Anaerobic Respiration

BiologyRespiration in PlantsFor NEET aspirants

Anaerobic respiration is the incomplete oxidation of glucose without oxygen, carried out by fermentation. This page covers alcoholic fermentation in yeast, lactic acid fermentation in some bacteria and in muscles during exercise, the enzymes involved, why NADH + H⁺ is reoxidised, the energy and ATP yield, the 13 per cent alcohol limit, and how anaerobic respiration compares with aerobic respiration. It follows the NCERT Class 11 chapter Respiration in Plants. NEET asks enzyme-product pairs, the energy fraction and the net ATP yield.

On this page1Fermentation2Alcoholic fermentation3Lactic acid fermentation4Energy yield5Fermentation vs aerobic6Exam essentials7Quick revision8Solved examples9Practice

Key Points at a Glance

★ Must learn Fermentation: incomplete oxidation of glucose under anaerobic conditions.
Occurs in many prokaryotes, unicellular eukaryotes and germinating seeds.
★ Must learn Yeast: pyruvic acid to ethanol + $C O_{2}$ by pyruvic acid decarboxylase and alcohol dehydrogenase.
Some bacteria make lactic acid from pyruvic acid.
★ Must learn Muscles during exercise (oxygen inadequate): pyruvic acid reduced to lactic acid by lactate dehydrogenase.
The reducing agent is $NADH + H^{+}$ , which is reoxidised to $NA D^{+}$ in both fermentations.
★ Must learn Energy released: less than 7 per cent of the energy in glucose; not all of it is trapped as ATP.
Net gain: 2 ATP per glucose fermented (all from glycolysis).
Yeasts poison themselves to death at about 13 per cent alcohol.
★ Must learn Fermentation: partial breakdown, net 2 ATP, slow NADH oxidation. Aerobic: complete breakdown, many more ATP, vigorous NADH oxidation.

1. Fermentation: Where It Fits

Glycolysis turns glucose into pyruvic acid in every organism.
Cells handle this pyruvic acid in three major ways: lactic acid fermentation, alcoholic fermentation and aerobic respiration.
The first two take place without oxygen; together they are the major pathways of anaerobic respiration.
★ Exam imp Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes.
★ Exam imp Fermentation also takes place in germinating seeds.

★ Very important Fermentation: the incomplete oxidation of glucose under anaerobic conditions, in which pyruvic acid is converted to ethanol and $C O_{2}$ , or to lactic acid.

Key idea

Fermentation is glycolysis followed by one extra conversion of pyruvic acid, all without oxygen.

2. Alcoholic Fermentation

In fermentation by yeast, glucose is incompletely oxidised under anaerobic conditions.
A set of reactions converts pyruvic acid to $C O_{2}$ and ethanol.
★ Exam imp Two enzymes catalyse these reactions: pyruvic acid decarboxylase and alcohol dehydrogenase.

Glycolysis breaks glucose down to pyruvic acid in the cytoplasm.
Pyruvic acid decarboxylase acts on pyruvic acid, and $C O_{2}$ is released.
Alcohol dehydrogenase completes the conversion to ethanol, using $NADH + H^{+}$ , which is reoxidised to $NA D^{+}$ .

Extra Depth: Pyruvic acid decarboxylase first removes $C O_{2}$ from pyruvic acid, forming acetaldehyde. Alcohol dehydrogenase then reduces acetaldehyde to ethanol using $NADH + H^{+}$ .

★ Exam imp Yeasts poison themselves to death when the alcohol concentration reaches about 13 per cent.
So the maximum alcohol content of a naturally fermented beverage is about 13 per cent.

Extra Depth: Beverages with more than about 13 per cent alcohol are obtained by distillation of the fermented liquid, which concentrates the alcohol.

Memory Trick

Alcohol needs two enzymes; lactic acid needs one. Alcoholic: pyruvic acid decarboxylase (removes $C O_{2}$ ) and alcohol dehydrogenase. Lactic: lactate dehydrogenase only, so no $C O_{2}$ is released.

Key idea

Yeast turns pyruvic acid into ethanol and CO₂, but dies when the alcohol reaches about 13 per cent.

3. Lactic Acid Fermentation

Some bacteria produce lactic acid from pyruvic acid.
★ Exam imp Animal cells, such as muscles during exercise, also do this when oxygen is inadequate for cellular respiration.
★ Exam imp In muscle, pyruvic acid is reduced to lactic acid by the enzyme lactate dehydrogenase.
In both lactic acid and alcoholic fermentation, the reducing agent is $NADH + H^{+}$ .
★ Exam imp This $NADH + H^{+}$ is reoxidised to $NA D^{+}$ in both processes.

Test yourself: hide labels

Figure 1: Major pathways of anaerobic respiration.

NADH + H^{+}

made when glyceraldehyde 3-phosphate is oxidised is used again to reduce pyruvic acid, so

NA D^{+}

is regenerated on both branches. Open arrows stand for several steps.

Alcoholic fermentation

Organism: yeast
Products: ethanol + $C O_{2}$
Enzymes: pyruvic acid decarboxylase, alcohol dehydrogenase
$C O_{2}$ released: yes

Lactic acid fermentation

Organisms: some bacteria; muscle cells in exercise
Product: lactic acid
Enzyme: lactate dehydrogenase
$C O_{2}$ released: no

Quick Recall: tap to check

Name the enzyme that reduces pyruvic acid to lactic acid in muscles.

Lactate dehydrogenase.

Which type of fermentation releases

C O_{2}

Alcoholic fermentation (pyruvic acid to ethanol +

C O_{2}

In Figure 1, what is the reducing agent on both branches from pyruvic acid?

NADH + H^{+}

, which is reoxidised to

NA D^{+}

When do muscle cells make lactic acid?

During exercise, when oxygen is inadequate for cellular respiration.

Key idea

Both fermentations use NADH + H⁺ to reduce pyruvic acid (or its product) and so regenerate NAD⁺.

4. Energy Yield and Drawbacks of Fermentation

★ Exam imp Not much energy is released in fermentation: less than seven per cent of the energy in glucose.
Not all of even this small amount is trapped as high-energy bonds of ATP.
★ Exam imp The processes are also hazardous: either acid or alcohol is produced.

Count per glucose fermented	Value
ATP synthesised in glycolysis	4
ATP used in glycolysis	2
ATP formed when pyruvic acid becomes ethanol or lactic acid	0
Net ATP gain	2
$NADH + H^{+}$ formed in glycolysis	2, reoxidised to $NA D^{+}$ during fermentation

★ Very important Net gain in fermentation: 2 ATP per glucose, whether the product is ethanol or lactic acid. Both ATP come from glycolysis.

Extra Depth: Reoxidising $NADH + H^{+}$ returns $NA D^{+}$ to the cell, so that the oxidation of glyceraldehyde 3-phosphate in glycolysis can keep going without oxygen.

Numbers to Remember

Less than 7 per cent of the energy in glucose is released in fermentation.
About 13 per cent alcohol kills yeast; this is the upper limit for naturally fermented beverages.
Net 2 ATP per glucose fermented to alcohol or lactic acid.
2 enzymes in alcoholic fermentation; 1 enzyme (lactate dehydrogenase) in lactic acid formation.
3 major fates of pyruvic acid, 2 of them anaerobic.

Memory Trick

Seven for energy, thirteen for ethanol. Fermentation releases under 7 per cent of glucose energy, and yeast stops at about 13 per cent ethanol.

Key idea

Fermentation is wasteful: under 7 per cent of glucose energy is released, only 2 ATP are gained, and acid or alcohol accumulates.

5. Fermentation and Aerobic Respiration Compared

To extract more of the energy in glucose and make many more ATP, cells need complete oxidation.
Aerobic respiration: complete oxidation of organic substances in the presence of oxygen, releasing $C O_{2}$ , water and a large amount of energy.
In eukaryotes these steps take place within the mitochondria, and they need $O_{2}$ .
Aerobic respiration is the most common type of respiration in higher organisms.

Feature	Fermentation (anaerobic)	Aerobic respiration
Breakdown of glucose	Partial	Complete, to $C O_{2}$ and $H_{2} O$
Net ATP per glucose	Only 2 (glucose to pyruvic acid)	Many more (38 in theory)
Oxidation of NADH to $NA D^{+}$	Rather slow	Very vigorous
Oxygen	Not needed	Needed
Site in eukaryotes	Cytoplasm	Cytoplasm (glycolysis) and mitochondria
End products	Ethanol + $C O_{2}$ , or lactic acid	$C O_{2}$ + $H_{2} O$
Energy released	Less than 7 per cent of glucose energy	A large amount

NEET Focus

"Rather slow" belongs to fermentation; "very vigorous" belongs to aerobic respiration. Statement questions swap these, and also claim that lactic acid fermentation releases $C O_{2}$ or that fermentation gives 4 ATP.

Memory Trick

Fermentation is Partial, Two, Slow: partial breakdown, 2 ATP, slow NADH oxidation. Aerobic respiration is the opposite on all three: complete breakdown, many ATP, vigorous NADH oxidation.

Tips and Tricks

Spot the process by its end products: ethanol + $C O_{2}$ means yeast (alcoholic); lactic acid alone means bacteria or exercising muscle; $C O_{2}$ + $H_{2} O$ means aerobic respiration.

Quick Recall: tap to check

Which process oxidises NADH to

NA D^{+}

very vigorously?

Aerobic respiration; in fermentation the oxidation is rather slow.

Where do the steps of complete oxidation take place in eukaryotes?

Within the mitochondria, using

O_{2}

What are the end products of complete oxidation of glucose?

C O_{2}

and water, with a large amount of energy.

Key idea

Fermentation stops at partial breakdown with 2 ATP; aerobic respiration oxidises glucose completely for far more ATP.

6. Exam Essentials

Pairs to Match

List I	List II
Yeast	Ethanol + $C O_{2}$
Some bacteria	Lactic acid from pyruvic acid
Muscles during exercise	Lactic acid when oxygen is inadequate
Pyruvic acid decarboxylase	Alcoholic fermentation; $C O_{2}$ released
Alcohol dehydrogenase	Ethanol formed
Lactate dehydrogenase	Pyruvic acid reduced to lactic acid
$NADH + H^{+}$	Reducing agent, reoxidised to $NA D^{+}$
About 13 per cent alcohol	Yeasts poison themselves to death
Less than 7 per cent	Share of glucose energy released by fermentation
Net 2 ATP	Gain from fermenting one glucose
Germinating seeds	Carry out fermentation
Mitochondria	Site of complete oxidation in eukaryotes

Exceptions

Lactic acid fermentation releases no $C O_{2}$ ; only alcoholic fermentation does.
Animal cells are mostly aerobic, yet muscles during exercise carry out lactic acid fermentation.
Fermentation gives no extra ATP beyond glycolysis.
Even the small energy released (under 7 per cent) is not all trapped as ATP.
Yeast cannot make a naturally fermented beverage stronger than about 13 per cent alcohol.
NADH is oxidised slowly in fermentation, unlike the vigorous oxidation in aerobic respiration.

Examples to Remember

Example	What it shows
Yeast	Alcoholic fermentation
Some bacteria	Lactic acid fermentation
Muscle cells during exercise	Lactic acid fermentation
Germinating seeds	Fermentation under anaerobic conditions
Many prokaryotes and unicellular eukaryotes	Fermentation under anaerobic conditions

Memory Trick

Yeast gives Ethanol; muscle gives Lactic. Pair the first letters: Y-E for yeast-ethanol, and remember that exercising muscle feels the "lactic" burn.

7. Quick Revision

Pyruvic acid has three fates: lactic acid fermentation, alcoholic fermentation and aerobic respiration.
Fermentation is incomplete oxidation of glucose under anaerobic conditions.
It occurs in many prokaryotes, unicellular eukaryotes and germinating seeds.
Yeast converts pyruvic acid to ethanol and $C O_{2}$ using pyruvic acid decarboxylase and alcohol dehydrogenase.
Some bacteria make lactic acid from pyruvic acid.
Muscles during exercise reduce pyruvic acid to lactic acid by lactate dehydrogenase when oxygen is inadequate.
$NADH + H^{+}$ is the reducing agent and is reoxidised to $NA D^{+}$ in both processes.
Less than 7 per cent of glucose energy is released, and not all of it is trapped as ATP.
Fermentation is hazardous because acid or alcohol accumulates.
Net gain: 2 ATP per glucose fermented, all from glycolysis.
Yeasts die at about 13 per cent alcohol, so naturally fermented beverages hold at most about 13 per cent.
Aerobic respiration oxidises glucose completely in mitochondria using $O_{2}$ and yields a large amount of energy.
Fermentation: partial breakdown, 2 ATP, slow NADH oxidation. Aerobic: complete breakdown, many ATP, vigorous NADH oxidation.

8. Solved Examples

Solved Example 1

What is the net number of ATP synthesised when one molecule of glucose is fermented to alcohol or to lactic acid?
(A) 0
(B) 2
(C) 4
(D) 38

Solution:

Answer: (B). Glycolysis synthesises 4 ATP and uses 2, a net gain of 2. Converting pyruvic acid to ethanol or lactic acid forms no ATP; it only reoxidises $NADH + H^{+}$ . So the net gain is 2 ATP.

Solved Example 2

Match List I with List II.
List I: A. Yeast, B. Muscle cells during exercise, C. Pyruvic acid decarboxylase, D. Alcohol dehydrogenase
List II: I. Lactic acid by lactate dehydrogenase, II. Ethanol and

C O_{2}

from pyruvic acid, III. Forms ethanol, IV. Acts on pyruvic acid, releasing

C O_{2}

Choose the correct answer:
(A) A-II, B-I, C-IV, D-III
(B) A-I, B-II, C-IV, D-III
(C) A-II, B-I, C-III, D-IV
(D) A-II, B-IV, C-I, D-III

Solution:

Answer: (A). Yeast carries out alcoholic fermentation (II). Exercising muscle makes lactic acid with lactate dehydrogenase (I). The decarboxylase releases $C O_{2}$ (IV), and alcohol dehydrogenase forms ethanol (III).

Solved Example 3

Read the statements.
A. Fermentation releases less than seven per cent of the energy in glucose.
B. Lactic acid fermentation releases

C O_{2}

.
C.

NADH + H^{+}

is reoxidised to

NA D^{+}

in both alcoholic and lactic acid fermentation.
D. Yeasts die when the alcohol concentration reaches about 13 per cent.
E. Fermentation takes place in the mitochondria.
Choose the correct answer:
(A) A, C and D only
(B) A, B and C only
(C) B, D and E only
(D) C, D and E only

Solution:

Answer: (A). B is wrong: only alcoholic fermentation releases $C O_{2}$ . E is wrong: fermentation, like glycolysis, is cytoplasmic; mitochondria are the site of aerobic respiration.

Solved Example 4

Arrange the compounds in the order in which they appear during alcoholic fermentation.
A. Pyruvic acid
B. Glucose
C. Glyceraldehyde 3-phosphate
D. Ethanol +

C O_{2}

E. Phosphoenol pyruvic acid
Choose the correct answer:
(A) B, C, E, A, D
(B) B, E, C, A, D
(C) B, C, A, E, D
(D) C, B, E, A, D

Solution:

Answer: (A). Glucose (B) is broken down to glyceraldehyde 3-phosphate (C), then via 3-phosphoglyceric acid to phosphoenol pyruvic acid (E) and pyruvic acid (A). Yeast then forms ethanol + $C O_{2}$ (D).

Solved Example 5

Which of the following is NOT true of fermentation?
(A) Glucose is only partially broken down
(B) It gives a net gain of 2 ATP per glucose
(C) NADH is oxidised to

NA D^{+}

very vigorously
(D) Either acid or alcohol is produced

Solution:

Answer: (C). In fermentation NADH is oxidised to $NA D^{+}$ rather slowly; the vigorous oxidation is a feature of aerobic respiration.

Solved Example 6

Statement I: The maximum alcohol content of a naturally fermented beverage is about 13 per cent.
Statement II: Yeasts poison themselves to death when the alcohol concentration reaches about 13 per cent.
Choose the correct answer:
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correct

Solution:

Answer: (A). Because yeast dies at about 13 per cent alcohol, natural fermentation cannot push the alcohol content beyond this.

9. Practice Questions

Practice Questions

Match List I with List II. List I: A. Less than 7 per cent, B. About 13 per cent, C. Net 2 ATP, D. Lactate dehydrogenase. List II: I. Gain from fermenting one glucose, II. Alcohol level that kills yeast, III. Enzyme in exercising muscle, IV. Share of glucose energy released by fermentation. Options:
(A) A-IV, B-II, C-I, D-III
(B) A-II, B-IV, C-I, D-III
(C) A-IV, B-I, C-II, D-III
(D) A-I, B-II, C-IV, D-IIIAnswer: (A). These are the four numbers and the enzyme to remember for fermentation.
Which statements are correct? A. Fermentation occurs in germinating seeds. B. Alcohol dehydrogenase acts in lactic acid fermentation. C. Fermentation is hazardous because acid or alcohol is produced. D. Muscles make lactic acid only when oxygen is plentiful. Options:
(A) A and C only
(B) A, B and C only
(C) B and D only
(D) A, C and D onlyAnswer: (A). B is wrong: lactic acid needs lactate dehydrogenase. D is wrong: muscles make lactic acid when oxygen is inadequate.
Arrange the events of lactic acid formation in exercising muscle: A. Pyruvic acid is reduced by lactate dehydrogenase. B. Glucose is broken down by glycolysis. C. The regenerated $NA D^{+}$ is reused to oxidise glyceraldehyde 3-phosphate. D. Oxygen supply becomes inadequate. Options:
(A) D, B, A, C
(B) B, D, C, A
(C) D, A, B, C
(D) B, A, D, CAnswer: (A). Oxygen falls short during exercise, glycolysis gives pyruvic acid, lactate dehydrogenase reduces it while reoxidising $NADH + H^{+}$ , and the $NA D^{+}$ formed returns to glycolysis.
Which of the following is NOT a product of any fermentation pathway?
(A) Ethanol
(B) Lactic acid
(C) $C O_{2}$
(D) Water as the end product of complete oxidationAnswer: (D). Complete oxidation to $C O_{2}$ and water happens only in aerobic respiration.
Statement I: In alcoholic fermentation, pyruvic acid is converted to $C O_{2}$ and ethanol. Statement II: Lactic acid fermentation releases $C O_{2}$ . Options:
(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correctAnswer: (C). Lactic acid fermentation forms lactic acid only; no $C O_{2}$ is released.
Which enzymes convert pyruvic acid to ethanol and $C O_{2}$ in yeast?
(A) Lactate dehydrogenase and invertase
(B) Pyruvic acid decarboxylase and alcohol dehydrogenase
(C) Hexokinase and alcohol dehydrogenase
(D) Pyruvate dehydrogenase and citrate synthaseAnswer: (B). These two enzymes catalyse alcoholic fermentation.
Differentiate between aerobic respiration and fermentation.Answer: Aerobic respiration needs oxygen, oxidises glucose completely to $C O_{2}$ and water in the cytoplasm and mitochondria, oxidises NADH vigorously and gives many ATP. Fermentation needs no oxygen, breaks glucose down only partly to ethanol + $C O_{2}$ or lactic acid, oxidises NADH slowly and gives only 2 ATP.
Distinguish between aerobic respiration and anaerobic respiration.Answer: Aerobic respiration uses $O_{2}$ , is complete and releases a large amount of energy. Anaerobic respiration (fermentation) occurs without $O_{2}$ , is incomplete and releases less than 7 per cent of the energy in glucose.
Distinguish between glycolysis and fermentation.Answer: Glycolysis is the breakdown of glucose to pyruvic acid; it is common to aerobic and anaerobic respiration and gives the 2 net ATP. Fermentation is glycolysis followed by the conversion of pyruvic acid to ethanol + $C O_{2}$ or lactic acid, which forms no extra ATP but reoxidises $NADH + H^{+}$ .
What is the maximum concentration of alcohol in naturally fermented beverages, and how are stronger beverages obtained?Answer: About 13 per cent, because yeasts poison themselves to death at this level. Stronger beverages are obtained by distillation of the fermented liquid.

Common Mistakes to Avoid

Watch out

Writing that lactic acid fermentation releases $C O_{2}$ . Only alcoholic fermentation does.
Pairing lactate dehydrogenase with yeast. Yeast uses pyruvic acid decarboxylase and alcohol dehydrogenase; lactate dehydrogenase acts in muscles.
Giving the net ATP of fermentation as 4. Four ATP are made in glycolysis, but two are used, so the net gain is 2.
Thinking fermentation adds ATP after glycolysis. The conversion of pyruvic acid to ethanol or lactic acid forms no ATP.
Stating that fermentation releases about 70 per cent of glucose energy. It releases less than 7 per cent.
Saying NADH is oxidised vigorously in fermentation. It is oxidised rather slowly; vigorous oxidation is aerobic.
Assuming only microbes ferment. Muscle cells during exercise and germinating seeds also ferment.
Writing that yeast can make 20 per cent alcohol naturally. Yeast dies at about 13 per cent.

Frequently Asked Questions

What is anaerobic respiration?

Anaerobic respiration is the incomplete oxidation of glucose without oxygen. After glycolysis, pyruvic acid is converted either to ethanol and carbon dioxide (alcoholic fermentation) or to lactic acid (lactic acid fermentation). It occurs in many prokaryotes, unicellular eukaryotes and germinating seeds, and in muscles during exercise.

What happens in alcoholic fermentation?

In alcoholic fermentation by yeast, glucose is broken down to pyruvic acid by glycolysis. The enzymes pyruvic acid decarboxylase and alcohol dehydrogenase then convert pyruvic acid into carbon dioxide and ethanol. NADH + H+ formed in glycolysis is reoxidised to NAD+ in the process.

Why do muscles produce lactic acid during exercise?

During exercise, the oxygen supply can become inadequate for cellular respiration in muscle cells. The pyruvic acid from glycolysis is then reduced to lactic acid by the enzyme lactate dehydrogenase, with NADH + H+ acting as the reducing agent and being reoxidised to NAD+.

How many ATP are produced in fermentation?

Fermentation gives a net gain of only 2 ATP per glucose. Glycolysis makes 4 ATP but uses 2, and the later conversion of pyruvic acid to ethanol or lactic acid forms no ATP. Less than seven per cent of the energy in glucose is released.

Why is fermentation considered hazardous?

Fermentation produces either acid or alcohol, and both can harm the cell as they accumulate. For example, yeasts poison themselves to death when the alcohol concentration reaches about 13 per cent. Fermentation is also inefficient, releasing less than seven per cent of the energy in glucose.

What is the maximum alcohol concentration in naturally fermented beverages?

It is about 13 per cent, because yeasts die when the alcohol concentration reaches this level. Beverages with a higher alcohol content are obtained by distilling the fermented liquid, which concentrates the alcohol. The limit also shows why fermentation is called hazardous: its own product poisons the organism.

How does fermentation differ from aerobic respiration?

Fermentation breaks glucose down only partly, gives a net gain of 2 ATP and oxidises NADH to NAD+ rather slowly. Aerobic respiration oxidises glucose completely to carbon dioxide and water, generates many more ATP and oxidises NADH very vigorously, using oxygen in the mitochondria.

Which points of Anaerobic Respiration are most asked in NEET?

NEET can ask any point of the NCERT text, but common points are the enzymes of alcoholic and lactic acid fermentation, which organism makes which product, the reoxidation of NADH + H+, the figure of less than seven per cent energy, the 13 per cent alcohol limit, the net gain of 2 ATP and the fermentation versus aerobic comparison.

Previous year questions on Anaerobic Respiration

1 question from past papers, each with a step-by-step solution.

NEET 2022, Biology Q18

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