BiologyCBSE Class 11 · Respiration in Plants

Krebs Cycle, Electron Transport, the ATP Balance Sheet and RQ

Updated 17 October 2026 · Fact-checked

In the mitochondrial matrix, acetyl CoA joins oxaloacetic acid in the Krebs cycle, giving CO2, NADH, FADH2 and ATP (GTP). Electrons from NADH and FADH2 pass along the electron transport system on the inner membrane to oxygen, forming water and ATP by oxidative phosphorylation. The textbook balance sheet is 38 ATP per glucose. RQ is the volume of CO2 released divided by the volume of O2 consumed.

Understand Krebs Cycle, ETS, ATP Balance and RQ

Krebs (TCA) cycle. Takes place in the mitochondrial matrix. Acetyl CoA (2C) + oxaloacetic acid (OAA, 4C) → citric acid (6C) (citrate synthase). Citric acid is oxidised through a series of steps, losing two CO₂ (decarboxylations) and regenerating OAA.

Per acetyl CoA: 3 NADH, 1 FADH₂, 1 GTP (→ ATP) by substrate-level phosphorylation, 2 CO₂.
Per glucose (two acetyl CoA): 6 NADH, 2 FADH₂, 2 ATP, 4 CO₂.

Electron transport system (ETS) and oxidative phosphorylation. On the inner mitochondrial membrane. Electrons from NADH and FADH₂ pass through complexes I to IV: NADH dehydrogenase (I), succinate dehydrogenase with FADH₂ (II), cytochrome bc₁ (III), cytochrome c oxidase (IV), with ubiquinone and cytochrome c as carriers. Oxygen is the final electron acceptor and combines with electrons and protons to form water. The energy released pumps protons into the intermembrane space; they flow back through ATP synthase (F₀F₁), making ATP by oxidative phosphorylation. In the older textbook accounting, 1 NADH gives 3 ATP and 1 FADH₂ gives 2 ATP.

Respiratory balance sheet (the textbook's 38 ATP).
| Step | ATP |
| Glycolysis net (substrate level) | 2 |
| Glycolysis 2 NADH x 3 | 6 |
| Link reaction 2 NADH x 3 | 6 |
| Krebs 6 NADH x 3 | 18 |
| Krebs 2 FADH₂ x 2 | 4 |
| Krebs substrate level | 2 |
| Total | 38 |
Realistically less is made because of transport costs and proton leakage, but 38 is the theoretical balance sheet used here.

Amphibolic pathway. Respiration is both catabolic (breaks down carbohydrates, fats, proteins) and anabolic (intermediates are used for synthesis, e.g. acetyl CoA for fatty acids, α-ketoglutarate for amino acids).

Respiratory quotient (RQ). RQ = volume of CO₂ evolved ÷ volume of O₂ consumed.

Carbohydrates: RQ = 1 (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O).
Fats: RQ less than 1 (about 0.7; tripalmitin: 2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O + energy, so RQ = 102 ÷ 145 = 0.7).
Proteins: about 0.9.
Organic acids (oxalic acid): RQ above 1 (2 oxalic acid + O₂ → 4CO₂ + 2H₂O: RQ = 4).
Anaerobic respiration: RQ tends to infinity (CO₂ without oxygen consumed).

Draw and label: the Krebs cycle (OAA, citric acid, α-ketoglutaric acid, succinic acid, malic acid, OAA) with NADH, FADH₂, CO₂ released; and the ETS complexes.

Krebs Cycle, ETS, ATP Balance and RQ: key facts

Krebs per glucose: $6 N A D H ∣2 F A D H 2∣2 A T P ∣4 C O 2$; Two turns
Yield per carrier: $1 N A D H = 3 A T P ∣1 F A D H 2 = 2 A T P$; Textbook values
Total: $2 + 6 + 6 + 18 + 4 + 2 = 38 A T P$; Per glucose
RQ: CO2 / O2
carbohydrate 1
fat 0.7
protein 0.9
anaerobic infinity; Respiratory quotient

How to answer Krebs Cycle, ETS, ATP Balance and RQ questions

Questions ask for the ATP total, the count of carriers or an RQ.

1Count carriers per glucose in each stage.
2Multiply NADH by 3 and FADH2 by 2 (textbook values); add substrate-level ATP.
3State the final electron acceptor and the location of each stage.
4For RQ, write the balanced equation and divide CO2 by O2.
5Interpret RQ against the substrate.

Krebs Cycle, ETS, ATP Balance and RQ 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.

Germinating seeds

Suppose germinating seeds in a closed flask give an RQ of 0.7, while those in another flask give 1.

What substrates are likely respired in each?

Try it first, then show the solutionSolution

RQ of 1 points to carbohydrates.
RQ of about 0.7 points to fats as the main respiratory substrate (oil seeds).
Fats require more O2 per CO2, hence the lower value.

Answer: 1: carbohydrates; 0.7: fats.

Memory hook: More O2 needed means RQ below 1.

Tricks to remember Krebs Cycle, ETS, ATP Balance and RQ

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

6-2-2
Krebs per glucose: 6 NADH, 2 FADH2, 2 ATP.
Use it when: Counting
O2 is last
Oxygen is the final acceptor.
Use it when: ETS
3-2 yield
NADH 3 ATP, FADH2 2 ATP.
Use it when: Balance sheet

Krebs Cycle, ETS, ATP Balance and RQ: solved examples

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

Example 1: Krebs yield · 2 marks · Very short answerBoard level

List what two turns of the Krebs cycle (one glucose) give in terms of reduced carriers, ATP and gas.

Try it first, then show the solutionSolution

6 NADH, 2 FADH2 (1 mark).
2 ATP (GTP) and 4 CO2 (1 mark).

Answer: 6 NADH, 2 FADH2, 2 ATP, 4 CO2.

Example 2: ATP balance sheet · 4 marks · Short answerBoard level

Prepare the balance sheet of ATP from one glucose in aerobic respiration, using NADH = 3 ATP and FADH2 = 2 ATP. Show the total.

Try it first, then show the solutionSolution

Glycolysis: 2 ATP net + 2 NADH x 3 = 6, total 8 (1 mark).
Link: 2 NADH x 3 = 6 (1 mark).
Krebs: 6 NADH x 3 = 18, 2 FADH2 x 2 = 4, 2 ATP, total 24 (1 mark).
Sum: 8 + 6 + 24 = 38 ATP (1 mark).

Answer: 38 ATP.

Example 3: RQ of tripalmitin · 3 marks · Short answerHarder board level

2 C51H98O6 + 145 O2 -> 102 CO2 + 98 H2O. Calculate RQ and interpret.

Try it first, then show the solutionSolution

RQ = CO2 / O2 = 102 / 145 (1 mark).
= 0.703, i.e. about 0.7 (1 mark).
Less than 1 because fats have little oxygen and need more O2 for oxidation (1 mark).

Answer: RQ about 0.7.

Example 4: RQ of organic acid · 3 marks · Short answerHarder board level

2 (COOH)2 + O2 -> 4 CO2 + 2 H2O. Find the RQ and say what kind of substrate gives such RQ.

Try it first, then show the solutionSolution

RQ = 4 / 1 (1 mark).
= 4 (1 mark).
Organic acids are already oxidised, need little O2, so RQ above 1 (1 mark).

Answer: RQ = 4.

Example 5: Cyanide poisoning · 5 marks · Long answerHOTS / competency

Suppose cyanide blocks cytochrome c oxidase. Predict the effect on NADH oxidation, proton pumping, ATP made by oxidative phosphorylation and oxygen use, and why the cell still makes some ATP.

Try it first, then show the solutionSolution

Electrons cannot reach oxygen, so the chain backs up and NADH is not reoxidised (1 mark).
Proton pumping and the gradient fall (1 mark).
Oxidative phosphorylation stops and O2 consumption falls (1 mark).
Glycolysis can still give a little ATP by substrate level phosphorylation if NAD+ is regenerated by fermentation (1 mark).
Krebs cycle stalls because NAD+ is not regenerated; the cell is starved of ATP (1 mark).

Answer: ATP collapses; glycolysis only.

Where students lose marks in Krebs Cycle, ETS, ATP Balance and RQ

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

Mistake: FADH2 as 3 ATP
Carrier yields swapped.
Wrong working
1 FADH2 gives 3 ATP.
Right working
FADH2 gives 2; NADH gives 3.
Do this: NADH 3.
Usually costs: 1 mark.
Mistake: ETS in matrix
Location wrong.
Wrong working
ETS occurs in the matrix.
Right working
Inner mitochondrial membrane.
Do this: Membrane.
Usually costs: 1 mark.
Mistake: RQ of fat above 1
Direction mistaken.
Wrong working
Fat RQ is above 1.
Right working
Fat RQ is about 0.7.
Do this: Fat below 1.
Usually costs: 1 mark.

Exam tips

Make the ATP table in the answer.
State the textbook yields used.
Write the balanced equation for RQ.
Name the final electron acceptor.

Practice questions on Krebs Cycle, ETS, ATP Balance and RQ

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 final electron acceptor in the ETS is
(a) NAD+ (b) FAD (c) oxygen (d) cytochrome c
Hint
Last in chain.
Answer
(c).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The RQ for carbohydrates is 1.
Reason (R): Equal volumes of CO2 are released and O2 consumed when glucose is oxidised.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
6 CO2 for 6 O2.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
Calculate the RQ for the oxidation of glucose, and state the RQ in anaerobic respiration.
Hint
Equation.
Answer
C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O gives 6/6 = 1. In anaerobic respiration RQ tends to infinity.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a seed sample gives out 8 mL CO2 while taking in 10 mL O2.
(a) Find RQ. (1)
(b) Name the likely substrate. (1)
(c) Give another substrate with RQ above 1. (2)
Hint
8/10.
Answer
(a) RQ = 8/10 = 0.8. (b) A mixture, between fat (0.7) and carbohydrate (1), or protein (about 0.9). (c) An organic acid such as oxalic acid.
Q5 · 4 marks · Long answerHOTS / competency
Suppose a cell oxidises 3 glucose molecules aerobically. Find the NADH, FADH2, CO2 and ATP (balance sheet) totals, and compare with the same 3 glucose fermented to lactic acid.
Hint
Multiply by 3.
Answer
NADH: glycolysis 2 + link 2 + Krebs 6 = 10 per glucose, so 30; FADH2: 2 x 3 = 6; CO2: 2 (link) + 4 (Krebs) = 6 per glucose, so 18; ATP = 38 x 3 = 114. Fermented to lactic acid: net 2 x 3 = 6 ATP, no CO2. Aerobic yields 19 times as much.

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.Where is the Krebs cycle?Show answer
Mitochondrial matrix.
2.Final electron acceptor?Show answer
Oxygen.
3.ATP from one NADH?Show answer
3 (textbook).
4.RQ of carbohydrate?Show answer
1.
5.What is oxidative phosphorylation?Show answer
ATP synthesis driven by ETS.
6.What is amphibolic?Show answer
Both breaking down and building up.

Krebs Cycle, ETS, ATP Balance and RQ: frequently asked questions

What is the TCA cycle?

Another name for the Krebs cycle.

How much ATP does one glucose give?

38 in the textbook balance sheet.

What is RQ?

Ratio of CO2 evolved to O2 used.

Why is respiration amphibolic?

Intermediates are used for synthesis as well.

What is ATP synthase?

An enzyme making ATP using a proton gradient.