5.7.13 Respiratory Quotient (RQ)

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

RQ = CO₂ produced / O₂ consumed

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

RQ Table

• These values can be compared to experimental data to determine what substances is being used for respiration

Calculating the RQ

• 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

Calculating the RQ for anaerobic respiration

• Anaerobic respiration is respiration that takes place without oxygen but still produces 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

Investigating RQs using respirometers

• Respirometers are used to measure and investigate the rate of oxygen consumption during respiration in organisms
• They can also be used to calculate respiratory quotients
• The experiments usually involve organisms such as seeds or invertebrates
• The manometer fluid moves according to how much oxygen has been consumed  and carbon dioxide produced

The typical set-up of a respirometer

Equation for calculating change in gas volume

• The volume of oxygen consumed (cm³ min^-1) can be worked out using the diameter of the capillary tube r (cm) and the distance moved by the manometer fluid h (cm) in a minute using the formula:

πr²h

Method

• Measure oxygen consumption
  • Set up the respirometer and run the experiment with soda-lime present in both tubes
  • Use the manometer reading to calculate the change in gas volume within a given time, x cm³ min^-1; this is the oxygen consumption
• Reset the apparatus:
  • Allow air to re-enter the tubes via the screw cap and reset the manometer fluid using the syringe
• Run the experiment again: 
  • Remove the soda-lime from both tubes 
  • The manometer reading this time will be influenced by give you both oxygen consumed and carbon dioxide produced, y cm³ min^-1
    • Note respiring different substrates leads to less carbon dioxide being produced as carbon atoms may end up in other compounds that are not carbon dioxide. Less carbon dioxide is given off by the respiring organism, than oxygen consumed so pressure will drop and the liquid in the manometer will move toward the organism, y cm³ min^-1
  • Use the manometer readings to calculate the difference between reading 1 and reading 2, giving you carbon dioxide produced, x - y cm³ min^-1
• Note that when the volumes for each x and y are the same, the level of manometer fluid will not change and y will be 0, making the RQ 1

Calculations

• x tells us the volume of oxygen consumed by respiration within a given time
• y tells us the volume of oxygen consumed by respiration and carbon dioxide produced by respiration within a given time (total volume of gas produced, minus oxygen consumed, x, within a given time)
• The two measurements x and y can be used to calculate the RQ

RQ Equation for Respirometer experiment.

Analysis

• Respirometers can be used in experiments to investigate how different factors affect the RQ of organisms over time
  • E.g. temperature – using a series of water baths

• When an RQ value changes it means the substrate being respired has changed
• Some cells may also be using a mixture of substrates in respiration e.g. An RQ value of 0.85 suggests both carbohydrates and lipids are being used
  • This is because the RQ of glucose is 1 and the RQ of lipids is 0.7

• Under normal cell conditions the order in which substrates are used in respiration: carbohydrates, lipids then proteins
• The RQ can also give an indication of under or overfeeding:
  • An RQ value of more than 1 suggests excessive carbohydrate/calorie intake
  • An RQ value of less than 0.7 suggests underfeeding