Compare substrates and calculate respiratory quotient
Compare the energy values of respiratory substrates and calculate respiratory quotient from equations for respiration.
Section 12.1 · Energy
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- ATP transfers energy for cellular work, and enzymes control metabolic reactions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which substrate releases more energy per gram: lipid or carbohydrate?
What you will learn
- Compare carbohydrate, lipid and protein energy values
- Define and calculate respiratory quotient (RQ)
Build the explanation
Lipids release more energy per unit mass than carbohydrates or proteins because their molecules are more reduced and contain many energy-rich C–H bonds. Carbohydrates and proteins have lower and broadly similar energy values per gram; exact values depend on the substrate and convention used. Respiratory quotient (RQ) is the ratio of carbon dioxide molecules produced to oxygen molecules taken in during respiration: RQ = CO₂ produced ÷ O₂ consumed. Use the balanced equation to identify the gas coefficients.
Apply it carefully
RQ is a ratio of molecule amounts, not a mass ratio. Use coefficients only after balancing the equation, and keep numerator and denominator in the correct order. An RQ of 1 is expected for a carbohydrate such as glucose oxidised completely; mixed substrates or incomplete conditions can alter measured values.
Pause and explain
For glucose respiration, what is the RQ when equal numbers of CO₂ and O₂ molecules appear in the equation?
Worked example
For C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, calculate RQ.
Show the worked solution
- Take CO₂ produced divided by O₂ consumed.
- Use the coefficients for the gases.
Answer RQ = 6 ÷ 6 = 1.0.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why lipids have a higher energy value per gram than carbohydrates, then define RQ.
Give me a hint
Use relative reduction for the first part and a gas ratio for the second.
Compare my reasoning
- Lipids are more reduced and contain more energy-rich C–H bonds per unit mass.
- RQ = molecules of CO₂ produced ÷ molecules of O₂ consumed.
Lipids generally release more energy per unit mass than carbohydrates because they are more reduced and contain more energy-rich C–H bonds. RQ is CO₂ produced divided by O₂ consumed during respiration.
Look for these in your work
- Relative energy value correct
- Chemical reason given
- RQ order correct
A substrate is respired according to 2C₃H₆O₃ + 6O₂ → 6CO₂ + 6H₂O. Calculate RQ.
Give me a hint
Use the balanced gas coefficients.
Compare my reasoning
- CO₂ produced = 6 molecules.
- O₂ consumed = 6 molecules; 6 ÷ 6 = 1.0.
RQ = CO₂ produced ÷ O₂ consumed = 6 ÷ 6 = 1.0.
Look for these in your work
- Numerator correct
- Denominator correct
- Ratio correct
A balanced substrate equation has 4CO₂ and 5O₂. Calculate RQ and explain what an RQ below 1 means mathematically.
Give me a hint
Calculate the ratio first; do not infer a unique substrate from a number alone.
Compare my reasoning
- RQ = 4 ÷ 5.
- RQ = 0.80, so fewer CO₂ molecules are produced than O₂ molecules consumed.
RQ = 4/5 = 0.80. This means that, for the equation or measurements supplied, fewer carbon dioxide molecules are produced than oxygen molecules are consumed. It does not by itself identify a unique respiratory substrate.
Look for these in your work
- Calculation correct
- Meaning of ratio stated
- No unsupported substrate identification
Common mistakes
- Reversing CO₂ and O₂ in the RQ ratio
- Using masses instead of molecule amounts
- Treating one RQ value as proof of a unique substrate
What can you explain now?
Which substrate releases more energy per gram: lipid or carbohydrate?
Compare with the explanation
Lipid
Lipids are more reduced and have more energy-rich carbon–hydrogen bonds per unit mass.
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Make it stick
How do you calculate RQ, and why are lipids high-energy respiratory substrates?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.