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7.1.6Unit 6: Practical Skills in Biology II

Respiratory Quotient — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 22 of 48 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Part 1 of 5What RQ Measures

Definition
  • Definition RQ is the ratio of carbon dioxide produced to oxygen consumed by an organism in the same period of time.
Formula
  • Formula RQ = volume (or moles) of CO₂ produced ÷ volume (or moles) of O₂ taken in, and it has no units.
Why moles work
  • Why moles work Equal numbers of gas molecules occupy equal volumes, so the coefficients in a balanced equation give the ratio directly.
Link to substrate
  • Link to substrate Different molecules contain different proportions of hydrogen and oxygen, so each needs a different amount of oxygen.
Practical use
  • Practical use RQ lets us infer the fuel in use without analysing the tissues themselves.

Part 2 of 5Typical Values and Their Meaning

Typical Values and Their Meaning
Carbohydrate
  • Carbohydrate Glucose gives an RQ of 1.0 because six CO₂ are released for each six O₂ used.
Lipid
  • Lipid Fats contain proportionally more hydrogen, so more oxygen is needed and the RQ is about 0.7.
Protein
  • Protein Protein respiration gives an RQ of about 0.9, between lipid and carbohydrate.
Mixed fuels
  • Mixed fuels A value between 0.7 and 1.0, such as 0.85, suggests that carbohydrate and lipid are being used together.
Above 1.0
  • Above 1.0 More CO₂ is released than O₂ taken in, which indicates anaerobic respiration or oversupply of carbohydrate.
SubstrateTypical RQ
Carbohydrate1.0
Protein0.9
Lipid0.7

Part 3 of 5Calculating RQ From an Equation

1
Write the molecular formula of the substrate and an unbalanced equation with oxygen on the left and carbon dioxide and water on the right.
2
Balance carbon first by setting the number of CO₂ equal to the number of carbon atoms in the substrate.
3
Balance hydrogen by giving water half as many molecules as there are hydrogen atoms in the substrate.
4
Count oxygen atoms on the right, subtract those already in the substrate, and halve the result to find the number of O₂ molecules needed.
5
Divide the coefficient of CO₂ by the coefficient of O₂ to obtain the RQ.

Part 4 of 5Worked Example

Question

  • Palmitic acid has the formula C₁₆H₃₂O₂.
  • Calculate the RQ when it is fully respired aerobically.

  1. UnbalancedC₁₆H₃₂O₂ + O₂ → CO₂ + H₂O.
  2. Carbon
    • 16 CO₂.
    • Hydrogen: 32 H atoms give 16 H₂O.
  3. Oxygen on the right = 32 + 16 = 48 atoms; the acid supplies 2, so 46 come from O₂, which is 23 O₂.
  4. BalancedC₁₆H₃₂O₂ + 23O₂ → 16CO₂ + 16H₂O.
  5. RQ = 16 ÷ 23 = 0.696.

Answer RQ = 0.70 (to two significant figures), typical of a lipid.

Part 5 of 5Exam Checklist & Tips

7.1.6 – Key Points for the Exam

  • Respiratory quotient Carbon dioxide produced divided by oxygen consumed during respiration.
  • Respiratory substrate The molecule, such as glucose or a fatty acid, that is broken down to release energy.
  • Anaerobic respiration Release of energy without oxygen, which can give a very high or undefined RQ.
  • Balanced equation An equation with equal numbers of each atom on both sides, required before calculating RQ.

Common Mistakes

  • Calculating RQ from an unbalanced equation, which gives a wrong ratio.
  • Inverting the formula to O₂ ÷ CO₂, which gives the reciprocal.
  • Giving RQ units; it is a ratio of two volumes so it has none.
  • Assuming a value of 0.85 means a single fuel when it indicates a mixture of substrates.

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