Q.When does anaerobic respiration occur in man and yeast?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Comparison of Aerobic and Anaerobic Respiration
The Core Idea: Why Your Muscles Burn
Imagine you're sprinting for a bus. After a few seconds, your legs start to ache and feel heavy. That burning sensation is a direct signal from your body about the fundamental choice it makes between two ways of getting energy.
Every living cell needs energy, stored in a molecule called ATP. Think of ATP as a rechargeable battery. Respiration is the process of recharging that battery by breaking down glucose. The question is: how do you break it down? The answer depends entirely on whether oxygen is available.
The Two Paths: With Oxygen vs. Without
Aerobic respiration is the "with oxygen" path. It's the main, efficient, long-distance runner's method. Most of the time, your cells use this. It's a complete burn.
Anaerobic respiration (often called fermentation in microorganisms) is the "without oxygen" path. It's the emergency, short-burst, sprinter's method. It kicks in when oxygen can't reach your muscle cells fast enough.
In humans, anaerobic respiration produces lactic acid. In yeast and some bacteria, it produces ethanol and carbon dioxide. Both are forms of fermentation, but the end products differ.
The Precise Comparison
Let's lay out the differences clearly. The three key battlegrounds are: oxygen, end products, and energy yield.
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Oxygen Requirement | Requires oxygen | Does not require oxygen |
| Site in Cell | Mitochondria (the "powerhouse") | Cytoplasm (the cell's fluid) |
| Glucose Breakdown | Complete (fully oxidised) | Incomplete (partially broken) |
| End Products | Carbon dioxide (CO2) + Water (H2O) | Lactic acid (animals) OR Ethanol + CO2 (yeast/plants) |
| Energy (ATP) Yield | High (~36-38 ATP per glucose) | Low (~2 ATP per glucose) |
| Efficiency | Very efficient (~40% of energy captured) | Very inefficient (~2% of energy captured) |
Why the Energy Difference is So Huge
This is the most important "why". Glucose is a high-energy molecule. Aerobic respiration strips it completely bare, extracting almost every last bit of energy. It's like burning a log to ash — you get all the heat.
Anaerobic respiration only takes the first, easy step (glycolysis). It leaves most of the energy locked inside the partially broken molecule (lactic acid or ethanol). It's like taking a single bite out of the log and throwing the rest away. That's why you get only 2 ATP instead of 36-38 ATP.
Aerobic respiration yields about 18 times more ATP per glucose molecule than anaerobic respiration. This is the single most critical fact for exams.
The Real-World Consequence: The Oxygen Debt …
Anaerobic respiration (fermentation) is used when the oxygen supply to a cell is not enough to run the full aerobic pathway.
- In man: it occurs in muscle cells during vigorous exercise, when oxygen delivery cannot keep pace with the muscle's energy demand, so pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
- In yeast: it occurs whenever the yeast is growing under anaerobic conditions, converting pyruvic acid to CO2 and ethanol via pyruvic acid decarboxylase and alcohol dehydrogenase. …
Anaerobic respiration switches on in man's muscles when oxygen is in short supply during exertion, and in yeast whenever oxygen is unavailable.
Fermentation, the anaerobic route for handling pyruvic acid, becomes the operating pathway whenever aerobic respiration cannot proceed for lack of oxygen. The circumstances that trigger it differ between man and yeast, but the underlying reason — an inadequate oxygen supply relative to demand — is the same.
- In man: anaerobic respiration occurs in muscle cells specifically during exercise, when the muscles are working hard enough that oxygen cannot be delivered fast enough to sustain aerobic respiration. Under this oxygen shortfall, pyruvic acid produced by glycolysis is reduced to lactic acid, a reaction catalysed by lactate dehydrogenase.
- In yeast: fermentation is the normal route under anaerobic conditions generally, not just during a burst of activity. Pyruvic acid is converted to CO2 and ethanol, through the successive action of pyruvic acid decarboxylase and alcohol dehydrogenase. …
Step 1. Recall that anaerobic respiration (fermentation) takes over whenever oxygen supply cannot meet a cell's energy demand.
Step 2. In man, identify the tissue/situation: skeletal muscle during vigorous exercise, when O2 delivery cannot keep pace with demand.
Step 3. Identify the product in muscle: pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
Step 4. In yeast, identify the condition: whenever the yeast is growing under anaerobic conditions (not tied to a burst of activity). …
- AP EAPCET 2022Set ap-2022-07-12-AN1 markMCQQ.Which product is commonly seen in both aerobic and anaerobic respiration ? (A) Pyruvic acid (B) Malic acid (C) Citric acid (D) Succinic acid
›Reveal solutionSolution
Glycolysis, common to both aerobic and anaerobic respiration, always ends in pyruvic acid — the pathways only diverge after this shared intermediate.
Concept and Intuition
Cellular respiration always begins with glycolysis, a universal, oxygen-independent pathway in the cytoplasm that splits one glucose molecule into two molecules of pyruvic acid (pyruvate), with a net gain of 2 ATP and 2 NADH. What happens next depends on oxygen availability: under aerobic conditions, pyruvate is transported into the mitochondrion, converted to acetyl-CoA, and fully oxidized via the Krebs cycle and electron transport chain (yielding citric acid, succinic acid, malic acid, etc., as intermediates along the way). Under anaerobic conditions (fermentation), pyruvate is instead reduced to lactic acid (in muscle/some bacteria) or converted to ethanol and CO2 (in yeast) to regenerate NAD+. So pyruvic acid, being the universal glycolysis end-product, is the one substance common to both routes.
Step-by-Step Solution
- Recall that both aerobic and anaerobic respiration start with the same glycolysis pathway.
- Glycolysis converts glucose into pyruvic acid regardless of oxygen availability. …
- AP EAPCET 2021Set ap-2021-09-03-FN1 markMCQQ.Three major ways in which different cells handle pyruvic acid are as follows, except ____ (A) Lactic acid fermentation (B) Photorespiration (C) Aerobic respiration (D) Alcoholic fermentation
›Reveal solutionSolution
This tests knowledge of what happens to pyruvate after glycolysis — the three genuine fates are the two fermentations and aerobic respiration; photorespiration is unrelated.
Concept and Intuition
Glycolysis breaks glucose down to pyruvate in the cytoplasm, and pyruvate is then handled in one of three ways depending on oxygen availability and organism type: (1) reduced to lactic acid (anaerobic, e.g. in muscle cells during vigorous exercise or certain bacteria), (2) converted to ethanol + CO2 (alcoholic fermentation, e.g. yeast), or (3) fully oxidised aerobically by entering the mitochondrion for the link reaction, Krebs cycle and electron transport chain. Photorespiration, by contrast, is a light-dependent process in chloroplasts/peroxisomes/mitochondria where RuBisCO fixes O2 instead of CO2 onto RuBP — it does not process pyruvate at all and is unrelated to glycolytic end-product handling.
Step-by-Step Solution
- List the three genuine fates of pyruvate: lactic acid fermentation, alcoholic fermentation, aerobic (Krebs-cycle-based) respiration. …
- AP EAPCET 2021Set ap-2021-09-07-FN1 markMCQQ.Fermentation differs with aerobic respiration except one in the following? (A) Partial breakdown of glucose (B) Net gain is 2 ATP (C) NADH oxidation is slow (D) Glucose degrades into CO2 & H2O
›Reveal solutionSolution
The question asks which process is not a feature of fermentation compared to aerobic respiration. Fermentation involves partial breakdown, 2 ATP net gain, and slow NADH oxidation, but not complete degradation to CO₂ and H₂O — so the exception is (D).
Fermentation and aerobic respiration are both ways cells harvest energy from glucose, but they differ in their final electron acceptor and the extent of glucose breakdown. The key concept is oxidative phosphorylation: aerobic respiration uses oxygen to fully oxidize glucose to CO₂ and H₂O, yielding ~36–38 ATP. Fermentation is anaerobic; it only partially oxidizes glucose (to lactate or ethanol + CO₂) and relies on substrate-level phosphorylation for a net gain of just 2 ATP. NADH reoxidation in fermentation is slow because it happens via the final organic product, not the electron transport chain.
Let’s examine each option:
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Option (A): Partial breakdown of glucose
In fermentation, glucose is only partially broken down (e.g., to pyruvate, then to lactate or ethanol). Aerobic respiration fully oxidizes glucose. So this is a feature of fermentation, not an exception.
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Option (B): Net gain is 2 ATP
Fermentation yields exactly 2 ATP per glucose (from glycolysis). Aerobic respiration yields much more. So this is also a feature of fermentation.
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Option (C): NADH oxidation is slow
In fermentation, NADH is reoxidized by reducing pyruvate (or its derivative), a process that is slower than the rapid electron transport chain in respiration. This is indeed a characteristic of fermentation.
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Option (D): Glucose degrades into CO₂ & H₂O …
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- AP EAPCET 2021Set ap-2021-09-06-FN1 markMCQQ.Assertion (A): Anaerobic respiration yields less energy utilizing oxygen Reason (R): In anaerobic respiration, energy is released due to incomplete breakdown of organic molecules (A) Both A and R are correct and R is the correct explanation of A (B) Both A and R are correct and R is not the correct explanation of A (C) A is correct but R is wrong (D) A is wrong but R is correct
›Reveal solutionSolution
The assertion is false because anaerobic respiration yields less energy without using oxygen; the reason is true because incomplete breakdown releases less energy. So the correct option is (D).
Concept and Intuition
This question tests your understanding of cellular respiration and the role of oxygen. The key idea is that oxygen is the final electron acceptor in aerobic respiration, allowing the complete breakdown of glucose to carbon dioxide and water, which yields a large amount of ATP (about 36–38 molecules per glucose). In anaerobic respiration (or fermentation), oxygen is not used; instead, organic molecules (like pyruvate) act as electron acceptors, leading to incomplete breakdown (e.g., to lactic acid or ethanol). This incomplete breakdown releases far less energy—only 2 ATP per glucose. So the assertion gets the direction of energy yield and the role of oxygen exactly backwards.
Step-by-Step Reasoning
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Analyze the Assertion (A):
"Anaerobic respiration yields less energy utilizing oxygen."
- Anaerobic respiration, by definition, occurs without oxygen. The phrase "utilizing oxygen" is contradictory.
- It is true that anaerobic respiration yields less energy (only 2 ATP per glucose) compared to aerobic respiration (36–38 ATP). But the reason for less energy is the absence of oxygen, not its use.
- Therefore, the assertion is incorrect because it wrongly states that oxygen is used.
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Analyze the Reason (R):
"In anaerobic respiration, energy is released due to incomplete breakdown of organic molecules."
- This is correct. Without oxygen, the breakdown of glucose stops at intermediate products (e.g., lactic acid or ethanol), leaving much of the chemical energy untapped. …
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