When does anaerobic respiration occur in man and yeast?

NCERT Exemplar · Q13

Q. When does anaerobic respiration occur in man and yeast?

Telangana TsbieShortImportance★★★★★est

48% · 25/52 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

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.

Note

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 ( $CO_2$ ) + Water ( $H_2O$ )	Lactic acid (animals) OR Ethanol + $CO_2$ (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.

Important

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 …

Start your 14-day free trial to unlock the full solution →

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. …

Start your 14-day free trial to unlock the full solution →

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

TG EAPCET 2022Set ap-2022-07-31-AN1 markMCQ
$_2$
›Reveal solutionSolution
Assertion (A) is true — facultative and obligate anaerobes do exist today — but Reason (R) is false because not all organisms possess oxygen-dependent enzymatic machinery. The correct option is (C).

Why this question tests your understanding of metabolic diversity

Living organisms have evolved remarkably diverse strategies for extracting energy from glucose. The key distinction lies in whether they require oxygen, tolerate it, or are actually poisoned by it. This question probes whether you understand that metabolic machinery is not universal across life.

Let me examine each statement carefully.

Evaluating Assertion (A)

Assertion (A) claims that present-day organisms include both facultative and obligate anaerobes.

This is straightforwardly true. The microbial world displays a spectrum of oxygen relationships:

Obligate anaerobes (e.g., Clostridium botulinum, many methanogens) cannot survive in the presence of oxygen. They lack enzymes like catalase and superoxide dismutase to neutralize toxic oxygen radicals, so $O_2$ is lethal to them.

Facultative anaerobes (e.g., E. coli, yeast Saccharomyces) are metabolic opportunists. They preferentially use aerobic respiration when oxygen is available (higher ATP yield) but switch to fermentation or anaerobic respiration when it is absent.

Both groups are abundant in modern ecosystems — from deep-sea vents and anaerobic sediments (obligate anaerobes) to the human gut and soil (facultative anaerobes).

Assertion (A) is true.

Evaluating Reason (R)

Reason (R) claims that all living organisms possess enzymatic machinery for partial oxidation of glucose with the help of $O_2$ .

This is false, and the word "all" is the fatal flaw.

The phrase "partial oxidation of glucose with the help of $O_2$ " describes aerobic respiration — specifically glycolysis followed by the citric acid cycle and the electron transport chain, where oxygen serves as the terminal electron acceptor.

However:

Obligate anaerobes lack this machinery. They do not possess a functional aerobic electron transport chain. Many lack cytochrome oxidases and other components required to use $O_2$ . Instead, they rely entirely on fermentation or anaerobic respiration (using $NO_3^-$ , $SO_4^{2-}$ , or other acceptors). …
Start your 14-day free trial to see the full solution →

←Previous questionQ12 — F1 particles participate in the synthesis of ________.→Next questionQ14 — Which of the following will release more energy on oxidation? Arrange …

Q.When does anaerobic respiration occur in man and yeast?

🎓Unlock everything free for 14 days

Unlock everything free for 14 days