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7.1.8Unit 3: Practical Skills in Biology I

Core Practical 16: Respirometer to Calculate RQ — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 25 of 27 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Respirometer Set-up

Organisms
  • Organisms Germinating seeds or small invertebrates are placed in a test tube because they respire actively.
Carbon dioxide absorber
  • Carbon dioxide absorber Soda lime or potassium hydroxide removes CO₂, so any volume change is due to oxygen uptake alone.
Manometer
  • Manometer A capillary tube with coloured fluid shows pressure change as the fluid moves toward the tube where gas volume falls.
Control tube
  • Control tube A matching tube containing glass beads of equal volume compensates for changes in temperature and air pressure.
Water bath
  • Water bath Keeps the temperature constant so gas volume changes come from respiration, not heating.

Part 2 of 5Procedure and Calculations

1
With soda lime present, run the apparatus for a fixed time and measure how far the fluid moves.
2
Convert the distance to volume using V = πr²h, where r is the capillary radius and h is the distance moved, then divide by time to get x in cm³ min⁻¹.
3
Reset the fluid and remove the soda lime from both tubes, so carbon dioxide stays in the gas space.
4
Repeat the run and calculate y, the net change in gas volume per minute, taking a decrease as negative.
5
Without soda lime the change is CO₂ produced minus O₂ used, so CO₂ produced = x + y and RQ = (x + y) ÷ x.

Part 3 of 5Interpreting Results

Interpreting Results
RQ of one
  • RQ of one Equal CO₂ output and O₂ uptake suggests that carbohydrate is the substrate.
RQ near 0.7
  • RQ near 0.7 Mostly lipid is respired, which is typical of oil-rich seeds as they germinate.
Changing RQ
  • Changing RQ A change in RQ over time or temperature shows that a different substrate is now being used.
Reliability
  • Reliability Repeat the readings, use the average and make sure the apparatus is leak-proof and equilibrated.

Part 4 of 5Worked Example

Question

  • In a respirometer run with soda lime, larvae used 3.0 cm³ of oxygen per minute (x).
  • Without soda lime the gas volume changed by −0.6 cm³ per minute (y).
  • Calculate the RQ.

  1. CO₂ produced = x + y = 3.0 + (−0.6) = 2.4 cm³ min⁻¹.
  2. RQ = CO₂ ÷ O₂ = 2.4 ÷ 3.0 = 0.80.

Answer RQ = 0.80, suggesting a mixture of carbohydrate and lipid.

Part 5 of 5Exam Checklist & Tips

7.1.8 – Key Points for the Exam

  • Respirometer Apparatus that measures gas volume changes caused by respiring organisms.
  • Soda lime A solid that absorbs carbon dioxide, used so that only oxygen uptake is measured.
  • Manometer A fluid-filled tube that shows pressure or volume change by movement of the fluid.
  • Net gas change The overall volume change when CO₂ produced is partly offset by O₂ taken in.

Common Mistakes

  • Forgetting to remove the soda lime for the second run.
  • Taking the fluid movement as the volume without using V = πr²h.
  • Not equalising the control tube volume, which allows temperature and pressure changes to affect results.
  • Using the wrong sign for y; a fall in gas volume is negative.

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