Cellular Respiration Revision Notes

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Chartered Biologist (CBiol MRSB)

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Former WJEC & Edexcel Examiner

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25+ Years Teaching

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Overview Key Terms Glycolysis Link Reaction Krebs Cycle Oxidative Phosphorylation ATP Yield & Anaerobic Other Respiratory Substrates & Exam Boards Common Mistakes FAQ

Why Cellular Respiration Trips Up So Many Students

Cellular respiration is, alongside photosynthesis, the topic where the most marks are won and lost in A2 Biology. It has four stages, each in a specific location, each producing specific molecules – and examiners expect you to keep them straight. The students who lose marks are not the ones who don’t know respiration happens; they’re the ones who put the Krebs cycle in the cytoplasm, confuse NAD with NADP, or say “glucose enters the mitochondrion” (it doesn’t – pyruvate does).

From years of teaching and examining this topic, the pattern is clear: respiration rewards precision about location and product. Where does each stage happen? What goes in, what comes out, and how much ATP? Get those three things right for each stage and the marks follow. On this page I’ll take you through glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation in the exact order and detail examiners want – plus anaerobic respiration and the use of other respiratory substrates.

Board check: Respiration is examined in AQA 3.5.2, OCR A 5.2.2, OCR B (Module 5), Edexcel A (Salters-Nuffield) 7.3–7.8, Edexcel B Topic 7, and WJEC/Eduqas (A2, “Respiration releases chemical energy in biological processes”). All boards require the four stages and chemiosmosis. WJEC/Eduqas and OCR A emphasise the chemiosmotic theory and the energy budget; Edexcel A names the lactate “oxygen debt” recovery.

Key Terminology – The Words That Earn Marks

Examiners reject loose wording in respiration more than almost anywhere else. Learn these exactly.

Cellular respiration The enzyme-controlled release of energy from organic molecules (e.g. glucose) in cells, used to synthesise ATP. It can be aerobic (using oxygen) or anaerobic (without oxygen).

Glycolysis The splitting of one 6-carbon glucose into two 3-carbon pyruvate molecules, in the cytoplasm, producing a net gain of ATP and reduced NAD. It is anaerobic and is the first stage of both aerobic and anaerobic respiration.

Link reaction The reaction in which pyruvate is oxidised to acetate and combined with coenzyme A to form acetyl coenzyme A, releasing CO₂ and reduced NAD. It occurs in the mitochondrial matrix.

Krebs cycle A cycle of oxidation-reduction reactions in the mitochondrial matrix that produces reduced coenzymes (NAD and FAD), ATP by substrate-level phosphorylation, and CO₂.

Oxidative phosphorylation The synthesis of ATP using energy released as electrons pass down the electron transfer chain, driven by chemiosmosis across the inner mitochondrial membrane. It requires oxygen as the final electron acceptor.

Chemiosmosis The flow of protons (H⁺) down their electrochemical gradient through ATP synthase, which is coupled to the synthesis of ATP from ADP and P_i.

Respiratory substrate An organic molecule that can be oxidised in respiration to release energy – usually glucose, but also lipids and amino acids.

Examiner reject list: Do NOT confuse NAD (respiration) with NADP (photosynthesis). Do NOT say “glucose enters the mitochondrion” – it is pyruvate. Do NOT call the electron transfer chain a “Krebs cycle” product – it is a separate stage. Do NOT write “energy is produced” – energy is released and transferred to ATP (energy is never created).

Stage 1 – Glycolysis (in the cytoplasm)

Glycolysis is common to all respiration, aerobic or anaerobic, and happens in the cytoplasm. It needs no oxygen.

The Steps Examiners Want to See

Phosphorylation of glucose Glucose (6C) is phosphorylated to glucose phosphate using 2 ATP. This activates the glucose and makes it more reactive (the “investment” of ATP).

Splitting into triose phosphate The 6C phosphorylated sugar is split into two 3-carbon triose phosphate molecules.

Oxidation to pyruvate Each triose phosphate is oxidised to pyruvate (3C). Hydrogen is removed and accepted by NAD to form reduced NAD, and ATP is produced.

Net yield Per glucose: 4 ATP made − 2 ATP used = net 2 ATP, plus 2 reduced NAD and 2 pyruvate.

The numbers to quote: net 2 ATP, 2 reduced NAD, 2 pyruvate per glucose. “Substrate-level phosphorylation” describes the direct ATP made here and in the Krebs cycle (as opposed to oxidative phosphorylation later).

Stage 2 – The Link Reaction (mitochondrial matrix)

If oxygen is present, pyruvate is actively transported into the mitochondrial matrix, where the link reaction occurs.

Pyruvate is oxidised to acetate Pyruvate (3C) is decarboxylated (losing CO₂) and dehydrogenated (losing hydrogen to NAD, forming reduced NAD), producing acetate (2C).

Acetyl coenzyme A forms Acetate combines with coenzyme A to form acetyl coenzyme A (acetyl CoA), which carries the acetate into the Krebs cycle.

Per glucose (two pyruvate): the link reaction happens twice, producing 2 acetyl CoA, 2 CO₂ and 2 reduced NAD. No ATP is made directly here.

Common error: Students forget the link reaction occurs twice per glucose (because glycolysis made two pyruvate). When asked for totals per glucose, double everything from the link reaction onwards.

Stage 3 – The Krebs Cycle (mitochondrial matrix)

The Krebs cycle is a series of oxidation-reduction reactions in the matrix that strips hydrogen (and its electrons) from the acetate, loading the reduced coenzymes that feed the final stage.

Acetyl CoA enters the cycle The 2C acetate from acetyl CoA combines with a 4-carbon molecule to form a 6-carbon molecule. Coenzyme A is released and reused.

Decarboxylation and dehydrogenation The 6C molecule is gradually converted back to the 4C molecule, losing 2 CO₂ (decarboxylation) and hydrogen (dehydrogenation) per turn.

Reduced coenzymes and ATP made Per turn: 3 reduced NAD, 1 reduced FAD, and 1 ATP (by substrate-level phosphorylation). The 4C molecule is regenerated so the cycle continues.

Per glucose (two turns): the cycle turns twice, giving 6 reduced NAD, 2 reduced FAD, 2 ATP and 4 CO₂. The reduced NAD and FAD now carry hydrogen to the electron transfer chain.

Stage 4 – Oxidative Phosphorylation (inner mitochondrial membrane)

This is where most ATP is made, on the inner mitochondrial membrane (cristae), using the reduced NAD and FAD from the earlier stages. It requires oxygen.

Reduced coenzymes release hydrogen Reduced NAD and FAD are oxidised, releasing hydrogen atoms, which split into protons (H⁺) and electrons (e⁻).

Electrons pass down the electron transfer chain Electrons are passed along a chain of carriers in the inner membrane in a series of redox reactions, releasing energy at each step.

Protons are pumped & chemiosmosis occurs The energy is used to pump protons from the matrix into the intermembrane space, creating an electrochemical gradient. Protons then diffuse back through ATP synthase, driving the synthesis of ATP (chemiosmosis).

Oxygen is the final electron acceptor At the end of the chain, electrons and protons combine with oxygen to form water. Without oxygen, the chain backs up and stops – this is why aerobic respiration needs oxygen.

Why oxygen matters (a top marking point): oxygen is the final electron acceptor. If it is absent, the electron transfer chain cannot pass on its electrons, reduced NAD/FAD cannot be re-oxidised, and the whole aerobic pathway halts. This is the link to anaerobic respiration below.

ATP Yield & Anaerobic Respiration

The ATP balance sheet (per glucose)

Stage	Location	ATP (direct)	Reduced NAD	Reduced FAD	CO₂
Glycolysis	Cytoplasm	net 2	2	0	0
Link reaction (x2)	Matrix	0	2	0	2
Krebs cycle (x2)	Matrix	2	6	2	4
Oxidative phosphorylation	Inner membrane	~26–28	–	–	0

Total: the theoretical maximum is often quoted as ~38 ATP per glucose, though many modern textbooks and boards use ~30–32 ATP to account for the cost of transporting molecules into the mitochondrion. Quote the figure your specification/textbook uses, and state it as approximate.

Anaerobic respiration (no oxygen)

Without oxygen, oxidative phosphorylation stops, so reduced NAD cannot be re-oxidised by the electron transfer chain. To keep glycolysis going (and keep making a little ATP), the cell must regenerate oxidised NAD another way:

In animals (and bacteria) – lactate fermentation Pyruvate accepts hydrogen from reduced NAD, forming lactate and regenerating oxidised NAD. This lets glycolysis continue. Lactate builds up (causing fatigue) and is later oxidised back to pyruvate when oxygen returns (the “oxygen debt”).

In plants and yeast – alcoholic fermentation Pyruvate is decarboxylated to ethanal, which accepts hydrogen from reduced NAD to form ethanol and CO₂, regenerating oxidised NAD.

Common error: Anaerobic respiration still relies on glycolysis for its ATP – only 2 ATP per glucose, far less than aerobic. The fermentation step makes no extra ATP; its only job is to regenerate oxidised NAD so glycolysis can keep running.

Other Respiratory Substrates & the Respiratory Quotient

Glucose is the usual substrate, but lipids and proteins can also be respired (AQA, WJEC/Eduqas, OCR A).

Lipids Hydrolysed to glycerol and fatty acids. Glycerol is converted to triose phosphate (entering glycolysis); fatty acids are broken down to acetyl groups that enter the Krebs cycle. Lipids release more energy per gram than carbohydrates because they contain more hydrogen.

Proteins / amino acids Deaminated, then the carbon skeletons enter respiration at glycolysis, the link reaction or the Krebs cycle depending on the amino acid.

Respiratory quotient (RQ) – WJEC/Eduqas & others: RQ = CO₂ produced ÷ O₂ consumed. Carbohydrate ≈ 1.0, protein ≈ 0.9, lipid ≈ 0.7. A low RQ suggests fat is being respired; RQ above 1.0 indicates some anaerobic respiration is occurring. You may be asked to calculate RQ from gas-exchange data using a respirometer.

Exam Board Comparison – What Your Board Requires

This is the table no other revision site provides. Use it to check exactly what your board requires.

Subtopic	AQA	OCR A	OCR B	Edexcel A	Edexcel B	WJEC / Eduqas
Glycolysis	✔	✔	✔	✔	✔	✔
Link reaction	✔	✔	✔	✔	✔	✔
Krebs cycle	✔	✔	✔	✔	✔	✔
Oxidative phosphorylation	✔	✔	✔	✔	✔	✔
Chemiosmotic theory (named)	✔	✔	✔	✔	✔	✔
Names of intermediates required	Some	Some	❌	Some	Some	❌
Anaerobic (lactate & ethanol)	✔	✔	✔	✔	✔	✔
Lipids/proteins as substrates	✔	✔	Implicit	✔	✔	✔
Respiratory quotient (RQ)	❌	✔	❌	✔	✔	✔
Respirometer practical	✔	✔	✔	✔	✔	✔

Eduqas/OCR B note: WJEC/Eduqas and OCR B state that the names of intermediate compounds and electron carriers are not required – focus on the principles (where, what goes in/out, chemiosmosis). AQA/Edexcel expect named molecules like pyruvate, acetyl CoA, NAD, FAD.

8 Common Mistakes from Examiner Reports

These are the errors I see again and again. Every one of them costs marks.

#	The mistake	The correction
1	“Glucose enters the mitochondrion”	Pyruvate enters the mitochondrion (by active transport). Glucose is split into pyruvate in the cytoplasm first.
2	Confusing NAD and NADP	Respiration uses NAD (and FAD). NADP is for photosynthesis.
3	Putting the Krebs cycle in the cytoplasm	Glycolysis = cytoplasm. Link reaction & Krebs = matrix. Oxidative phosphorylation = inner membrane.
4	Forgetting the link reaction/Krebs happen TWICE	Two pyruvate per glucose, so double the products from the link reaction onwards.
5	“ATP synthase makes ATP from electrons”	ATP synthase uses the proton gradient (chemiosmosis), built by electrons moving down the chain.
6	Saying anaerobic respiration “makes ATP in fermentation”	Fermentation makes no ATP – it regenerates oxidised NAD so glycolysis (the only ATP source) can continue.
7	“Oxygen is used in the Krebs cycle”	Oxygen is only used at the end of the electron transfer chain, as the final electron acceptor.
8	Writing “energy is produced”	Energy is released and transferred to ATP – it cannot be created. Examiners penalise “produced” here.

Lost in the Four Stages of Respiration?

Respiration is all about location and product – exactly the kind of thing tutoring drills until it’s automatic. If glycolysis, the link reaction, Krebs and oxidative phosphorylation blur together, I’ll teach you the structure that fits every respiration question and the language examiners reward.

Tyrone John • CBiol MRSB • Former WJEC/Eduqas & Edexcel Examiner • 25+ Years Teaching A-Level Biology

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Frequently Asked Questions – Cellular Respiration

Where does each stage of respiration take place?

Glycolysis takes place in the cytoplasm. The link reaction and the Krebs cycle take place in the mitochondrial matrix. Oxidative phosphorylation (the electron transfer chain and chemiosmosis) takes place on the inner mitochondrial membrane, also called the cristae. Naming the correct location for each stage is one of the most common marking points in respiration questions.

How much ATP is made per glucose in aerobic respiration?

The theoretical maximum is often quoted as about 38 ATP per glucose: a net 2 from glycolysis, 2 from the Krebs cycle (substrate-level phosphorylation), and the rest from oxidative phosphorylation. Many modern textbooks and exam boards use a lower figure of about 30 to 32 ATP to account for the energy cost of transporting molecules into the mitochondrion. Quote whichever figure your specification uses and describe it as approximate.

Why does aerobic respiration need oxygen?

Oxygen is the final electron acceptor at the end of the electron transfer chain. It accepts electrons and protons to form water. Without oxygen, the electrons have nowhere to go, so the chain backs up and stops. This means reduced NAD and FAD cannot be re-oxidised, the matrix runs out of oxidised coenzymes, and the link reaction and Krebs cycle halt. Only glycolysis can then continue, via anaerobic respiration.

What is the difference between substrate-level and oxidative phosphorylation?

Substrate-level phosphorylation is the direct transfer of a phosphate group to ADP during glycolysis and the Krebs cycle, making a small amount of ATP. Oxidative phosphorylation makes most of the ATP: it uses the energy released as electrons pass down the electron transfer chain to pump protons and create a gradient, and ATP synthase then makes ATP as the protons flow back (chemiosmosis). Oxidative phosphorylation requires oxygen; substrate-level phosphorylation does not.

What happens in anaerobic respiration?

Without oxygen, oxidative phosphorylation stops, so reduced NAD cannot be re-oxidised by the electron transfer chain. To keep glycolysis going, the cell regenerates oxidised NAD by fermentation. In animals, pyruvate accepts hydrogen to form lactate; in plants and yeast, pyruvate forms ethanol and carbon dioxide. Fermentation itself makes no ATP. The only ATP comes from glycolysis, giving just 2 ATP per glucose, far less than aerobic respiration.

What is chemiosmosis?

Chemiosmosis is the process that makes ATP in oxidative phosphorylation. As electrons pass down the electron transfer chain, the energy released is used to pump protons from the matrix into the intermembrane space, creating an electrochemical gradient. The protons then diffuse back into the matrix through the enzyme ATP synthase, and this flow drives the synthesis of ATP from ADP and inorganic phosphate. The same chemiosmotic principle operates in photosynthesis.

Can fats and proteins be used in respiration?

Yes. Lipids are hydrolysed to glycerol and fatty acids: glycerol is converted to triose phosphate and enters glycolysis, while fatty acids are broken down into acetyl groups that enter the Krebs cycle. Lipids release more energy per gram than carbohydrates because they contain more hydrogen. Proteins are deaminated and their carbon skeletons enter respiration at glycolysis, the link reaction or the Krebs cycle depending on the amino acid.

What is the respiratory quotient (RQ)?

The respiratory quotient is the ratio of carbon dioxide produced to oxygen consumed in respiration (RQ = CO₂ produced ÷ O₂ consumed). It indicates which substrate is being respired: carbohydrate gives an RQ of about 1.0, protein about 0.9, and lipid about 0.7. An RQ above 1.0 suggests some anaerobic respiration is also occurring. RQ can be measured using a respirometer and is required by WJEC/Eduqas, OCR A and Edexcel.

Written by Tyrone John

CBiol MRSB • Former WJEC/Eduqas & Edexcel Examiner • PGCE • 25+ Years Teaching A-Level Biology • Published Scientific Research

Tyrone has over 25 years of experience teaching A-Level Biology and is a Chartered Biologist and member of the Royal Society of Biology. As a former examiner for WJEC/Eduqas and Edexcel, he has first-hand knowledge of how mark schemes are applied and what examiners look for in student answers. Learn more →