What you need to know
- 7.6respiratory quotientRQ = CO₂ produced ÷ O₂ used. It is about 1.0 for carbohydrate, 0.7 for lipid and 0.9 for protein. is carbon dioxide produced divided by oxygen consumed.
Typical values 1, 0.7 and 0.9 are teaching comparisons for carbohydrate, lipid and protein, not measurements from a named experiment. Protein is rarely the main fuel. A ratio is not a significance test. Core Practical 16 collects the volumes.
By the end, you should be able to…
- Calculate RQ from volumes or moles of the two gases.
- Compare typical carbohydrate and lipid values using the chemistry, not a slogan.
- Refuse a claim that one RQ proves the only substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction..
Build your understanding
A ratio, not a rate
respiratory quotientRQ = CO₂ produced ÷ O₂ used. It is about 1.0 for carbohydrate, 0.7 for lipid and 0.9 for protein., RQ, is the carbon dioxide produced divided by the oxygen consumed in the same time. Volumes may be used if temperature and pressure are the same, because equal volumes of those gases then contain equal numbers of molecules. Moles work directly. RQ has no unit if both gases are in the same unit.
RQ is not the rate of respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP.. Rate is oxygen used, or carbon dioxide produced, per unit time, often per gram. Two samples can share an RQ and still respire at different speeds. Do not divide the gases the wrong way up: oxygen on the bottom.
Why carbohydrate and lipid differ
For glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate., C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, so RQ = 6/6 = 1. A carbohydrate is already partly oxidised. A typical lipid is more reduced: more of the oxygen consumed goes into forming water from hydrogen, and less carbon dioxide is released per oxygen used. A typical lipid RQ is about 0.7. That 0.7 is a teaching value for a fat such as a triglycerideA lipid made of one glycerol and three fatty acids joined by ester bonds. It is an energy store., not a result you measured.
Protein gives a teaching value around 0.9, but amino-acid breakdown also involves nitrogen excretion, and protein is not usually the main respiratory substrateAn organic molecule that is broken down in respiration to release energy, such as glucose, fatty acids or amino acids.. A value between 0.7 and 1 is consistent with a mixture. It does not prove that only one fuel was used, and two volumes are not a statistical test.
When the number misleads
If a tissue or a yeast culture releases carbon dioxide by an anaerobic route while using little oxygen, RQ can rise above 1. That is not “extra carbohydrate”. Read the conditions before you name a substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction..
A worked illustrative pair: in 10 minutes a seed sample used 8.0 cm³ of oxygen and produced 5.6 cm³ of carbon dioxide. RQ = 5.6/8.0 = 0.7. That is consistent with lipid being an important substrate in that run. It is invented teaching data. It does not prove lipid was the only substrate, and it says nothing about a second jar unless you are given one.
Follow the process
If you are given a molecular formula, balance the aerobic equation first → the coefficient of CO₂ is the molecules produced and the coefficient of O₂ is the molecules consumed → RQ = CO₂ produced ÷ O₂ consumed. Equal volumes of those gases contain equal numbers of molecules at the same temperature and pressure, so measured volumes can replace the coefficients. Glucose is 6/6 = 1. A more reduced fuel uses more oxygen relative to the carbon dioxide released, so the RQ is below 1.
Worked substrate calculation, not a respirometer reading: linoleic acid, C18H32O₂ + 25O₂ → 18CO₂ + 16H₂O, so RQ = 18/25 = 0.72. Balance before you divide. If a measured RQ is above 1, check for anaerobic carbon dioxide before you invent a new fuel. A volume per minute is a rate, not an RQ.
NOT: one ratio does not prove a single substrate, and it is not a significance test.
- Same time and the same units
- Carbohydrate about 1
- Lipid about 0.7
- A mixture can sit between them
Two traps this diagram blocks
- Do not treat an RQ as a diagnosis of a diet.
- Do not invert the ratio.
From the diagram alone
Question
A tissue has an RQ of 0.85. What can you conclude, and what must you not conclude? [3]
Show a model answer
The value is between the usual figures for lipid and carbohydrate, so both substrates may be in use. It does not prove that only one substrate is being respired, and it is not a statistical test.
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: the RQ is 0.7. Y: more oxygen was consumed than carbon dioxide produced. Z: that fits a lipid better than glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate., if the conditions were aerobic. Incomplete: “The organism is respiring slowly.” Improved: “RQ is a ratio, not a rate. 0.7 means the gas volumes fit a lipid; the speed is the volume per minute.”
Write the division
CO₂ on top. If you get a number above about 1, check for anaerobic carbon dioxide before you invent a new substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction..
Keep rate separate
cm³ per minute is a rate. A bare 0.7 is an RQ.
Key explanations
- The two gas volumes must be measured over the same time, in the same units.
Avoid this misconception
The claim: An RQ of 0.7 proves the organism was respiring only lipid, and the result is statistically significant.
The correction: 0.7 is consistent with lipid-rich respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. in that interval. Other substrates can contribute. One ratio has no significance test attached.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Define RQ.
Show suggested answer
Carbon dioxide produced divided by oxygen consumed, over the same time.
Practice question
Retrieval and explanation · suggested answer
Calculate RQ if 12 cm³ of oxygen is used and 12 cm³ of carbon dioxide is produced.
Show suggested answer
12/12 = 1, which matches carbohydrate as a typical value.
Practice question
Retrieval and explanation · suggested answer
Why is a lipid RQ lower than a carbohydrate RQ?
Show suggested answer
Lipids are more reduced, so more oxygen is used relative to the carbon dioxide released. A typical teaching value is about 0.7, compared with 1 for glucose.
Practice question
Retrieval and explanation · suggested answer
A yeast culture has RQ 1.4. Give one cautious interpretation.
Show suggested answer
Carbon dioxide production exceeded oxygen use, which can happen if anaerobic ethanol fermentation is releasing extra CO₂. It is not automatically “more than carbohydrate”.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Illustrative data: jar A uses 10 cm³ oxygen and produces 9 cm³ carbon dioxide. Jar B uses 4 cm³ oxygen and produces 3.6 cm³ carbon dioxide. A student says B has the lower RQ and therefore respired less lipid. Check the arithmetic and the claim.
Show a hint
Divide before you compare the jars. Then look at rates.
Explore the reasoning
RQ of A is 0.9. RQ of B is 3.6/4 = 0.9. The quotients match. B used less oxygen, so its rate was lower if the time and mass were the same, but that is not a different substrate. Neither value is a significance test, and 0.9 does not name a single fuel.
Recall this lesson
Lesson recall questions
Question 1 of 5: RQ = volume of ________ produced / volume of ________ consumed.
Respiratory quotient
Practice question
Retrieval and explanation · suggested answer
RQ = volume of ________ produced / volume of ________ consumed.
Lesson: 7A 5: Respiratory Substrates and Respiratory Quotient
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Yeast is a cell with a metabolism, not an RQ chart
7A · Respiratory Substrates and Respiratory QuotientThe Microbiology Society’s introduction treats microorganisms as living things you can investigate. It does not publish a table of respiratory quotients. Yeast can be the material in a…Specification 7.6Read & use the scienceClose article
Why this matters
The Microbiology Society’s introduction treats microorganisms as living things you can investigate. It does not publish a table of respiratory quotients. Yeast can be the material in a respirometer or a dye tube. Any RQ still has to be calculated from the two gases you measured. 7.6: RQ is CO₂ produced divided by O₂ consumed. The organism only supplies the gases. A measured gas or ATP change cannot, on its own, identify every intermediate enzyme or show which step changed.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.6 Respiratory quotient and the gases used to calculate it.
Use the science
Why does “it is yeast, so the RQ is 1” fail?
Explore a possible response
The substrate and whether oxygen is available change the gases. Yeast on glucose in air can be near 1. The same yeast fermenting, or a seed using lipid, will not match that assumption. You calculate the ratio from data.
Explore the source
- Microbiology Society: what is microbiologyMicrobiology Society
A fuel leaves a signature in two gases
7A · Respiratory Substrates and Respiratory QuotientIndirect calorimetry compares oxygen consumed with carbon dioxide produced to investigate fuel use. The respiratory quotient is the carbon dioxide volume divided by the oxygen volume, so a…Specification 7.6Read & use the scienceClose article
Why this matters
Indirect calorimetry compares oxygen consumed with carbon dioxide produced to investigate fuel use. The respiratory quotient is the carbon dioxide volume divided by the oxygen volume, so a single gas reading is not enough. The NCBI physiology review explains why carbohydrate and lipid oxidation yield different ratios, but a person’s measured breath ratio can also be altered by non-steady exercise and ventilation. That makes an RQ a useful clue rather than a perfect label saying “only fat” or “only carbohydrate”. Laboratory physiologists must match the measurement period and conditions before interpreting a change. The classroom calculation is the first step in that reasoning.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.6 Respiratory quotient and the gases used to calculate it.
Use the science
Two equal-length trials have the same oxygen uptake but different carbon dioxide output. Which has the higher RQ, and what cannot be concluded?
Explore a possible response
The trial with more carbon dioxide has the higher ratio. That alone does not prove exclusive use of one fuel, because mixed substrates and measurement conditions can affect the observed gases.
Explore the source
- Physiology of respiratory quotientNCBI Bookshelf