁻¹ min⁻¹
Show answer and explanation

Correct answer: 0.10 cm³ g⁻¹ min⁻¹

12/(30×4.0) = 0.10 cm³ g⁻¹ min⁻¹.

3. If experimental and control vessels shift equally, what is the best conclusion?

  1. The shift may be background temperature or pressure change, not demonstrated O₂ uptake
  2. The sample definitely respires rapidly
  3. RQ is definitely 1
Show answer and explanation

Correct answer: The shift may be background temperature or pressure change, not demonstrated O₂ uptake

Matching control movement indicates the signal is not specific to respiration.

Academic references

  1. Cambridge IGCSE Biology 0610 — official syllabus and resources
  2. Cambridge AS & A Level Biology 9700 — official syllabus
  3. OpenStax Biology 2e — peer-reviewed foundational biology

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

Content publisher
WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer