NCERT grounding
Section 12.3 of NCERT Biology Class 11 introduces fermentation as the anaerobic alternative to aerobic respiration. The text states: "In fermentation, say by yeast, the incomplete oxidation of glucose is achieved under anaerobic conditions by sets of reactions where pyruvic acid is converted to CO₂ and ethanol. The enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions." NCERT explicitly notes that "less than seven per cent of the energy in glucose is released" — a figure directly tested in NEET 2022.
"In both lactic acid and alcohol fermentation not much energy is released; less than seven per cent of the energy in glucose is released and not all of it is trapped as high energy bonds of ATP."
NCERT Biology Class 11, Chapter 12, §12.3
What is fermentation?
Fermentation is the anaerobic breakdown of pyruvate that follows glycolysis when oxygen is absent or insufficient. It is not a separate pathway for generating ATP; its biochemical purpose is to regenerate NAD⁺ from NADH so that glycolysis — the only ATP-yielding pathway available under anaerobic conditions — can continue.
Fermentation occurs in many prokaryotes, unicellular eukaryotes (yeast), and in specific tissues of higher organisms (skeletal muscle during intense exercise). In plants, fermentation operates in waterlogged roots and in germinating seeds before the aerobic machinery is fully established.
Energy released from glucose during fermentation
Both alcoholic and lactic acid fermentation release less than 7% of the chemical energy stored in glucose. The remainder stays locked in the end products — ethanol or lactate — because glucose is only partially oxidised. Source: NCERT §12.3; tested NEET 2022.
Alcoholic fermentation
Alcoholic fermentation occurs primarily in yeast (Saccharomyces) and converts pyruvate to ethanol in two enzymatic steps.
Alcoholic Fermentation — Reaction Sequence
-
Step 1
Pyruvate → Acetaldehyde
Pyruvate decarboxylase removes CO₂ from pyruvate, forming acetaldehyde (ethanal). Mg²⁺ is a cofactor.
CO₂ released -
Step 2
Acetaldehyde → Ethanol
Alcohol dehydrogenase reduces acetaldehyde to ethanol, oxidising NADH to NAD⁺ in the process.
NADH → NAD⁺ -
Net
Glucose → 2 Ethanol + 2 CO₂
Overall: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ + 2 ATP
2 ATP (glycolysis only)
The two-step sequence is critical for NEET: pyruvate decarboxylase acts first (CO₂ released here, not during ethanol formation), and alcohol dehydrogenase acts second (ethanol formed here, NAD⁺ regenerated here). Students who conflate the two enzymes or reverse the order will choose the wrong option on a matching or assertion question.
Lactic acid fermentation
Lactic acid fermentation converts pyruvate directly to lactic acid (lactate) in a single enzymatic step. It operates in animal muscle cells during vigorous exercise, in bacteria such as Lactobacillus, and in certain plant tissues under oxygen deprivation.
Figure 1. In lactic acid fermentation, pyruvate is reduced directly to lactate by lactate dehydrogenase. NADH is oxidised to NAD⁺ in the same step. Unlike alcoholic fermentation, no CO₂ is released.
The accumulation of lactic acid in muscle tissue lowers pH and contributes to the sensation of muscle fatigue during sustained anaerobic exercise. In bacteria such as Lactobacillus, the same pathway is exploited industrially to produce yogurt and cheese.
Why NAD⁺ regeneration matters
The step in glycolysis where 3-phosphoglyceraldehyde (PGAL) is oxidised to 1,3-bisphosphoglycerate (BPGA) requires NAD⁺ as the electron acceptor, reducing it to NADH. A cell contains only a finite pool of NAD⁺. If this pool is not regenerated, the oxidation step stalls, glycolysis halts, and ATP production from glycolysis ceases entirely — a lethal outcome for an anaerobic cell.
To understand why this matters, trace the coupling between glycolysis and fermentation step by step. Glycolysis converts one glucose molecule into two pyruvate molecules. At step 6 of glycolysis, the enzyme glyceraldehyde-3-phosphate dehydrogenase oxidises PGAL to 1,3-bisphosphoglycerate (BPGA), simultaneously reducing NAD⁺ to NADH. Each glucose molecule forces two NAD⁺ molecules into NADH — one per PGAL molecule. If those two NADH molecules cannot be re-oxidised back to NAD⁺, the enzyme has nothing to reduce in the next round, and glycolysis stops dead at step 6.
Under aerobic conditions this is not a problem: NADH is funnelled into the mitochondrial electron transport system, which passes electrons ultimately to O₂, regenerating NAD⁺ with a substantial ATP bonus. But when O₂ is absent, the electron transport system cannot run. Fermentation solves the problem by providing an alternative electron acceptor — either acetaldehyde (in alcoholic fermentation) or pyruvate itself (in lactic acid fermentation). The reduction of these acceptors oxidises NADH back to NAD⁺, at zero ATP cost but with the full "payment" of glycolysis-derived ATP intact.
Figure 2. The NAD⁺/NADH cycle. Glycolysis consumes NAD⁺ (converting it to NADH); fermentation regenerates it by using NADH to reduce acetaldehyde or pyruvate. Without the return arrow, glycolysis halts at step 6.
Core principle: Fermentation does not produce ATP. Its exclusive biochemical function is to regenerate NAD⁺ from NADH, keeping the NAD⁺ pool available so glycolysis can continue and produce 2 ATP per glucose under anaerobic conditions.
Without fermentation
HALT
Glycolysis outcome
NAD⁺ pool depleted; PGAL oxidation blocked; glycolysis stops at step 6.
ATP production = 0; cell cannot sustain life anaerobically.
With fermentation
RUNS
Glycolysis outcome
NADH oxidised back to NAD⁺; glycolysis continues; 2 ATP produced per glucose.
Toxic end-products (ethanol/lactate) accumulate — the trade-off for survival.
NEET 2018 conceptComparison: alcoholic vs lactic acid fermentation
| Parameter | Alcoholic Fermentation | Lactic Acid Fermentation |
|---|---|---|
| Key enzyme | Pyruvate decarboxylase → Alcohol dehydrogenase | Lactate dehydrogenase |
| End product | Ethanol (C₂H₅OH) + CO₂ | Lactic acid (C₃H₆O₃) |
| CO₂ released? | Yes — during pyruvate → acetaldehyde step | No |
| Net ATP per glucose | 2 (from glycolysis only) | 2 (from glycolysis only) |
| NAD⁺ regenerated? | Yes — by alcohol dehydrogenase | Yes — by lactate dehydrogenase |
| Organisms | Yeast, some bacteria, plant roots (stress) | Muscle cells, Lactobacillus, some fungi |
| Number of enzymatic steps (pyruvate → product) | 2 steps | 1 step |
| Energy efficiency | <7% of glucose energy released | <7% of glucose energy released |
Fermentation vs aerobic respiration
Fermentation vs Aerobic Respiration — Key Distinctions
Fermentation
2 ATP
Net ATP per glucose
- Incomplete oxidation of glucose
- Pyruvate reduced to ethanol or lactate
- No O₂ required; no mitochondria needed
- NADH oxidised slowly back to NAD⁺
- Toxic end products accumulate
- Energy efficiency <7%
- Occurs in cytoplasm only
Aerobic Respiration
38 ATP
Net ATP per glucose (theoretical)
- Complete oxidation to CO₂ + H₂O
- Pyruvate enters mitochondria as acetyl CoA
- O₂ required as terminal electron acceptor
- NADH oxidised vigorously via ETS
- Harmless CO₂ and H₂O as products
- Energy efficiency ~40%
- Cytoplasm + mitochondria
Efficiency and energy recovery
The stark contrast between fermentation and aerobic respiration is best understood as an energy-capture problem. Glucose contains roughly 2,870 kJ of chemical energy per mole. Glycolysis recovers only about 2 ATP — corresponding to approximately 61.7 kJ mol⁻¹ (using ~30.5 kJ per mole of ATP under standard conditions). That is barely 2.2% of glucose's total energy, and NCERT rounds this up to "less than 7%" when accounting for the fact that not all of even this small amount is captured as high-energy bonds. The remainder of the energy in fermentation stays locked in the end product — ethanol or lactic acid — because glucose is only partially oxidised.
Energy comparison at a glance: Aerobic respiration captures energy in 36–38 ATP molecules (~40% efficiency). Alcoholic fermentation captures energy in only 2 ATP molecules and loses the chemical energy of ethanol to the environment. Lactic acid fermentation captures 2 ATP; the energy in lactic acid is technically recoverable if lactic acid is later oxidised aerobically (as happens in the liver — the Cori cycle), but during the anaerobic episode itself it is not harvested.
Alcoholic fermentation
2 ATP
per glucose · ~3.5% efficiency
Energy fate: chemical energy locked in ethanol is lost — ethanol is excreted by yeast and cannot be recovered by it.
CO₂ also lost as gas — carries no usable energy.
Only ~3.5% of glucose energy capturedLactic acid fermentation
2 ATP
per glucose · ~3.5% efficiency
Energy fate: chemical energy remains in lactic acid — not lost in the same way as ethanol, but also not captured as ATP during the anaerobic phase.
No CO₂ — all carbon from glucose is retained in lactate.
NCERT §12.3 conceptAerobic respiration
36–38 ATP
per glucose · ~40% efficiency
Energy fate: glucose is completely oxidised to CO₂ + H₂O; most energy is captured via NADH → ETS → oxidative phosphorylation.
Requires O₂ as terminal electron acceptor and functional mitochondria.
~19× more ATP than fermentationWhy does it matter for NEET? Questions often frame fermentation as "inefficient" and ask you to calculate or compare yields. Remember: the NCERT phrase is "less than 7 per cent" (not exactly 3.5%) because fermentation does not even capture all the glycolysis ATP with perfect efficiency under cellular conditions. Use "less than 7%" when the question asks for the NCERT figure; use "~3.5%" only if the question asks for a calculation.
Industrial uses of fermentation
The industrial applications of fermentation arise directly from its chemistry: alcoholic fermentation produces CO₂ (useful for leavening bread) and ethanol (useful in beverages and biofuels); lactic acid fermentation produces lactic acid (useful for food preservation and flavour).
Baking
Agent: Saccharomyces cerevisiae (baker's yeast)
Useful product: CO₂ — bubbles trapped in gluten network cause dough to rise (leavening)
Ethanol produced co-currently is entirely evaporated during baking (oven temperatures exceed 100 °C). The bread retains no alcohol.
Wine & Beer
Agent: Saccharomyces cerevisiae (brewer's yeast)
Useful product: Ethanol (retained in solution, up to ~13% v/v)
CO₂ escapes from open vats or is captured for carbonation in beer. Beyond ~13%, accumulated ethanol denatures yeast enzymes, halting fermentation — a self-limiting process.
NEET conceptDairy (Yogurt / Cheese)
Agent: Lactobacillus spp. (lactic acid fermentation)
Useful product: Lactic acid — lowers pH of milk
Acidified milk proteins (caseins) denature and coagulate (curdle) — this is the physical basis of yogurt-setting and cheese curd formation. No CO₂ is released; characteristic tangy taste arises from dissolved lactic acid.
Biofuel (Gasohol)
Agent: Yeast (alcoholic fermentation)
Useful product: Ethanol blended with petrol
Used commercially in Brazil (sugarcane ethanol) and many other countries. Reduces dependence on fossil fuels. The ethanol is distilled from the fermentation broth to reach fuel-grade concentration.
Worked examples
During alcoholic fermentation in yeast, which enzyme directly reduces acetaldehyde to ethanol, and what is oxidised in the same reaction?
Answer: Alcohol dehydrogenase reduces acetaldehyde to ethanol. In the same reaction, NADH is oxidised to NAD⁺. This is the key regeneration step that allows glycolysis to continue. Pyruvate decarboxylase acts at the earlier step (pyruvate to acetaldehyde), not here.
Calculate the net ATP gain when one molecule of glucose is fermented to lactic acid. Show each phase.
Answer:
Investment phase (glycolysis steps 1–3): 2 ATP consumed.
Payoff phase (glycolysis steps 6–10): 4 ATP produced (substrate-level
phosphorylation).
Net from glycolysis: 4 − 2 = 2 ATP.
Fermentation (lactic acid formation): 0 ATP produced — this step only
regenerates NAD⁺.
Total net ATP = 2 per glucose. This is identical for alcoholic
fermentation; the ATP yield does not differ between the two types.
A student claims that CO₂ is produced during the lactic acid fermentation of glucose. Evaluate this claim.
Answer: The claim is incorrect for the fermentation stage. CO₂ is released during glycolysis only if we consider the complete pathway to CO₂ and H₂O under aerobic conditions; however, glycolysis itself does not release CO₂ either. In lactic acid fermentation, pyruvate is directly reduced to lactate — no decarboxylation occurs, so no CO₂ is produced. CO₂ is released in alcoholic fermentation (during the pyruvate → acetaldehyde step) but not in lactic acid fermentation.
Why cannot naturally fermented beverages exceed approximately 13% alcohol concentration?
Answer: Ethanol is the end product of yeast's own alcoholic fermentation. As ethanol accumulates in the medium, it denatures yeast enzymes and disrupts the yeast cell membrane, ultimately killing the yeast. This self-poisoning effect limits natural fermentation to approximately 13% ethanol. Higher-concentration spirits require distillation, which concentrates the ethanol after fermentation has stopped.
In a laboratory experiment, a microorganism is grown on glucose under anaerobic conditions. After 24 hours, the culture medium is tested: it contains ethanol and releases a gas that turns lime water milky. (a) Which type of fermentation is occurring? (b) Which two enzymes catalyse the conversion of pyruvate to the end product? (c) A second microorganism grown identically shows no gas production from pyruvate, but its medium turns acidic and causes milk to curdle. Which fermentation is this, and what single enzyme is responsible?
Answers:
(a) Alcoholic fermentation. The two clues are
ethanol (the end product) and CO₂ (the gas that turns lime water milky,
released during pyruvate → acetaldehyde). Lactic acid fermentation would
produce no gas and no ethanol.
(b) Step 1: pyruvate decarboxylase (pyruvate
→ acetaldehyde + CO₂). Step 2: alcohol dehydrogenase
(acetaldehyde + NADH → ethanol + NAD⁺).
(c) Lactic acid fermentation. No CO₂ is
released (no decarboxylation step). The acid produced is lactic acid, which lowers
milk pH and causes casein proteins to denature and curdle. The single enzyme is
lactate dehydrogenase (pyruvate + NADH → lactic acid + NAD⁺).
Lactobacillus is the classical organism performing this reaction in dairy applications.
If the complete combustion of one mole of glucose releases 2,870 kJ of energy, and each mole of ATP stores approximately 30.5 kJ under cellular conditions, calculate the approximate percentage of glucose energy captured as ATP during alcoholic fermentation. How does this compare with aerobic respiration (38 ATP)?
Answer:
Fermentation: 2 ATP × 30.5 kJ = 61 kJ captured.
Efficiency = (61 ÷ 2870) × 100 ≈ 2.1%.
NCERT rounds this to "less than 7%" to account for the fact that cellular
conditions reduce theoretical yields.
Aerobic respiration: 38 ATP × 30.5 kJ = 1,159 kJ captured.
Efficiency = (1159 ÷ 2870) × 100 ≈ 40.4%.
Conclusion: Aerobic respiration is approximately 19× more
efficient at capturing glucose energy as ATP than fermentation.
Common confusion & NEET traps
Alcoholic vs Lactic Acid — Most-Tested Distinctions
Alcoholic
- 2 enzymes: pyruvate decarboxylase + alcohol dehydrogenase
- CO₂ released (step 1)
- Ethanol product — toxic to yeast at ~13%
- Organisms: yeast, some plant root cells
Lactic Acid
- 1 enzyme: lactate dehydrogenase
- CO₂ NOT released
- Lactic acid product — causes muscle soreness
- Organisms: muscle cells, Lactobacillus