Respiratory Quotient (Edexcel International A Level Biology)

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Respiratory Quotient

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

RQ = CO2 ÷ O2

RQ Equation

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

Glucose RQ

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

RQ 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:

RQ Maths Equation

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

Worked example

Linoleic acid (a fatty acid found in nuts) has the molecular formula C18H32O2. Calculate the RQ value of this lipid.

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

Worked example

During ethanol fermentation in lettuce roots, glucose is partially broken down. This process is represented by the following equation:

glucose → ethanol + carbon dioxide

Calculate the RQ value.

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)

Exam Tip

Make sure the respiration equation you are working with is fully balanced before you start doing any calculations to find out the RQ value.

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.

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