This MCQ module is based on: Fermentation Aerobic Entry
TOPIC 52 OF 90
Fermentation Aerobic Entry
🎓 Class 11
Biology
CBSE
Theory
Ch 12 – Respiration in Plants
⏱ ~14 min
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This assessment will be based on: Fermentation Aerobic Entry
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Fermentation and the Entry into Aerobic Respiration
Part 1 left two molecules of pyruvic acid sitting in the cytoplasm with most of the glucose's energy still locked inside them. What the cell does next depends entirely on whether oxygen is available. This part follows the anaerobic road first — fermentation — and then opens the aerobic road by taking pyruvate into the mitochondrion.
12.3 Fermentation
Fermentation is the incomplete oxidation of glucose achieved under anaerobic conditions by sets of reactions that dispose of pyruvic acid without oxygen.
Alcoholic fermentation — the yeast route
In fermentation, say by yeast, pyruvic acid is converted to CO₂ and ethanol. The enzymes pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions.
Lactic acid fermentation — the bacterial and muscle route
Other organisms like some bacteria produce lactic acid from pyruvic acid. In animal cells also, like muscles during exercise, when oxygen is inadequate for cellular respiration, pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
The one thing both routes have in common. The reducing agent is NADH + H⁺, which is reoxidised to NAD⁺ in both the processes. That is the real purpose of fermentation. Glycolysis cannot continue without a supply of NAD⁺, and with no oxygen there is no electron transport system to regenerate it — so the cell dumps the electrons onto pyruvate itself, sacrificing the pyruvate to recover the NAD⁺.
Why fermentation is a poor deal
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. There is a second drawback too: the processes are hazardous — either acid or alcohol is produced, and both are toxic to the cell that makes them.
In-text question — what is the net ATP synthesised when one glucose is fermented to alcohol or lactic acid? Work it out exactly as the chapter directs — count what is synthesised and deduct what was utilised during glycolysis. Synthesised: 2 ATP at BPGA → PGA and 2 ATP at PEP → pyruvate = 4 ATP. Utilised: 2 ATP in the two phosphorylation steps. Net = 2 ATP. And note that the fermentation steps themselves yield no additional ATP at all — they exist only to regenerate NAD⁺. So the entire energy harvest from a glucose molecule under anaerobic conditions is the same 2 ATP that glycolysis already gave.
In-text questions — the yeast alcohol limit, and how stronger drinks are made. Yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent. It follows that the maximum concentration of alcohol in a naturally fermented beverage is about 13 per cent — fermentation stops there because the fermenting organism has been killed by its own product. This is why naturally fermented drinks such as wines and beers fall at or below that figure. Beverages of higher alcohol content are obtained by distillation — the fermented liquid is heated so that ethanol, which boils at a lower temperature (about 78°C) than water, vaporises first and is condensed separately, concentrating the alcohol well above what any yeast could survive. This is a purely physical separation performed after fermentation has ended; no organism produces such concentrations.
📐 Activity 12.2 — Demonstrate alcoholic fermentation
Setup. Take a conical flask; add about 100 mL of 10% sugar (glucose or cane sugar) solution and a teaspoon of baker's yeast. Fit the mouth with a one-holed stopper carrying a delivery tube whose other end dips into a test tube of freshly prepared lime water. Keep the flask warm (about 30–35°C). Set up an identical flask with boiled (dead) yeast as the control.
Predict: which flask will bubble, what will happen to the lime water, and what will the flask smell like after an hour? Also predict whether the same experiment would work if you were demonstrating lactic acid fermentation instead.
Observations. The live-yeast flask bubbles steadily; the gas turns the lime water milky, identifying it as CO₂. After some time the flask develops the characteristic smell of ethanol. The boiled-yeast control shows neither bubbling nor milkiness, proving the gas comes from living metabolism and not from the sugar solution itself.
The chemistry you have shown. Pyruvic acid is converted to CO₂ and ethanol by pyruvic acid decarboxylase (which removes the CO₂, leaving acetaldehyde) and alcohol dehydrogenase (which reduces acetaldehyde to ethanol, reoxidising NADH + H⁺ to NAD⁺).
Why lactic acid fermentation could not be shown this way. It releases no CO₂ — pyruvic acid, a 3-carbon acid, is simply reduced to lactic acid, also 3-carbon, with no decarboxylation. There would be no gas to bubble into the lime water. You would have to detect the falling pH with an indicator instead. That single difference — whether a carbon is lost as CO₂ — is the cleanest way to tell the two fermentations apart.
A last observation worth making. Leave the flask for a few days and fermentation slows and stops, even with sugar remaining. The yeast has been poisoned by its own alcohol at about 13 per cent.
| Feature | Alcoholic fermentation | Lactic acid fermentation |
|---|---|---|
| End products | Ethanol + CO₂ | Lactic acid only |
| CO₂ released? | Yes | No |
| Enzymes | Pyruvic acid decarboxylase and alcohol dehydrogenase | Lactate dehydrogenase |
| Carbon count of product | 2 C (ethanol) + 1 C (CO₂) | 3 C (lactic acid) |
| Occurs in | Yeast and many microbes | Some bacteria; animal muscle during exercise when O₂ is inadequate |
| Net ATP per glucose | 2 — the same for both; the fermentation steps add none | |
| Role of NADH | The reducing agent, reoxidised to NAD⁺ in both processes | |
| Hazard | Alcohol accumulates; yeast dies at about 13 per cent | Acid accumulates |
🎯 Interactive: Which fate for the pyruvate?
Fate of pyruvic acid: Converted to CO2 and ethanol
Pyruvic acid decarboxylase removes CO2 to give acetaldehyde, and alcohol dehydrogenase reduces it to ethanol while reoxidising NADH to NAD+. Net yield stays at 2 ATP per glucose, and less than seven per cent of the energy in glucose is released.
12.4 Aerobic Respiration
What, then, is the process by which organisms can carry out complete oxidation of glucose and extract the energy stored to synthesise a larger number of ATP molecules needed for cellular metabolism? In eukaryotes these steps take place within the mitochondria, and this requires O₂.
Aerobic respiration is the process that leads to a complete oxidation of organic substances in the presence of oxygen, and releases CO₂, water and a large amount of energy present in the substrate. This type of respiration is most common in higher organisms.
For aerobic respiration to take place within the mitochondria, the final product of glycolysis, pyruvate, is transported from the cytoplasm into the mitochondria.
The two crucial events in aerobic respiration.
• The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO₂.
• The passing on of the electrons removed as part of the hydrogen atoms to molecular O₂, with simultaneous synthesis of ATP.
• The complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO₂.
• The passing on of the electrons removed as part of the hydrogen atoms to molecular O₂, with simultaneous synthesis of ATP.
And note where each happens. The first process takes place in the matrix of the mitochondria, while the second process is located on the inner membrane of the mitochondria. Hold on to this division — it is the reason the two halves of aerobic respiration (Krebs' cycle and the ETS) are treated as separate sections in Part 3.
The link reaction — pyruvate to acetyl CoA
Pyruvate, which is formed by the glycolytic catabolism of carbohydrates in the cytosol, after it enters the mitochondrial matrix undergoes oxidative decarboxylation by a complex set of reactions catalysed by pyruvic dehydrogenase. The reactions catalysed by pyruvic dehydrogenase require the participation of several coenzymes, including NAD⁺ and Coenzyme A.
Pyruvic acid + CoA + NAD+ —pyruvate dehydrogenase, Mg2+→
Acetyl CoA + CO2 + NADH + H+
Acetyl CoA + CO2 + NADH + H+
During this process, two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (produced from one glucose molecule during glycolysis). The acetyl CoA then enters a cyclic pathway, the tricarboxylic acid cycle, more commonly called Krebs' cycle after the scientist Hans Krebs, who first elucidated it.
| Item | Detail |
|---|---|
| Site | Mitochondrial matrix |
| Enzyme complex | Pyruvic (pyruvate) dehydrogenase, requiring Mg²⁺ |
| Coenzymes needed | NAD⁺ and Coenzyme A |
| Type of reaction | Oxidative decarboxylation — a carbon is lost and hydrogens are removed |
| Carbon change | Pyruvic acid 3C → acetyl CoA 2C, releasing 1 CO₂ |
| Per glucose | 2 acetyl CoA + 2 CO₂ + 2 NADH + H⁺ |
| ATP made here | None directly — the energy is banked in NADH |
The first carbon dioxide of respiration. Glycolysis released no CO₂ at all. This link reaction is where the first CO₂ of aerobic respiration appears — one per pyruvate, two per glucose. The remaining four carbons of the original glucose are still held in the two acetyl CoA molecules, and the Krebs' cycle will strip them away in Part 3.
Fermentation compared with aerobic respiration
- Fermentation accounts for only a partial breakdown of glucose, whereas in aerobic respiration it is completely degraded to CO₂ and H₂O.
- In fermentation there is a net gain of only two molecules of ATP for each molecule of glucose degraded to pyruvic acid, whereas many more molecules of ATP are generated under aerobic conditions.
- NADH is oxidised to NAD⁺ rather slowly in fermentation, however the reaction is very vigorous in the case of aerobic respiration.
🎯 Competency-Based Questions
Scenario: A rice field is flooded for several days so that the roots are waterlogged. A researcher samples the root tissue and finds ethanol accumulating, the rate of sugar consumption sharply increased, and the plants showing poor growth despite ample sugar reaching the roots from the shoot.
Q1. Explain why ethanol accumulates in the waterlogged roots. L3 Apply
Water fills the air spaces around the roots, so O₂ becomes unavailable. Without oxygen, pyruvate cannot be taken into the aerobic route, so the cells fall back on alcoholic fermentation: pyruvic acid is converted to CO₂ and ethanol by pyruvic acid decarboxylase and alcohol dehydrogenase. The real driver is the need to reoxidise NADH + H⁺ to NAD⁺ so glycolysis can keep running — the ethanol is the waste left behind by that recycling.
Q2. Why is sugar consumption sharply increased, and why is growth poor despite plenty of sugar? L4 Analyse
Because the yield per glucose has collapsed. Fermentation gives a net gain of only two molecules of ATP per glucose and releases less than seven per cent of the energy in glucose, whereas aerobic respiration degrades glucose completely to CO₂ and H₂O and generates many more ATP. To obtain the same amount of ATP the tissue must therefore burn through far more sugar — hence the increased consumption. Growth is poor because even that increased consumption cannot match the aerobic output, so little ATP is left over for synthesis after maintenance. The accumulating ethanol is additionally hazardous to the tissue.
Q3. Fill in the blanks: Pyruvate entering the mitochondrial matrix undergoes ______ catalysed by ______, requiring the coenzymes ______ and ______. Per glucose this yields ______ acetyl CoA, ______ CO₂ and ______ NADH. L1 Remember
oxidative decarboxylation; pyruvic (pyruvate) dehydrogenase; NAD⁺; Coenzyme A; 2; 2; 2.
Q4. A gas sensor over two sealed anaerobic cultures detects CO₂ from one and none from the other, though both consume glucose at the same rate. Identify the cultures and justify. L4 Analyse
The CO₂-producing culture is carrying out alcoholic fermentation (for example yeast), because pyruvic acid is converted to CO₂ and ethanol, the CO₂ being removed by pyruvic acid decarboxylase. The silent culture is carrying out lactic acid fermentation (for example lactic acid bacteria), in which pyruvic acid is simply reduced to lactic acid by lactate dehydrogenase — a 3-carbon acid becoming a 3-carbon acid, with no decarboxylation and therefore no CO₂. Equal glucose consumption is expected, since both give the same net 2 ATP and neither fermentation step yields extra ATP. Testing the medium's pH, or smelling for ethanol, would confirm the identification.
Q5. “Fermentation is simply a less efficient version of respiration, so an organism that ferments is a primitive failure.” Evaluate. L5 Evaluate
The efficiency claim is correct: fermentation is only a partial breakdown of glucose, gives a net gain of only two ATP, releases less than seven per cent of the energy in glucose, oxidises NADH to NAD⁺ rather slowly, and is hazardous because acid or alcohol is produced. But the verdict “primitive failure” does not follow, for three reasons. (i) Fermentation is not an alternative to glycolysis but a continuation of it — its function is to regenerate NAD⁺ so that glycolysis, which yields the only ATP available, can keep running. Without it, an anaerobic cell would stop within seconds. (ii) It is the only option in the environments where it is used: waterlogged roots, deep mud, the interior of a seed, an over-exercised muscle. Efficiency is irrelevant where the efficient route is unavailable. (iii) Every organism, ourselves included, retains this machinery, and our muscles use it whenever oxygen supply lags behind demand — so it is a universal reserve capacity, not a relic. A fair summary: fermentation is an energetically poor but indispensable emergency pathway, and the honest measure of a pathway is not its yield but whether the organism survives without it.
🧠 Assertion–Reason Questions
For each pair choose: (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.
Assertion (A): Fermentation is essential for a cell respiring anaerobically, even though it yields no extra ATP.
Reason (R): NADH + H⁺ is the reducing agent in both fermentations and is reoxidised to NAD⁺, without which glycolysis could not continue.
Answer: A. Both are true and the reason is the correct explanation. Regenerating NAD⁺ is the whole function of the fermentation steps.
Assertion (A): Naturally fermented alcoholic beverages cannot exceed about 13 per cent alcohol.
Reason (R): Yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent.
Answer: A. The limit is set by the fermenting organism's own tolerance. Higher concentrations are reached only by distillation, a physical process applied after fermentation ends.
Assertion (A): Carbon dioxide is first released during glycolysis.
Reason (R): Pyruvate undergoes oxidative decarboxylation in the mitochondrial matrix to form acetyl CoA, CO₂ and NADH + H⁺.
Answer: D. The assertion is false — glycolysis releases no CO₂; all six carbons of glucose remain in the two pyruvate molecules. The reason is true and identifies where the first CO₂ of aerobic respiration actually appears.
Coming next. Part 3 takes up Sections 12.4.1 and 12.4.2 — the tricarboxylic acid cycle from citrate synthase to the regeneration of oxaloacetic acid, and then the electron transport system with its five complexes, the role of oxygen as the final hydrogen acceptor, and oxidative phosphorylation through F₁–F₀ ATP synthase.
Frequently Asked Questions - Fermentation and the Entry into Aerobic Respiration
What is fermentation?
It is the incomplete oxidation of glucose under anaerobic conditions. In yeast, pyruvic acid is converted to CO2 and ethanol by pyruvic acid decarboxylase and alcohol dehydrogenase; in some bacteria and in animal muscle it is reduced to lactic acid by lactate dehydrogenase.
Why is fermentation necessary if it produces no extra ATP?
Because NADH + H+ must be reoxidised to NAD+ for glycolysis to continue, and with no oxygen there is no electron transport system to do it. The cell therefore dumps the electrons onto pyruvate itself, which is why ethanol or lactic acid is left behind.
How much energy does fermentation release?
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. The net gain is only 2 ATP per glucose, the same as glycolysis alone, since the fermentation steps add none.
Why is fermentation described as hazardous?
Because either acid or alcohol is produced, and both are toxic to the cell making them. Yeasts poison themselves to death when the alcohol concentration reaches about 13 per cent.
What is the maximum alcohol content of a naturally fermented beverage, and how are stronger drinks made?
About 13 per cent, because yeasts die at that concentration and fermentation stops. Beverages with more alcohol are obtained by distillation, a physical separation in which ethanol vaporises before water and is condensed separately after fermentation has ended.
What is the difference between alcoholic and lactic acid fermentation?
Alcoholic fermentation gives ethanol and CO2 using pyruvic acid decarboxylase and alcohol dehydrogenase. Lactic acid fermentation gives only lactic acid using lactate dehydrogenase and releases no CO2, since a 3-carbon acid becomes another 3-carbon acid.
What is aerobic respiration?
It is the process that leads to complete oxidation of organic substances in the presence of oxygen, releasing CO2, water and a large amount of the energy present in the substrate. In eukaryotes it takes place within the mitochondria and is most common in higher organisms.
What are the two crucial events of aerobic respiration and where do they occur?
First, the complete oxidation of pyruvate by stepwise removal of all the hydrogen atoms, leaving three molecules of CO2, which happens in the mitochondrial matrix. Second, the passing of the electrons removed with those hydrogens to molecular O2 with simultaneous ATP synthesis, which is located on the inner mitochondrial membrane.
How is pyruvate converted to acetyl CoA?
By oxidative decarboxylation in the mitochondrial matrix, catalysed by pyruvic dehydrogenase with Mg2+ and requiring the coenzymes NAD+ and Coenzyme A. Per glucose this gives 2 acetyl CoA, 2 CO2 and 2 NADH + H+.
How does fermentation compare with aerobic respiration?
Fermentation is only a partial breakdown of glucose while aerobic respiration degrades it completely to CO2 and H2O. Fermentation nets just 2 ATP per glucose against many more aerobically, and NADH is oxidised to NAD+ rather slowly in fermentation but very vigorously in aerobic respiration.
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