Respiratory Quotient (RQ) (Cambridge International A-Level Biology)

Spec point — 12.1.5 · state that the respiratory quotient (RQ) is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in, as a result of respiration

Respiratory quotient (RQ)

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

$\mathrm{RQ}=\frac{\mathrm{CO}_{2}}{O_{2}}$

The formula for the Respiratory Quotient

RQ values of different respiratory substrates

• Carbohydrates, lipids and proteins have different typical RQ values

• This is because the number of carbon-hydrogen bonds differs in each type of biological molecule

  • A higher number of 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 by chemiosmosis
  • More oxygen is therefore required to break down the molecule (in the last step of oxidative phosphorylation to form water)

• When glucose is respired aerobically, equal volumes of carbon dioxide are produced and oxygen taken in, meaning it has an RQ value of 1

Exam Hint 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 RQs

Spec point — 12.1.6 · calculate RQ values of different respiratory substrates from equations for respiration

• 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

• 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

RQ for a lipid

Linoleic acid (a fatty acid found in nuts) has the molecular formula C₁₈H₃₂O₂

Step 1: Create a respiration equation

C₁₈H₃₂O₂ + O₂→ CO₂ + H₂O

Step 2: Balance the equation

C × 18 C × 1

H × 32 H × 2

O × 4 O × 3

Step 3: Create the full equation

C₁₈H₃₂O₂ + 25O₂→ 18CO₂ + 16H₂O

Step 3: Use the RQ formula

CO_ 2 / (straight O)_ 2=RQ

$\frac{18}{25}=0.72$

Calculating the RQ for anaerobic respiration

• Anaerobic respiration is respiration that takes place without oxygen but does produce a small amount of ATP

• Depending on the organism anaerobic respiration in cells can be done via lactate or ethanol fermentation

  • Mammalian muscle cells use lactate fermentation
  • Plant tissue cells and yeast use ethanol fermentation

• The RQ cannot be calculated for anaerobic respiration in muscle cells because no oxygen is used and no carbon dioxide is produced during lactate fermentation

• For yeast cells the RQ tends towards infinity as no oxygen is used while carbon dioxide is still being produced

Worked Example

RQ for Anaerobic Respiration

Ethanol fermentation in lettuce roots

glucose → ethanol + carbon dioxide (+ energy)

Step 1: Create the respiration equation

C₆H₁₂O₆→ C₂H₅OH + CO₂(+ energy)

Step 2: Balance the equation

C₆H₁₂O₆→ 2C₂H₅OH + 2CO₂+ energy

Step 3: Use the RQ formula

CO_ 2 / (straight O)_ 2=RQ

2 / 0=∞ (∞)

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