5.7.13 Respiratory Quotient Flashcards
What is 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
Why do different respiratory substrates have different RQ’s?
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
What is glucose RQ?
Look at sme page
How do you calculate RQ?
The respiratory quotient is calculated from respiration equations
It involves comparing the ratios of carbon dioxide given out to oxygen taken in
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
How do you calculate 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
How can you investigate RQ 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
What is the 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
What is the method for investigating RQ using different respiratory substrates?
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
Look at sme page
What is the analysis for the method which investigates RQ using different respiratory substrates?
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
