What you need to know
- 7.1–7.6Original mixed practice on aerobic respirationRespiration using oxygen: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. It releases much more ATP than anaerobic respiration., glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD., lactateThe product of anaerobic respiration in animals, formed when pyruvate is reduced by reduced NAD., Krebs products, chemiosmosisMaking ATP as H⁺ ions diffuse back down their gradient through ATP synthase. and RQ.
No mark allocations. Illustrative volumes are invented. Enzyme names and Krebs-acid names stay out.
By the end, you should be able to…
- Place each stage in the cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles. or the mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place..
- Separate substrate-level ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. from oxidative phosphorylationMaking ATP using energy from electrons passed along the electron transport chain, with oxygen as the final electron acceptor..
- Calculate an RQ and limit the claim.
Build your understanding
Order the pathway
A safe sequence is: glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD. in the cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles.; link reactionThe stage in the mitochondrial matrix in which pyruvate is decarboxylated and dehydrogenated and joined to coenzyme A to form acetyl coenzyme A. and Krebs cycleA cycle of reactions in the mitochondrial matrix in which acetyl groups are broken down, releasing CO₂ and making reduced NAD, reduced FAD and ATP. in the matrix; electron transport and ATP synthaseThe enzyme in the inner mitochondrial membrane and thylakoid membrane that makes ATP as H⁺ ions flow through it. on the inner membrane. Carbon dioxide appears in the link reaction and the cycle. Water appears at the end of the chain. lactateThe product of anaerobic respiration in animals, formed when pyruvate is reduced by reduced NAD. appears only when animal cells reoxidise NAD without oxygen.
Numbers you may use
Net two ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. per glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. in glycolysis. One substrate-level ATP per Krebs turn. No required grand total. RQ = CO₂/O₂. Illustrative example: 7.0 cm³ CO₂ and 10.0 cm³ O₂ gives RQ 0.7, consistent with lipid, not proof of only lipid.
Make your reasoning visible
Location first
Many wrong answers put the Krebs cycleA cycle of reactions in the mitochondrial matrix in which acetyl groups are broken down, releasing CO₂ and making reduced NAD, reduced FAD and ATP. in the cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles..
Avoid this misconception
The claim: Every stage uses oxygen and every stage makes the same amount of ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi..
The correction: Oxygen is the final electron acceptor. ATP yield is small in glycolysisThe first stage of respiration, in the cytoplasm: glucose is phosphorylated and split into two pyruvate, giving a net 2 ATP and 2 reduced NAD. and the cycle, and large only when the proton gradient drives ATP synthaseThe enzyme in the inner mitochondrial membrane and thylakoid membrane that makes ATP as H⁺ ions flow through it..
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Name the only 7A stage that occurs in the cytoplasm.
Show suggested answer
Glycolysis.
Practice question
Retrieval and explanation · suggested answer
State two products of the link reaction.
Show suggested answer
Carbon dioxide and reduced NAD. The two-carbon group is passed to coenzyme A.
Practice question
Retrieval and explanation · suggested answer
Give one role of oxygen in aerobic respiration.
Show suggested answer
It is the final electron acceptor on the electron transport chain and is reduced to water.
Practice question
Retrieval and explanation · suggested answer
Illustrative: 4.5 cm³ CO₂ produced, 5.0 cm³ O₂ used. Calculate RQ and comment.
Show suggested answer
RQ = 0.9. That sits between typical lipid and carbohydrate values, so a mixture is possible. It does not prove the substrate or a significant difference from another sample.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A cell poison blocks ATP synthase but the inner membrane stays intact and oxygen is still present. Suggest what happens to the proton gradient and to carbon dioxide production after a short time. State the assumption.
Show a hint
If protons cannot return, the gradient builds until pumping becomes difficult.
Explore the reasoning
Assume the carriers cannot keep pumping against a stalled gradient. Electron transport slows, oxygen use falls, and reduced coenzymes are not reoxidised, so the Krebs cycle and carbon dioxide production fall. The assumption is that a maximum gradient stops further pumping. You have not been given a named poison to memorise.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare the overall products of aerobic respiration with simply naming ‘glucose burning’.
Aerobic respiration overview
Practice question
Retrieval and explanation · suggested answer
Compare the overall products of aerobic respiration with simply naming ‘glucose burning’.
Lesson: 7A 1: Respiration in Cells
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Same gases, different process
7A · Exam PracticeNHLBI’s lung pages describe oxygen entering the blood and carbon dioxide leaving. Use that only as the body-level movement of the gases that 7A produces and consumes. It…Specification 7.6Read & use the scienceClose article
Why this matters
NHLBI’s lung pages describe oxygen entering the blood and carbon dioxide leaving. Use that only as the body-level movement of the gases that 7A produces and consumes. It is not a mark scheme for chemiosmosis. 7.1–7.6: the cell pathway, not ventilation. A measured gas or ATP change cannot, on its own, identify every intermediate enzyme or show which step changed. This is an extension to the lesson, not another fact to memorise.
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
Which gas volume would you still need, besides carbon dioxide, before an RQ could be calculated from a breathing experiment?
Explore a possible response
Oxygen consumed in the same period. Carbon dioxide alone is not an RQ.
Explore the source
- NHLBI: how the lungs workUS National Heart, Lung, and Blood Institute
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