BiologyISC Class 11 · Respiration in Plants

Electron Transport System, ATP Yield, the Amphibolic Pathway and Respiratory Quotient

Updated 17 October 2026 · Fact-checked

The electron transport system on the inner mitochondrial membrane passes electrons from NADH and FADH₂ to oxygen and uses the proton gradient to make ATP by oxidative phosphorylation. A glucose molecule gives a total of 36 to 38 ATP in aerobic respiration. The respiratory quotient RQ = volume of CO₂ released/volume of O₂ used depends on the substrate.

Understand Electron Transport, ATP Yield and Respiratory Quotient

Electron transport system (ETS). In the inner mitochondrial membrane: Complex I (NADH dehydrogenase) takes electrons from NADH, Complex II (succinate dehydrogenase) from FADH₂, ubiquinone (coenzyme Q) carries them to Complex III (cytochrome bc₁), cytochrome c carries them to Complex IV (cytochrome c oxidase, with Cu), where oxygen is the final electron acceptor and forms water: ½O₂ + 2H⁺ + 2e⁻ → H₂O. The energy released pumps protons into the intermembrane space.

Oxidative phosphorylation. The proton gradient drives the return of H⁺ to the matrix through ATP synthase (F₀–F₁), making ATP from ADP and Pi. In the traditional count each NADH gives 3 ATP and each FADH₂ gives 2 ATP.

ATP count per glucose (traditional). Glycolysis: 2 ATP + 2 NADH (6 ATP). Link reaction: 2 NADH (6 ATP). Krebs cycle: 2 ATP + 6 NADH (18 ATP) + 2 FADH₂ (4 ATP). Total = 2 + 6 + 6 + 2 + 18 + 4 = 38 ATP gross; if 2 ATP are used to move glycolytic NADH into the mitochondria, the net is 36. Modern estimates are lower because ATP/NADH ratios are less than 3. In anaerobic respiration only 2 ATP per glucose.

Amphibolic pathway. Respiration serves both catabolism and anabolism: fatty acids are broken to acetyl CoA, proteins to amino acids that enter at different points, and intermediates (acetyl CoA, OAA) are drawn off for biosynthesis of fatty acids and amino acids.

Respiratory quotient (RQ). RQ = volume of CO₂ evolved/volume of O₂ consumed. Carbohydrates: 1 (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O). Fats: less than 1 (about 0.7; palmitic acid C₁₆H₃₂O₂ + 23O₂ → 16CO₂ + 16H₂O gives 16/23 = 0.7). Proteins: about 0.9. Organic acids: more than 1 (malic acid 1.33; oxalic acid 4). Anaerobic respiration: infinite, because O₂ is not used.

Electron Transport, ATP Yield and Respiratory Quotient: key facts

ETS complexes: Complex I NADH dehydrogenase
II succinate dehydrogenase
III cyt bc₁
IV (cyt c oxidase) → O₂ → H₂O; Ubiquinone and cytochrome c are mobile carriers.
ATP yield: $N A D H \to 3 A T P ∣ F A D H_{2} \to 2 A T P (t r a d i t i o na l)$; Glucose total: 38 ATP gross, 36 net.
RQ: $R Q = C O_{2} e v o l v e d / O_{2} u se d$; Carbohydrate 1 | Fat about 0.7 | Protein about 0.9.
Special RQ values: $M a l i c a c i d : 4/3 = 1.33∣ O x a l i c a c i d : 4∣ A na er o bi c : in f ini t e$; RQ above 1 for acids.

Complex I	NADH dehydrogenase
II	succinate dehydrogenase
III	cyt bc₁
	IV (cyt c oxidase) → O₂ → H₂O

Electron Transport, ATP Yield and Respiratory Quotient in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Which food needs more oxygen?

Suppose a germinating seed contains only fat and respires it completely, and another seed contains only starch.

Compare the RQ in the two cases and explain what it tells about oxygen consumption.

Try it first, then show the solutionSolution

Starch gives glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, RQ = 6/6 = 1.
Fat (palmitic acid, as an example): C₁₆H₃₂O₂ + 23O₂ → 16CO₂ + 16H₂O, RQ = 16/23 = 0.70.
Fats contain relatively less oxygen than carbohydrates, so more O₂ is needed per CO₂ released and the RQ is less than 1.

Answer: RQ 1 for the starchy seed and about 0.7 for the fatty seed.

Memory hook: A lower RQ means more oxygen is used per CO₂ produced.

Tricks to remember Electron Transport, ATP Yield and Respiratory Quotient

Short memory aids. Say each one aloud once, then use it on a question the same day.

Fat: low, acid: high
Fats have an RQ below 1; organic acids above 1.
Use it when: Substrate identification.
Final electron acceptor: oxygen
O₂ takes electrons at Complex IV to make water.
Use it when: ETS questions.

Electron Transport, ATP Yield and Respiratory Quotient: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Final electron acceptor · 1 mark · MCQBoard level

The final acceptor of electrons in the respiratory chain is:
(a) cytochrome c
(b) NAD⁺
(c) oxygen
(d) ubiquinone

Try it first, then show the solutionSolution

Oxygen accepts electrons at Complex IV to form water.

Answer: (c) oxygen

Example 2: RQ of glucose · 3 marks · Short answerBoard level

Define respiratory quotient and calculate it for glucose and for palmitic acid, C₁₆H₃₂O₂ + 23O₂ → 16CO₂ + 16H₂O.

Try it first, then show the solutionSolution

RQ = volume of CO₂ evolved/volume of O₂ consumed. (1 mark)
Glucose: 6CO₂/6O₂ = 1. (1 mark)
Palmitic acid: 16/23 = 0.70. (1 mark)

Answer: RQ = 1 for glucose; 0.70 for palmitic acid.

Example 3: ATP count · 4 marks · Short answerHarder board level

Calculate the ATP produced in the complete aerobic oxidation of one molecule of glucose using 3 ATP per NADH and 2 ATP per FADH₂. Give the net value if 2 ATP are spent in transporting glycolytic NADH.

Try it first, then show the solutionSolution

Glycolysis: 2 ATP + 2 NADH × 3 = 8 ATP. (1 mark)
Link reaction: 2 NADH × 3 = 6 ATP. (1 mark)
Krebs cycle: 2 ATP + 6 NADH × 3 + 2 FADH₂ × 2 = 2 + 18 + 4 = 24 ATP. (1 mark)
Total = 8 + 6 + 24 = 38 ATP; net 36 ATP. (1 mark)

Answer: 38 ATP gross; 36 ATP net.

Example 4: RQ of different substrates · 4 marks · Short answerHarder board level

Find the RQ for (i) malic acid, C₄H₆O₅ + 3O₂ → 4CO₂ + 3H₂O, (ii) oxalic acid, 2C₂H₂O₄ + O₂ → 4CO₂ + 2H₂O. What RQ would you expect for anaerobic respiration, and for a seed whose RQ is 0.9?

Try it first, then show the solutionSolution

(i) 4/3 = 1.33. (1 mark)
(ii) 4/1 = 4. (1 mark)
Anaerobic respiration releases CO₂ without using O₂, so the RQ is infinity. (1 mark)
An RQ near 0.9 suggests protein as the substrate. (1 mark)

Answer: 1.33; 4; infinite; protein.

Example 5: Oxidative phosphorylation · 5 marks · Long answerHOTS / competency

A student has isolated mitochondria and adds a poison that makes the inner membrane freely permeable to protons. Predict what happens to (i) the oxygen consumption, (ii) the ATP synthesis, (iii) the heat produced, and explain with the chemiosmotic mechanism. Name the final product of the ETS.

Try it first, then show the solutionSolution

The proton gradient normally drives ATP synthase; if protons leak back freely the gradient collapses. (1.5 marks)
ATP synthesis falls to near zero. (1 mark)
Electron transport and oxygen consumption continue (even increase) because the pumping is no longer restrained by the gradient. (1 mark)
The energy appears as heat, so heat production increases. (1 mark)
The final product is water. (0.5 mark)

Answer: ATP synthesis falls, O₂ use continues, heat rises, water is formed.

Where students lose marks in Electron Transport, ATP Yield and Respiratory Quotient

The slips examiners see most often, and what to write instead.

Mistake: Writing RQ as O₂/CO₂
Inverting the ratio.
Do this: RQ = CO₂ evolved/O₂ consumed.
Usually costs: 2 marks.
Mistake: Giving 4 ATP for glycolysis net
Not subtracting the ATP used.
Do this: Net 2 ATP.
Usually costs: 1 mark.
Mistake: Forgetting FADH₂ in the ATP count
Counting only NADH.
Do this: Include 2 FADH₂ × 2 ATP from the Krebs cycle.
Usually costs: 1 to 2 marks.

Exam tips

Draw the ETS with complexes, mobile carriers and ATP synthase.
Show the ATP count as a table.
Write balanced equations before computing RQ.
State the RQ values for carbohydrates, fats and proteins.

Practice questions on Electron Transport, ATP Yield and Respiratory Quotient

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The RQ of carbohydrates is:
(a) 0.7
(b) 0.9
(c) 1.0
(d) more than 1
Hint
Equal volumes of CO₂ and O₂.
Answer
(c) 1.0
Q2 · 2 marks · Short answerBoard level
Name the four complexes of the electron transport system.
Hint
I to IV.
Answer
Complex I (NADH dehydrogenase), II (succinate dehydrogenase), III (cytochrome bc₁) and IV (cytochrome c oxidase).
Q3 · 3 marks · Short answerHarder board level
Why does the RQ of fats (about 0.7) differ from that of carbohydrates?
Hint
Think about the oxygen content of the molecules.
Answer
Fats contain less oxygen than carbohydrates, so more atmospheric oxygen is needed for their oxidation; the CO₂ released is less than the O₂ taken in.
Q4 · 3 marks · Short answerHarder board level
What is meant by an amphibolic pathway? Give an example.
Hint
Both breakdown and synthesis.
Answer
A pathway that functions in both catabolism and anabolism. The Krebs cycle: it breaks down acetyl CoA and its intermediates are used to make amino acids and fatty acids.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Anaerobic respiration releases less energy than aerobic respiration. Reason (R): Pyruvate is not completely oxidised in anaerobic respiration. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Compare the ATP yields.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a student measures the gas exchange of germinating seeds and finds that 100 mL of O₂ is used and 70 mL of CO₂ is released.
(i) Find the RQ. (1 mark)
(ii) Identify the likely substrate. (1 mark)
(iii) State what RQ would indicate carbohydrate respiration. (1 mark)
(iv) State the RQ if only CO₂ were released with no oxygen used. (1 mark)
Hint
RQ = CO₂/O₂.
Answer
(i) 70/100 = 0.7. (ii) Fat. (iii) 1.0. (iv) Infinity (anaerobic respiration).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Final product of ETS?Show answer
Water.
2.RQ of fats?Show answer
About 0.7.
3.ATP per NADH (traditional)?Show answer
3.
4.Site of ETS?Show answer
Inner mitochondrial membrane.

Electron Transport, ATP Yield and Respiratory Quotient: frequently asked questions

Why is the ATP count now given as 30 to 32 instead of 38?

Modern measurements give fewer than 3 ATP per NADH and 2 per FADH₂ because some of the proton gradient is used for transport; the syllabus count of 36 to 38 is a simplified value.

Why is oxygen necessary for aerobic respiration?

It accepts the electrons at the end of the ETS; without it NADH cannot be reoxidised and the cycle stops.

What is an uncoupler?

A substance that makes the membrane leak protons, so respiration continues without ATP synthesis, producing heat.