Respiratory Quotient (Edexcel International A Level Biology)

• The respiratory quotient (RQ) is the ratio of carbon dioxide molecules produced to oxygen molecules taken in during respiration

RQ = CO₂ ÷ O₂

The formula for the Respiratory Quotient

RQ values of different respiratory substrates

• Carbohydrates, lipids and proteins have different typical RQ values
• This is because of the number of carbon-hydrogen bonds differs in each type of biological molecule
  • More carbon-hydrogen bonds means that more hydrogen atoms can be used to create a proton gradient
  • More hydrogens means that more ATP molecules can be produced
  • More oxygen is therefore required to breakdown the molecule (in the last step of oxidative phosphorylation to form water)
• When glucose is aerobically respired equal amounts of carbon dioxide are produced to oxygen taken in, meaning it has an RQ value of 1

During the breakdown of glucose, six molecules of oxygen is used and six molecules of carbon dioxide is produced. This results in an RQ value of 1 (6CO₂ ÷ 6O₂)

RQ Values For Different Respiratory Substrates Table

• Some questions may ask you to suggest what substrate is being respired during an experiment based on the RQ value – so make yourself familiar with the values in the table

Calculating RQ values

• The respiratory quotient is calculated from respiration equations
• It involves comparing the ratios of carbon dioxide given out to oxygen taken in
• The formula for this is:

Equation to calculate the RQ value of a respiratory substrate

• For aerobic respiration the RQ will typically be less than 1 since oxygen is being used to break down the substrate 
• Should the RQ of an organism be greater than 1, it may indicate that anaerobic respiration is occurring, since very little to no oxygen is used
• If you know the molecular formula of the substrate being aerobically respired then you can create a balanced equation to calculate the RQ value
• In a balanced equation the number before the chemical formula can be taken as the number of molecules/moles of that compound
  • This is because the same number of molecules of any gas take up the same volume e.g. 12 molecules of carbon dioxide take up the same volume as 12 molecules of oxygen
• Glucose has a simple 1:1 ratio and RQ value of 1 but other substrates have more complex ratios leading to different RQ values

Step 1: Write down the respiration equation

C₁₈H₃₂O₂ + O₂ → CO₂ + H₂O

Step 2: Balance the equation 

C₁₈H₃₂O₂ + 25O₂ → 18CO₂ + 16H₂O

Step 3: Use the RQ formula

RQ = CO₂ ÷ O₂

RQ = 18 ÷ 25

RQ = 0.72

Step 1: Write down the respiration equation

C₆H₁₂O₆ → C₂H₅OH + CO₂

Step 2: Balance the equation

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Step 3: Use the RQ formula

RQ = CO₂ ÷ O₂

RQ = 2 ÷ 0

RQ = ∞ (infinity)