Chapters
In this article, we will discuss the respiratory quotient in detail. So, let us get started.
What is a Respiratory Quotient?
A respiratory quotient refers to the ratio of carbon dioxide molecules produced to the oxygen molecules consumed during the process of respiration.
The formula for respiratory quotient (RQ) is:
The respiratory quotient is not only used to determine the kind of respiratory substrate that an organism is respiring, but also helps to identify the type of respiration. Since RQ is the ratio, therefore, it has no unit.
RQ Values of Various Respiratory Substrates
Each respiratory substrate, for instance, proteins, lipids, and carbohydrates have varying RQ values. This is because the number of carbon-hydrogen bonds is different in each type of biological molecule.
- If there are more carbon-hydrogen bonds, then a greater number of hydrogen atoms can be employed to create a proton gradient.
- More hydrogen atoms tell us that more ATP molecules can be produced. Hence, more oxygen is needed to break down the molecule in the final stage of oxidative phosphorylation to create water.
When an organism respires glucose aerobically, then the same amounts of carbon dioxide are produced to oxygen consumed. It means that the RQ value in such a situation is equal to 1.
RQ of Glucose
The RQ values of carbohydrates, lipid, and protein are listed below:
- The RQ value of carbohydrates is 1.0
- The RQ value of lipid is 0.7
- The RQ value of protein is 0.9
How is RQ Calculated?
The RQ value is computed from equations of respiration. It includes comparing ratios of carbon dioxide produced to oxygen consumed. The following equation is used to calculate RQ value of a respiratory substrate:
To calculate the RQ value of the given substrate, you should know the molecular formula of the substrate being respired by the organism. After that, you can create a balanced equation to compute the RQ value.
In a balanced equation, the number before the chemical formula can be considered as the number of moles or molecules of that specific compound. This is because the equal number of molecules of any gas takes up an equal volume. For instance, 12 molecules of carbon dioxide will take up an equal volume as 12 molecules of oxygen.
The ratio of glucose is simple 1:1. As discussed above, the RQ value of glucose is 1, however, other substrates have complicated ratios which result in varying RQ values.
In the next section of the article, we will calculate the RQ value for a lipid.
RQ value for a lipid – Example
The molecular formula for the linoleic acid is 
. Linoleic acid is the fatty acid present in nuts. Follow these steps to determine the RQ value of linoleic acid.
Step 1 : Determine the equation of respiration
The respiration equation of the linoleic acid is:
Step 2 – Balance the equation in step 1
Step 3 : Use the RQ formula to determine the respiratory quotient
RQ for Anaerobic Respiration
Anaerobic respiration is a type of respiration that does not require oxygen, however, it produces a tiny amount of ATP. Depending on the organism, anaerobic respiration in cells can take place through lactate or ethanol fermentation.
- Muscle cells in mammals employ lactate fermentation
- Tissue cells in plants and yeast use ethanol fermentation
For anaerobic respiration, we cannot calculate the RQ value because no oxygen is consumed, and no carbon dioxide is produced during lactate fermentation. The respiratory quotient for yeast cells tends towards infinity because no oxygen is consumed, but carbon dioxide is produced.
Investigating Respiratory Quotients Using Respirometers
Respirometers are employed to measure and investigate the rate of oxygen consumed in the respiring organism. These instruments can also be used to compute respiratory quotients. The experiments usually include organisms such as germinating seeds or invertebrates.
- 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
We can work out the volume of oxygen taken in by using the diameter of the capillary tube r (cm) and the distance covered by the manometer fluid h (cm) using the following formula:
Use a respirometer to calculate the respiratory quotient
Method
- To measure the oxygen consumption, set up the respirometer and experiment with a soda-lime present in both tubes. To determine the change in gas volume within a specific time, use a manometer,
. - Reset the apparatus so that air can enter the tubes again through a screw cap and reset the manometer fluid by using a syringe
- Run the experiment again by removing the soda-lime from both the tubes and employ a manometer reading to determine the change in gas volume in a specific time,
.
Calculations
- x depicts the oxygen volume taken in by respiration in a given time
- y depicts the difference between oxygen volume consumed by respiration in a specific time and the volume of carbon dioxide produced in that time.
The value of y can be positive or negative depending on the direction of movement of the manometer fluid (up shows the positive value and down shows the negative value)
- Both the measurements x and y can be used to determine RQ
Determining RQ value from a respirometer experiment example
Suppose, x = 
y = 
If the same volumes of oxygen are taken in and carbon dioxide is produced as in the glucose, then the manometer fluid will not move, and y will be zero.
RQ of Germinating Seeds
Use a simple respirometer to calculate the RQ of germinating seeds:
- All the carbon dioxide is absorbed by the sodium hydroxide in the apparatus from the start
- We can use potassium hydroxide instead of sodium hydroxide as they both absorb carbon dioxide
- As the germinating seeds employ oxygen and pressure falls in tube A so the manometer level closest to the seeds rises
- Sodium hydroxide absorbs any carbon dioxide excreted
- Use the syringe to return the manometer fluid to the normal
- Calculate the volume of oxygen used by measuring the gas volume required from the syringe to return the levels to the previous (original) values.
If sodium hydroxide is replaced by water, then the evolved carbon dioxide can be measured.
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