BiologyCBSE Class 11 · Respiration in Plants

Glycolysis, Fermentation and the Link Reaction

Updated 17 October 2026 · Fact-checked

Glycolysis (EMP pathway) occurs in the cytoplasm and breaks one glucose into two pyruvic acid molecules with a net gain of 2 ATP and 2 NADH. Pyruvic acid then goes to fermentation (alcoholic or lactic) in the absence of oxygen, or, in aerobic respiration, to the link reaction in mitochondria, forming acetyl CoA, CO2 and NADH.

Glycolysis in the cytoplasm gives 2 pyruvic acid, net 2 ATP and 2 NADH; without O₂ it ferments, with O₂ it enters the mitochondrion.

Illustration drawn for QuestionBuddy.

Understand Glycolysis, Fermentation and Link Reaction

Respiration is the oxidation of food to release energy, which is stored in ATP. Plants have no special breathing organs; gas exchange is by diffusion.

Glycolysis (Embden-Meyerhof-Parnas pathway, EMP) occurs in the cytoplasm of all living organisms, with or without oxygen. Glucose (6C) is partially oxidised to two molecules of pyruvic acid (3C).

Glucose is phosphorylated (by hexokinase) to glucose-6-phosphate using ATP, which isomerises to fructose-6-phosphate; a second ATP is used by phosphofructokinase to give fructose-1,6-bisphosphate. 2 ATP used.
Fructose-1,6-bisphosphate splits into two triose phosphates (DHAP and PGAL, interconvertible).
Each PGAL is oxidised, with NAD⁺ reduced to NADH + H⁺ and inorganic phosphate attached, giving 1,3-bisphosphoglyceric acid (BPGA): 2 NADH per glucose.
BPGA to 3-PGA gives ATP by substrate-level phosphorylation (2 ATP), and PEP to pyruvic acid gives 2 more ATP.
Count: ATP used 2; ATP made 4; net gain 2 ATP and 2 NADH per glucose.

Fate of pyruvic acid. Depends on the cell:

Fermentation (no oxygen): incomplete oxidation.
Alcoholic: pyruvic acid → acetaldehyde + CO₂ (pyruvic acid decarboxylase) → ethanol (alcohol dehydrogenase), with NADH oxidised to NAD⁺. In yeast. Used in brewing and baking.
Lactic acid: pyruvic acid → lactic acid (lactate dehydrogenase), no CO₂ released; in muscle cells during exercise when oxygen is limited, and in some bacteria.
Fermentation releases less than 7 per cent of the energy in glucose; the net gain remains 2 ATP; hazardous products (alcohol) limit yeast.
Aerobic respiration (oxygen): pyruvic acid enters mitochondria.

Link reaction (oxidative decarboxylation). In the mitochondrial matrix, pyruvic acid + CoA + NAD⁺ → acetyl CoA + CO₂ + NADH + H⁺, catalysed by pyruvate dehydrogenase, needing Mg²⁺. One glucose gives two pyruvic acids, so 2 acetyl CoA, 2 CO₂, 2 NADH. The acetyl CoA enters the Krebs cycle.

Table: fermentation versus aerobic. Site: cytoplasm versus mitochondria. Products: ethanol/lactic acid versus CO₂ and water. ATP: 2 versus up to 38.

Draw and label: flow chart of glucose → glucose-6-phosphate → fructose-1,6-bisphosphate → 2 PGAL → 2 pyruvic acid, marking ATP used and made.

Glycolysis, Fermentation and Link Reaction: key facts

Glycolysis: Glucose -> 2 pyruvic acid
net 2 ATP
2 NADH; Cytoplasm
ATP count: $U se d 2∣ ma d e 4∣ n e t 2$; Per glucose
Alcoholic fermentation: Pyruvic acid -> acetaldehyde + CO2 -> ethanol; Yeast
Link reaction: Pyruvic acid -> acetyl CoA + CO2 + NADH; Matrix

How to answer Glycolysis, Fermentation and Link Reaction questions

Questions ask for ATP count, site or product.

1Name the site: cytoplasm or mitochondrial matrix.
2Count ATP used, made and net.
3State the pathway from pyruvic acid for the condition given (oxygen present or absent).
4Include CO2 in alcoholic but not lactic fermentation.
5For link reaction, multiply by two per glucose.

Glycolysis, Fermentation and Link Reaction 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 rises

Suppose bread dough with yeast doubles in size in a warm place and smells slightly of alcohol.

Explain the rise and smell.

Try it first, then show the solutionSolution

Yeast ferments sugars in the dough.
Pyruvic acid goes to acetaldehyde and CO2, then to ethanol.
The CO2 forms bubbles that raise the dough; ethanol gives the smell and mostly evaporates when baked.

Answer: Alcoholic fermentation makes CO2 and ethanol.

Memory hook: Yeast: gas lifts, alcohol smells.

Tricks to remember Glycolysis, Fermentation and Link Reaction

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

2 in, 4 out, 2 net
Glycolysis: ATP used 2, made 4, net 2.
Use it when: ATP count
C-to-C
Cytoplasm for glycolysis, mitochondrial matrix for the link reaction.
Use it when: Sites
Lactic has no CO2
Lactic fermentation releases no CO2.
Use it when: Difference

Glycolysis, Fermentation and Link Reaction: solved examples

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

Example 1: ATP count in glycolysis · 3 marks · Short answerBoard level

Show the ATP used, ATP made and net ATP in glycolysis of one glucose, and the NADH formed.

Try it first, then show the solutionSolution

ATP used: 2 (hexokinase step and PFK step) (1 mark).
ATP made: 4 by substrate-level phosphorylation, net 2 (1 mark).
NADH formed: 2 (1 mark).

Answer: Used 2, made 4, net 2; 2 NADH.

Example 2: Alcoholic fermentation · 3 marks · Short answerBoard level

Write the steps of alcoholic fermentation from pyruvic acid and name the enzymes.

Try it first, then show the solutionSolution

Pyruvic acid to acetaldehyde + CO2 by pyruvic acid decarboxylase (1 mark).
Acetaldehyde to ethanol by alcohol dehydrogenase (1 mark).
NADH is oxidised to NAD+, allowing glycolysis to continue (1 mark).

Answer: Decarboxylase then dehydrogenase.

Example 3: Link reaction count · 3 marks · Short answerHarder board level

For 1 glucose, give the products of the link reaction and total NADH from glycolysis plus link reaction.

Try it first, then show the solutionSolution

Two pyruvic acids give 2 acetyl CoA (1 mark).
2 CO2 and 2 NADH released in the link reaction (1 mark).
Total NADH = 2 (glycolysis) + 2 (link) = 4 (1 mark).

Answer: 2 acetyl CoA, 2 CO2; 4 NADH.

Example 4: Muscle fatigue · 4 marks · Short answerHarder board level

During sprinting, muscle cells make lactic acid. Explain why, state the gain in ATP per glucose and why this limits performance.

Try it first, then show the solutionSolution

Oxygen supply is limited, so pyruvic acid is reduced to lactic acid (1 mark).
NADH is reoxidised, allowing glycolysis to continue (1 mark).
Net gain stays 2 ATP per glucose (1 mark).
Low yield and acid build-up limit sustained effort (1 mark).

Answer: Lactic acid; 2 ATP; fatigue.

Example 5: Why ferment? · 4 marks · Short answerHOTS / competency

Fermentation yields much less energy but organisms still use it. Explain the benefit of reoxidising NADH, and give the percentage energy limit mentioned in textbooks.

Try it first, then show the solutionSolution

Glycolysis needs NAD+ to continue (1 mark).
Fermentation regenerates NAD+ from NADH (1 mark).
So ATP production continues without oxygen (1 mark).
Less than 7 per cent of glucose energy is released (1 mark).

Answer: Regenerates NAD+; under 7 per cent energy.

Where students lose marks in Glycolysis, Fermentation and Link Reaction

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

Mistake: Net ATP 4
Gross taken as net.
Wrong working
Glycolysis nets 4 ATP.
Right working
Net is 2.
Do this: 4 minus 2.
Usually costs: 1 mark.
Mistake: CO2 in lactic
Pathways mixed.
Wrong working
Lactic fermentation releases CO2.
Right working
It does not.
Do this: No decarboxylation.
Usually costs: 1 mark.
Mistake: Link reaction in cytoplasm
Site wrong.
Wrong working
Link reaction in cytoplasm.
Right working
Mitochondrial matrix.
Do this: Matrix.
Usually costs: 1 mark.

Exam tips

Always state ATP used, made, net.
Name enzymes in fermentation.
Mention NAD+ regeneration.
Scale by two for per-glucose link reaction.

Practice questions on Glycolysis, Fermentation and Link Reaction

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 net gain of ATP in glycolysis is
(a) 2 (b) 4 (c) 8 (d) 38
Hint
Used 2, made 4.
Answer
(a).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Lactic acid fermentation does not release CO2.
Reason (R): Pyruvic acid is directly reduced to lactic acid.
(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
No decarboxylation.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
Where in the cell do (i) glycolysis and (ii) the link reaction occur?
Hint
Two places.
Answer
(i) Cytoplasm. (ii) Mitochondrial matrix.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a yeast culture is kept without air and gives off gas bubbles and an alcoholic smell.
(a) Name the process. (1)
(b) Name the gas. (1)
(c) Name the two enzymes. (2)
Hint
Fermentation.
Answer
(a) Alcoholic fermentation. (b) CO2. (c) Pyruvic acid decarboxylase and alcohol dehydrogenase.
Q5 · 4 marks · Long answerHOTS / competency
Suppose 5 glucose molecules undergo glycolysis. Find the total ATP used, made, net, and NADH formed. Then find the acetyl CoA produced if all pyruvate enters the link reaction.
Hint
Multiply by five.
Answer
ATP used = 10; made = 20; net = 10; NADH = 10; pyruvic acid = 10, so acetyl CoA = 10 (plus 10 CO2 and 10 NADH).

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 glycolysis?Show answer
Cytoplasm.
2.Net ATP in glycolysis?Show answer
2.
3.Enzyme of link reaction?Show answer
Pyruvate dehydrogenase.
4.Product of lactic fermentation?Show answer
Lactic acid.
5.Yeast ferments to?Show answer
Ethanol and CO2.
6.Energy released in fermentation?Show answer
Under 7 per cent.

Glycolysis, Fermentation and Link Reaction: frequently asked questions

What is glycolysis?

Breakdown of glucose to pyruvic acid.

What is fermentation?

Incomplete oxidation without oxygen.

What is acetyl CoA?

The 2C product of the link reaction.

Is glycolysis aerobic?

It needs no oxygen.

What is substrate-level phosphorylation?

ATP formed directly from a substrate.