7.2Topic 7 · Run for your Life

Respiration — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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Respiration Edexcel A Level Biology SNAB notes: Outline the stages of aerobic respiration, their locations and products.Aerobic respirationglucose + oxygen → carbon dioxide + water: C6H12O6 + 6O2 → 6CO2 + 6H2O (energy released used to make ATP).Lactate fermentationglucose → lacta…

Volume of gas in capillary
Glycolysis
Link reaction per pyruvate
Krebs per acetyl CoA
ATP is the immediate energy source
Oxidative phosphorylation

Section 1 of 24Respiration

ATP is the immediate energy source

ATP is the immediate energy source small, soluble, releases energy in small manageable amounts via hydrolysis by ATPase, and is rapidly regenerated.

Glycolysis

Glycolysis glucose is phosphorylated by 2 ATP to hexose bisphosphate, split into 2 triose phosphate, each oxidised (reducing NAD) to pyruvate, producing 4 ATP (net 2).

Pyruvate is actively transported into the mitochondrial matrix; link reaction removes CO₂ and hydrogen, forming acetyl CoA.

Krebs

Krebs acetyl (2C) + oxaloacetate (4C) → citrate (6C); successive decarboxylations and dehydrogenations regenerate oxaloacetate; enzymes are in the matrix.

Oxidative phosphorylation

Oxidative phosphorylation reduced NAD/FAD release H, which splits into H⁺ and e⁻; electrons pass along carriers (redox reactions), releasing energy to pump H⁺ into the intermembrane space.

The H⁺ gradient drives H⁺ through ATP synthase (chemiosmosis), producing ATP from ADP + P_i. Cristae folding increases surface area for this.

Without oxygen, electrons cannot leave the chain, reduced NAD cannot be reoxidised, and Krebs and link reaction stop; only glycolysis continues if NAD is regenerated.

In muscle, lactate dehydrogenase reduces pyruvate to lactate using reduced NAD. Lactate lowers pH (lactic acid) and contributes to fatigue.

Lactate is carried in blood to the liver, where it is oxidised back to pyruvate and can be converted to glucose/glycogen; this requires oxygen (oxygen debt).

Respirometer

Respirometer temperature must be controlled because gas volume changes with temperature; control tube compensates for pressure/temperature changes; soda lime absorbs CO₂ so only O₂ change is measured. Safety: soda lime is corrosive; treat invertebrates ethically.

Respiratory substrates other than glucose (fats, proteins) can enter at acetyl CoA or Krebs cycle; lipids release more energy per gram because they contain more hydrogen.

Mitochondrion structure

Outer membrane is smooth and permeable to small molecules; inner membrane is folded into cristae, less permeable (impermeable to H⁺) and holds the electron transport chain and ATP synthase.

The intermembrane space has low pH from accumulated protons;
- the matrix contains enzymes, 70S ribosomes and circular DNA.

Stepwise energy release

Glucose is oxidised in many enzyme-controlled steps so energy is released gradually in usable amounts; releasing it all at once would raise temperature enough to denature enzymes. The slowest enzyme-catalysed step limits the overall rate.

Why phosphorylate glucose

Adding two phosphates from ATP makes glucose more reactive (lowering activation energy for splitting) and prevents it diffusing back out of the cell.

Coenzyme A

Coenzyme A carries the 2C acetyl group from the link reaction to the Krebs cycle; it is released when acetyl combines with oxaloacetate and is reused. Acetyl CoA can also come from fatty acids and amino acids.

Section 2 of 24Process & sequence

Set up a respirometer with organisms (e.g. germinating seeds or woodlice) in one tube and a control tube with an equal volume of glass beads; place soda lime in both to absorb CO₂.

Put both tubes in a water bath at a set temperature and leave ~10 min to equilibrate, with taps open.

Close the taps and use a syringe to set the coloured liquid in the capillary tube to a marked starting point.

Record the distance moved by the liquid towards the organisms over a fixed time (e.g. 10 min).

Calculate volume of O₂ taken up = πr² × distance moved, and divide by time (and mass) to give a rate.

Repeat at least three times and at other temperatures, keeping mass of organisms and soda lime constant; calculate means.

Oxidative phosphorylationwrite these steps in this order; each step is a marking point.

Reduced NAD and FAD deliver hydrogen to the electron transport chain on the cristae.

Hydrogen splits into H⁺ and electrons; electrons pass along carriers.

Energy released pumps H⁺ into the intermembrane space.

H⁺ diffuse back through ATP synthase, making ATP from ADP + P_i.

Oxygen accepts electrons and H⁺ to form water.

Section 3 of 24Compare

Stages of aerobic respiration (per glucose)
Stage	Location	CO₂	Reduced NAD	Reduced FAD	ATP (direct)
Glycolysis	Cytoplasm	0	2	0	2 (net)
Link reaction	Mitochondrial matrix	2	2	0	0
Krebs cycle (2 turns)	Mitochondrial matrix	4	6	2	2
Oxidative phosphorylation	Inner membrane (cristae)	0	used	used	~28 (theoretical up to 34)

Total yield per glucose is often quoted as about 30–32 ATP in practice (theoretical maximum 38).

Aerobic vs anaerobic (lactate) respiration
Feature	Aerobic	Anaerobic (animals)
Oxygen	Required	Not required
Stages	Glycolysis, link, Krebs, oxidative phosphorylation	Glycolysis + pyruvate → lactate
Products	CO₂ and H₂O	Lactate
ATP per glucose	~30–38	2
Location	Cytoplasm + mitochondria	Cytoplasm

Section 4 of 24Worked examples

In a respirometer the liquid moved 32 mm in 10 min. Capillary radius = 0.50 mm; seeds' mass = 4.0 g. Calculate the rate of O₂ uptake in mm³ g⁻¹ min⁻¹.

V = πr²h = 3.142 × 0.50² × 32 = 25.1 mm³. Rate = 25.1 ÷ 10 ÷ 4.0 = 0.63 mm³ g⁻¹ min⁻¹.

0.70

An organism produces 45 cm³ CO₂ while consuming 64 cm³ O₂. Calculate RQ and suggest the substrate.

RQ = 45 ÷ 64 = 0.70 (2 s.f.) → lipid is the respiratory substrate.

32 ATP

Using 2.5 ATP per reduced NAD and 1.5 per reduced FAD, calculate the ATP yield from one glucose in aerobic respiration.

Reduced NAD = 2 + 2 + 6 = 10 → 25 ATP.
- Reduced FAD = 2 → 3 ATP.
- Direct ATP = 2 (glycolysis) + 2 (Krebs) = 4.
- Total = 25 + 3 + 4 = 32 ATP.

Section 5 of 24Keep going

Respiration Edexcel A Level Biology SNAB: what to do next

These Respiration Edexcel A Level Biology SNAB notes covered the key ideas, worked examples and exam technique for this topic. Check your Respiration Edexcel A Level Biology SNAB understanding with the practice questions below, then reread any Respiration Edexcel A Level Biology SNAB point you were unsure about before moving on.

Respiration Edexcel A Level Biology SNAB: revision checklist

Revise the main Respiration Edexcel A Level Biology SNAB definitions until you can write them from memory.

Practise Respiration Edexcel A Level Biology SNAB exam questions under timed conditions.

Link each Respiration Edexcel A Level Biology SNAB idea to a diagram or a worked example.

List the common Respiration Edexcel A Level Biology SNAB mistakes and how to avoid them.

Explain one Respiration Edexcel A Level Biology SNAB idea out loud as if teaching a friend.

Summarise Respiration Edexcel A Level Biology SNAB in five key points on a revision card.

Check which command words appear most often in Respiration Edexcel A Level Biology SNAB questions.

Practice: Respiration Edexcel A Level Biology SNAB practice questions

Previous lesson: 7.1 Muscles & Movement — 9BI0 Notes

Next lesson: 7.3 Exercise — 9BI0 Notes

Course: Edexcel A Level Biology A (snab) 9BI₀ — all units

Official syllabus: Pearson Edexcel A Levels

Section 6 of 24Aerobic respiration overview and mitochondrial structure

Glucose is the main respiratory substrate in cells. Aerobic respiration breaks down a respiratory substrate to produce ATP using oxygen. The overall equation is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (energy released is used to make ATP)

Glucose is broken down in stages, each a series of chemical reactions controlled by intracellular enzymes. This releases the energy trapped in the chemical bonds of glucose gradually rather than all at once, so more of it can be captured in ATP and less is lost as heat.

Mitochondria are the site of most of aerobic respiration. They have two phospholipid membranes:

Aerobic respiration overview and mitochondrial structure

Outer membrane

The outer membrane is permeable to several small molecules.

Inner membrane

The inner membrane is folded into cristae, which increase the surface area.

It is the site of the electron transport chain and is impermeable to hydrogen ions.
- (oxidative phosphorylation)

Matrix

The matrix is the fluid inside the inner membrane.

It contains the enzymes and coenzymes for the link reaction and Krebs cycle, and mitochondrial DNA and ribosomes.

Exam tip

you must give exact locations.
It is not enough to say the Krebs cycle occurs "in the mitochondria";
- it occurs in the matrix.

Section 7 of 24The four stages and their locations

Stage	Location	Oxygen needed?
Glycolysis	Cytoplasm (cytosol)	No
Link reaction	Mitochondrial matrix	Indirectly (needs the products of later stages to regenerate NAD)
Krebs cycle	Mitochondrial matrix	Indirectly
Oxidative phosphorylation	Inner mitochondrial membrane	Yes, as final electron acceptor

Section 8 of 24Glycolysis

Glycolysis is the first stage of respiration. It does not require oxygen, so it is the first step for both aerobic and anaerobic respiration. Glucose is only partially oxidised during glycolysis.

Phosphorylation

Phosphorylation glucose (6C) is phosphorylated using 2 ATP, making hexose bisphosphate.

This makes glucose more reactive and lowers the activation energy for the following reactions.
- (it is normally very stable)

Splitting

Splitting hexose bisphosphate splits into two molecules of triose phosphate (3C).

Oxidation

Oxidation each triose phosphate is oxidised by removing hydrogen, which is accepted by NAD to form reduced NAD (NADH).

Two reduced NAD are formed per glucose.

ATP formation

ATP formation each triose phosphate is converted to pyruvate (3C) with the production of 2 ATP by substrate-level phosphorylation.

Four ATP are made in total per glucose.

Although ATP is used at the start, 2 ATP are used and 4 are produced, so there is a net gain of 2 ATP per glucose molecule. The net yield of glycolysis per glucose is 2 pyruvate, 2 reduced NAD and a net 2 ATP.

Section 9 of 24The link reaction

The end product of glycolysis is pyruvate. It still contains a large amount of chemical energy that can be used to produce more ATP. If oxygen is available, pyruvate is actively transported into the mitochondrial matrix, where the enzymes and coenzymes needed for the link reaction are found.

pyruvate + NAD + coenzyme A → acetyl CoA + CO₂ + reduced NAD

The link reaction dehydrogenates (removes hydrogen) and decarboxylates (removes carbon dioxide from) the three-carbon pyruvate to produce the two-carbon acetyl coenzyme A, which can enter the Krebs cycle. No ATP is made directly in this step.

Every glucose gives two pyruvate molecules, so the link reaction happens twice per glucose. Per glucose it therefore produces 2 acetyl CoA, 2 CO₂ and 2 reduced NAD.

Summary of yields before the Krebs cycle (per glucose): net 2 ATP, 4 reduced NAD (2 from glycolysis, 2 from the link reaction) and 2 CO₂.

Section 10 of 24Worked examples

State where glycolysis and the Krebs cycle take place in a cell. [2]2 marks
Explain why glycolysis has a net yield of 2 ATP although 4 ATP are produced. [3]3 marks
Describe what happens to pyruvate in the link reaction. [3]3 marks

Section 11 of 24The Krebs cycle

The Krebs cycle takes place in the mitochondrial matrix. Acetyl CoA (2C) from the link reaction combines with oxaloacetate (4C) to form citrate (6C), and coenzyme A is released to be reused. Citrate is then converted back to oxaloacetate through a series of oxidation-reduction (redox) reactions.

Dehydrogenated

Citrate is dehydrogenated and decarboxylated in steps, releasing CO₂ and passing hydrogen to NAD and FAD.

Substrate-level phosphorylation

One molecule of ATP is made by substrate-level phosphorylation in each turn.

Regenerated

Oxaloacetate is regenerated at the end, so the cycle can accept another acetyl CoA. This is why it is called a cycle.

Products of one turn (one acetyl CoA): 3 reduced NAD, 1 reduced FAD, 1 ATP and 2 CO₂. Because each glucose gives two acetyl CoA, the cycle turns twice per glucose, giving six reduced NAD, two reduced FAD, two ATP and four CO₂. Be able to name key molecules such as oxaloacetate and citrate.

Section 12 of 24Oxidative phosphorylation and the chemiosmotic theory

Oxidative phosphorylation is the last stage of aerobic respiration. It takes place at the inner mitochondrial membrane and produces most of the ATP, and water from oxygen. It needs reduced NAD and reduced FAD from earlier stages.

Oxidative phosphorylation and the chemiosmotic theory

Reduced NAD and reduced FAD are oxidised at the electron transport chain, releasing hydrogen atoms that split into protons (H⁺) and electrons.

The electrons pass from carrier to carrier along the chain.

The carriers are positioned close together, allowing electrons to pass along.
At each step electrons lose energy.

The energy released is used to pump protons from the matrix into the intermembrane space.

The inner membrane is impermeable to H⁺, so a proton concentration gradient builds up.
- (electrochemical)

ATP synthase

Protons flow back down the gradient into the matrix through ATP synthase channels.

The flow of protons drives the synthesis of ATP from ADP and P_i (chemiosmosis).

Oxygen is the final electron acceptor

At the end of the chain, oxygen is the final electron acceptor. It combines with electrons and protons to form water.

Classical valuesabout 3 ATP per reduced NAD and 2 ATP per reduced FAD (modern estimates are lower, about 2.5 and 1.5).

Oxygen is vital because without it the chain stops, as electrons have nowhere to go. Reduced NAD and FAD cannot be oxidised back to NAD and FAD, so they cannot be used in further hydrogen transport and the Krebs cycle and link reaction also stop. Use "hydrogen" for atoms and "hydrogen ions/protons" for H⁺ carefully.

Section 13 of 24Practical: the respirometer

A respirometer measures the rate of oxygen consumption of living organisms such as germinating seeds, woodlice or maggots. Two tubes are set up: the test tube containing the organisms and a control tube (for example glass beads of equal volume) to compensate for changes in temperature and pressure.

Practical: the respirometer

The CO₂ produced is absorbed by soda lime or potassium hydroxide (KOH), so any change in gas volume is due to oxygen use only.

As oxygen is consumed, the volume of air in the tube falls and coloured manometer fluid moves towards the organisms.

The experiment is run for a set time (for example 30 minutes) and the distance moved by the fluid is recorded.

The volume of oxygen used = πr²h, where r is the radius of the capillary tube (diameter ÷ 2) and h is the distance moved by the fluid.

Practical: the respirometer

Rate = volume ÷ time, in cm³ min⁻¹.

Keep temperature constant (for example in a water bath) and treat living organisms with respect: do not harm them and return them to their habitat.

Section 14 of 24Anaerobic respiration: why it happens

Cells sometimes have little or no oxygen. The consequence is that there is no final acceptor of electrons from the electron transport chain, so the chain stops. The Krebs cycle and link reaction also stop because no NAD and FAD are regenerated. Only glycolysis can continue, provided NAD is regenerated by another route (fermentation).

Section 15 of 24Worked examples

Explain why oxygen is needed for aerobic respiration even though it is not used in the Krebs cycle. [3]3 marks
Calculate the volume of oxygen used by seeds if the manometer fluid moved 3.5 cm in 10 minutes in a capillary tube of diameter 1.0 mm. Give the rate in mm³ min⁻¹. [3]3 marks
Describe the role of the proton gradient in ATP synthesis. [3]3 marks

Section 16 of 24Why anaerobic respiration occurs

Cells can experience conditions with little or no oxygen, for example muscle cells during vigorous exercise or yeast in a sealed fermenter.

Why anaerobic respiration occurs

No final electron acceptor

Without oxygen there is no final electron acceptor at the end of the electron transport chain.

The chain stops, so reduced NAD and reduced FAD cannot be reoxidised, and without NAD the link reaction and Krebs cycle also stop.

Oxidative phosphorylation, which makes most ATP, therefore cannot take place.

Glycolysis can still continue because it does not need oxygen, but it needs a supply of NAD to oxidise triose phosphate. Anaerobic respiration is the set of reactions that regenerate NAD from reduced NAD so glycolysis can keep going and make a small net amount of ATP (2 ATP per glucose).

Section 17 of 24Lactate fermentation in animals

In mammalian muscle, pyruvate acts as the hydrogen acceptor. Pyruvate is reduced to lactate (lactic acid) by the enzyme lactate dehydrogenase, and reduced NAD is oxidised to NAD:

Lactate fermentation in animals

pyruvate + reduced NAD → lactate + NAD

The regenerated NAD returns to glycolysis, which allows continued ATP production at a rate of 2 ATP per glucose.

This is much less efficient than aerobic respiration,
which yields about 30 ATP per glucose,
because the glucose is only partly broken down and most chemical energy remains in lactate.
No carbon dioxide is produced.

The process is reversible. Lactate is not a final waste product; it still contains chemical energy.

Section 18 of 24What happens to lactate

Lactate (lactic acid) can build up in the cells after a period of time. Lactate lowers pH, which can inhibit enzymes and contribute to muscle fatigue, so it must be removed. It is carried in the blood to the liver. After lactate is produced two things can happen:

1Oxidised back to pyruvate

It can be oxidised back to pyruvate,

which is then channelled into the Krebs cycle for ATP production when oxygen becomes available again.
- (after the link reaction)

2Converted into glycogen

It can be converted into glycogen in the liver and stored, or converted to glucose.

The extra oxygen taken in after exercise, used to oxidise lactate and restore ATP and phosphocreatine stores, is called the oxygen debt (excess post-exercise oxygen consumption). This is why breathing remains deep and fast for some time after stopping exercise.

Section 19 of 24Anaerobic respiration during exercise

During intense exercise the muscles demand ATP faster than oxygen can be delivered by the blood.

Mitochondrial oxidative phosphorylation cannot keep pace, so muscle cells rely increasingly on glycolysis and lactate production.

This gives ATP quickly but only a small yield, so glycogen stores are used up rapidly.

Lactate builds up in the muscle and blood, the pH falls, and enzymes work less efficiently, which contributes to fatigue and cramp.

An athlete can only sustain anaerobic exercise for a short time,

which is why sprinters tire quickly compared with long-distance runners relying on aerobic respiration.

Training increases the number of mitochondria and capillaries in muscle and improves the oxygen supply, delaying the point (the lactate threshold) at which anaerobic respiration becomes dominant. After exercise, the heart rate and breathing rate stay raised so that extra oxygen can be supplied to repay the oxygen debt: lactate is oxidised, and ATP and phosphocreatine stores are restored.

Section 20 of 24Ethanol fermentation compared with lactate fermentation

Plants, fungi (such as yeast) and some bacteria use a different route. Pyruvate is first decarboxylated to ethanal (releasing CO₂), and ethanal is then reduced to ethanol by reduced NAD, regenerating NAD.

Feature	Lactate fermentation	Ethanol fermentation
Organisms	Animals, some bacteria	Yeast, plants
Pyruvate converted to	Lactate	Ethanal, then ethanol
CO₂ released	No	Yes
Reversible?	Yes	No
Net ATP per glucose	2	2

The ethanol pathway is irreversible, and ethanol is toxic at high concentration. Anaerobic respiration is useful in industry for brewing, baking and yoghurt making.

Section 21 of 24Worked examples

Explain why lactate is produced during anaerobic respiration. [3]3 marks
State two fates of lactate once oxygen is available. [2]2 marks
Compare the ATP yield of aerobic and anaerobic respiration and explain the difference. [3]3 marks

Section 22 of 24Key terms

Term	Meaning
Glycolysis	Partial oxidation of glucose to two pyruvate in the cytoplasm, with a net gain of 2 ATP.
Pyruvate	Three-carbon end product of glycolysis.
Acetyl CoA	Two-carbon compound formed in the link reaction that enters the Krebs cycle.
NAD	Coenzyme that accepts hydrogen to become reduced NAD.
Decarboxylation	Removal of carbon dioxide from a molecule.
Dehydrogenation	Removal of hydrogen atoms from a molecule.
Mitochondrial matrix	Fluid centre of the mitochondrion containing enzymes for the link reaction and Krebs cycle.
Cristae	Folds of the inner mitochondrial membrane that increase surface area.

Term	Meaning
Krebs cycle	Cyclic series of redox reactions in the matrix that regenerates oxaloacetate and produces reduced coenzymes, ATP and CO₂.
Oxaloacetate	Four-carbon acceptor molecule regenerated in the Krebs cycle.
Citrate	Six-carbon compound formed when acetyl CoA joins oxaloacetate.
Oxidative phosphorylation	ATP production at the inner mitochondrial membrane using energy from the electron transport chain.
Chemiosmosis	Synthesis of ATP driven by protons moving down their gradient through ATP synthase.
Respirometer	Apparatus for measuring oxygen consumption by living organisms.
Soda lime	Chemical used to absorb carbon dioxide in a respirometer.

Term	Meaning
Anaerobic respiration	ATP production without oxygen, using glycolysis plus fermentation.
Lactate	Product formed when pyruvate accepts hydrogen from reduced NAD in animal cells.
Fermentation	Regeneration of NAD from reduced NAD by reducing pyruvate or a derivative.
Oxygen debt	Extra oxygen needed after exercise to remove lactate and restore ATP stores.
Lactate dehydrogenase	Enzyme that converts pyruvate to lactate.
Ethanol	Alcohol produced by yeast and plants in anaerobic respiration.

Section 23 of 24Command terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Section 24 of 24Exam technique

Saying glycolysis needs oxygen. It occurs in the cytoplasm without oxygen and is common to aerobic and anaerobic respiration.

Quoting 4 ATP as the yield of glycolysis. The net yield is 2 ATP because 2 are used.

Giving the location of the Krebs cycle only as the mitochondria. The exact location (the matrix) is needed for the mark.

Forgetting that the link reaction happens twice per glucose because glucose yields two pyruvate.

Saying oxygen is used in the Krebs cycle. It is used only at the end of the electron transport chain.

Confusing hydrogen atoms with hydrogen ions. Protons are pumped; reduced NAD carries hydrogen atoms.

Forgetting to double Krebs cycle yields per glucose, since two acetyl CoA enter.

Using the diameter instead of the radius in πr²h for respirometer calculations.

Saying anaerobic respiration produces no ATP. Glycolysis still produces a net 2 ATP.

Saying the purpose of lactate formation is to make ATP. Its purpose is to regenerate NAD so glycolysis can continue.

Writing that carbon dioxide is released in lactate fermentation. It is released only in ethanol fermentation.

Thinking lactate causes muscle soreness days later; it is cleared within hours and its role is in fatigue.

Last updated: 18 October 2026