BiologyISC Class 11 · Respiration in Plants

Glycolysis, Fermentation, Link Reaction and the Krebs Cycle

Updated 17 October 2026 · Fact-checked

Glycolysis in the cytoplasm converts one glucose into two pyruvate with a net gain of 2 ATP and 2 NADH. In the absence of oxygen pyruvate is fermented to ethanol and CO₂ or to lactic acid; in its presence pyruvate enters the mitochondria, forms acetyl CoA in the link reaction, and is oxidised in the Krebs cycle to CO₂, NADH and FADH₂.

Understand Glycolysis, Fermentation and the Krebs Cycle

Glycolysis (EMP pathway). Occurs in the cytoplasm of all living cells. Glucose (6C) is phosphorylated twice using 2 ATP to fructose-1,6-bisphosphate, which splits into two 3-carbon sugars (glyceraldehyde-3-phosphate, PGAL, and dihydroxyacetone phosphate). Each PGAL is oxidised, with NAD⁺ reduced to NADH, and converted in steps to pyruvic acid, with production of 4 ATP (substrate-level phosphorylation). Net: 2 pyruvate, 2 ATP, 2 NADH per glucose.

Fermentation. In anaerobic conditions pyruvate is converted to ethanol and CO₂ (yeast; pyruvic acid decarboxylase and alcohol dehydrogenase) or to lactic acid (animal muscle in strenuous exercise; lactate dehydrogenase). The NADH is reoxidised to NAD⁺, which lets glycolysis continue. Only a small part of the energy of glucose is released (net 2 ATP per glucose).

Link reaction. In the mitochondrial matrix, pyruvate + CoA + NAD⁺ → acetyl CoA + CO₂ + NADH (pyruvate dehydrogenase). Two pyruvates per glucose give 2 acetyl CoA, 2 CO₂ and 2 NADH.

Krebs cycle (TCA cycle). In the matrix. Acetyl CoA (2C) condenses with oxaloacetic acid (4C) to citric acid (6C), which is oxidised stepwise, releasing 2 CO₂ with regeneration of OAA. Each turn yields 3 NADH, 1 FADH₂ and 1 ATP (or GTP). Per glucose there are 2 turns: 6 NADH, 2 FADH₂, 2 ATP and 4 CO₂.

Overall (before the electron transport system). Per glucose: glycolysis (2 ATP, 2 NADH) + link reaction (2 NADH) + Krebs cycle (2 ATP, 6 NADH, 2 FADH₂) = 4 ATP, 10 NADH, 2 FADH₂, and 6 CO₂ released.

Glycolysis, Fermentation and the Krebs Cycle: key facts

Glycolysis: $Gl u cose \to 2 p y r uv a t e + 2 A T P (n e t) + 2 N A D H$; 4 ATP made, 2 used; in the cytoplasm.
Fermentation: 1.Pyruvate
2.ethanol + CO₂ | Pyruvate
3.lactic acid; Yeast; muscle and lactic acid bacteria.
Link reaction: Pyruvate + CoA + NAD⁺ → acetyl CoA + CO₂ + NADH; Mitochondrial matrix.
Krebs cycle (per turn): $3 N A D H + 1 F A D H_{2} + 1 A T P + 2 C O_{2}$; Two turns per glucose.

Glycolysis, Fermentation and the Krebs Cycle 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.

Bread dough rising

Suppose a baker mixes yeast in dough and leaves it in a warm place, where the dough rises and smells of alcohol.

Explain the process and the products.

Try it first, then show the solutionSolution

Yeast respires anaerobically in the dough: glucose → 2 pyruvate (glycolysis) → 2 ethanol + 2 CO₂.
The CO₂ is trapped in the dough, making it rise.
Ethanol evaporates during baking; the NADH is reoxidised so that glycolysis continues.

Answer: Alcoholic fermentation: ethanol and CO₂ from glucose.

Memory hook: Fermentation regenerates NAD⁺ for glycolysis.

Tricks to remember Glycolysis, Fermentation and the Krebs Cycle

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

Glycolysis: 4 made, 2 used
A net gain of 2 ATP per glucose.
Use it when: Counting ATP.
TCA per glucose is doubled
Count 2 turns for every glucose.
Use it when: Krebs cycle products.

Glycolysis, Fermentation and the Krebs Cycle: solved examples

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

Example 1: Net ATP in glycolysis · 1 mark · MCQBoard level

The net gain of ATP in glycolysis per glucose molecule is:
(a) 1
(b) 2
(c) 4
(d) 8

Try it first, then show the solutionSolution

Four ATP are made and two are used: net 2.

Answer: (b) 2

Example 2: Glycolysis · 3 marks · Short answerBoard level

State the site and the end products of glycolysis and explain what is meant by substrate-level phosphorylation.

Try it first, then show the solutionSolution

Site: the cytoplasm. (0.5 mark)
End products per glucose: 2 pyruvic acid, 2 NADH and a net of 2 ATP. (1 mark)
Substrate-level phosphorylation is the direct transfer of a phosphate from a substrate to ADP to form ATP, without the electron transport chain. (1.5 marks)

Answer: Cytoplasm; 2 pyruvate + 2 NADH + 2 ATP net.

Example 3: Fermentation · 4 marks · Short answerHarder board level

Compare alcoholic fermentation and lactic acid fermentation. Why does fermentation not stop glycolysis?

Try it first, then show the solutionSolution

Alcoholic: pyruvate → ethanol + CO₂ in yeast. (1 mark)
Lactic acid: pyruvate → lactic acid in muscle and lactic acid bacteria (curd). (1 mark)
In both, NADH is oxidised back to NAD⁺, which is needed for glycolysis to continue. (1 mark)
Net 2 ATP is made per glucose. (1 mark)

Answer: Ethanol + CO₂ versus lactic acid; NAD⁺ is regenerated.

Example 4: Krebs cycle accounting · 4 marks · Short answerHarder board level

Calculate the NADH, FADH₂, ATP and CO₂ produced from two molecules of pyruvate in the link reaction and Krebs cycle together.

Try it first, then show the solutionSolution

Link reaction (2 pyruvates): 2 NADH and 2 CO₂. (1 mark)
Krebs cycle (2 turns): 6 NADH, 2 FADH₂, 2 ATP and 4 CO₂. (1.5 marks)
Totals: 8 NADH, 2 FADH₂, 2 ATP, 6 CO₂. (1.5 marks)

Answer: 8 NADH, 2 FADH₂, 2 ATP, 6 CO₂.

Example 5: Efficiency of fermentation · 5 marks · Long answerHOTS / competency

Taking the energy released by the complete oxidation of one mole of glucose as 2870 kJ and the energy of the phosphate bond in ATP as 30.6 kJ, calculate the percentage of energy retained as ATP in (i) fermentation or glycolysis (net 2 ATP) and (ii) aerobic respiration (38 ATP). State where the rest of the energy goes.

Try it first, then show the solutionSolution

(i) 2 × 30.6 = 61.2 kJ; efficiency = 61.2/2870 × 100 = 2.1%. (1.5 marks)
(ii) 38 × 30.6 = 1162.8 kJ; efficiency = 1162.8/2870 × 100 = 40.5%. (1.5 marks)
Aerobic respiration is about 19 times as efficient as glycolysis alone (40.5/2.1). (1 mark)
The rest is lost as heat. (1 mark)

Answer: 2.1% in glycolysis; 40.5% in aerobic respiration; the rest is lost as heat.

Where students lose marks in Glycolysis, Fermentation and the Krebs Cycle

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

Mistake: Net ATP stated as 4
Forgetting the two ATP used.
Do this: Net ATP in glycolysis is 2.
Usually costs: 1 mark.
Mistake: Counting a single turn of the Krebs cycle per glucose
Halving the products.
Do this: Two turns occur per glucose.
Usually costs: 1 to 2 marks.
Mistake: Placing glycolysis in mitochondria
Wrong location.
Do this: Glycolysis occurs in the cytoplasm.
Usually costs: 1 mark.

Exam tips

Draw a flow chart of respiration with the site of each stage.
Write products per glucose in a table.
Link fermentation to the regeneration of NAD⁺.
Use units (kJ, mol) correctly in energy calculations.

Practice questions on Glycolysis, Fermentation and the Krebs Cycle

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 link reaction takes place in the:
(a) cytoplasm
(b) mitochondrial matrix
(c) inner mitochondrial membrane
(d) chloroplast
Hint
Pyruvate enters the mitochondria.
Answer
(b) mitochondrial matrix
Q2 · 2 marks · Short answerBoard level
Write the equation of alcoholic fermentation of one mole of glucose.
Hint
Ethanol and CO₂.
Answer
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (with the net formation of 2 ATP).
Q3 · 3 marks · Short answerHarder board level
Which compound is formed when acetyl CoA combines with oxaloacetic acid, and how many carbons does it have?
Hint
The first compound of the Krebs cycle.
Answer
Citric acid, a six-carbon compound.
Q4 · 3 marks · Short answerHarder board level
Why is it said that the Krebs cycle is a cycle? Which compound is regenerated?
Hint
The starting acid returns.
Answer
Because the oxaloacetic acid that accepts acetyl CoA is regenerated at the end of the sequence, so the process can repeat.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Muscles produce lactic acid in strenuous exercise. Reason (R): Oxygen supply is inadequate, so pyruvate is reduced to lactic acid. 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
Think about oxygen and NAD⁺.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a yeast culture is grown with glucose with and without air.
(i) Name the products without air. (1 mark)
(ii) Name the products with air. (1 mark)
(iii) State the site of glycolysis. (1 mark)
(iv) State the net ATP from glycolysis alone. (1 mark)
Hint
Compare fermentation and aerobic respiration.
Answer
(i) Ethanol and CO₂. (ii) CO₂ and water (with much more ATP). (iii) The cytoplasm. (iv) Two.

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.Site of the Krebs cycle?Show answer
Mitochondrial matrix.
2.NADH per turn of Krebs cycle?Show answer
3.
3.End product in yeast fermentation?Show answer
Ethanol and CO₂.
4.First compound of the Krebs cycle?Show answer
Citric acid.

Glycolysis, Fermentation and the Krebs Cycle: frequently asked questions

Why is the cycle named the citric acid cycle?

Because citric acid is formed in its first step, when acetyl CoA joins oxaloacetic acid.

Do all cells carry out glycolysis?

Yes, it is the common first step of respiration in all living cells, aerobic or anaerobic.

Can plants ferment?

Plant tissues can respire anaerobically (for instance in waterlogged roots or germinating seeds) producing ethanol and CO₂.