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Gas exchange · respirometer data
Respiration measurements and RQ
Calculate respiratory quotient, volume changes in a capillary and respiration rates per gram from supplied measurements and assumptions.
Method and worked examples ↓Learn the method
How to approach these questions
- Use matching volume units for CO₂ and O₂ before finding RQ. RQ has no unit.
- A capillary radius is half its diameter. Squaring a diameter instead of a radius gives four times the correct area.
- Divide volume change by elapsed time to find a rate.
- To compare different masses of organisms, divide the rate by mass as well. Account for temperature, pressure and apparatus effects before attributing a movement entirely to respiration.
Worked examples
A sample produces 1.4 cm³ CO₂ while consuming 2.0 cm³ O₂. Calculate RQ.
RQ = 1.4 ÷ 2.0 = 0.70. Interpreting a substrate requires the experimental assumptions; this ratio alone is not proof of a particular fuel.
A capillary radius is 0.5 mm. Fluid moves 8 mm in 2 min. Use π = 3.142 to find the volume change per minute.
Volume = 3.142 × 0.5² × 8 = 6.284 mm³. Rate = 6.284 ÷ 2 = 3.14 mm³ min⁻¹ to 3 significant figures.
Keep practising
Topic map and study list · Practical data guides · Active recall decks
Course resources
AQA A-level Biology · OCR A A-level Biology · Textbook answers
Skill coverage follows the common A-level science subject content. Check your board’s current requirements: AQA 7402 and OCR A H420.