How would you demonstrate that yeast can respire both aerobically and anaerobically?

Exercise · Q17

Q. How would you demonstrate that yeast can respire both aerobically and anaerobically?

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Concept understanding — Fermentation

Fermentation: The Backup Energy Plan

Imagine you're sprinting. Your muscles are screaming for energy, burning through glucose faster than your lungs can deliver oxygen. Your cells have a problem: the usual energy-making machinery (the Krebs cycle and electron transport chain) is oxygen-hungry, and right now oxygen is in short supply.

What does the cell do? It doesn't shut down. It switches to a simpler, faster, but far less efficient plan — fermentation.

The Core Idea

Fermentation is an anaerobic (oxygen-free) process that takes the pyruvate produced by glycolysis and converts it into either ethanol (in yeast and some bacteria) or lactic acid (in animal muscles and some bacteria). Its only purpose is to regenerate a molecule called NAD⁺, which is essential for glycolysis to keep running.

Think of it this way: glycolysis is like a factory that produces a small amount of ATP but uses up NAD⁺ as a temporary worker. If that worker (NAD⁺) doesn't get recycled, the factory shuts down. Fermentation is the recycling plant — it takes the used worker (NADH) and converts it back into the fresh worker (NAD⁺) so glycolysis can keep producing ATP.

Important

Fermentation does not produce any ATP directly. The only ATP comes from glycolysis (2 ATP per glucose). Fermentation simply allows glycolysis to continue by recycling NAD⁺.

The Two Main Types

1. Lactic Acid Fermentation (in your muscles, and in bacteria that make yogurt)

Pyruvate is reduced directly to lactic acid by NADH. The NADH gets oxidised back to NAD⁺, and the lactic acid builds up — that's the burning sensation in your muscles during intense exercise.

2. Alcoholic Fermentation (in yeast, used in brewing and baking)

Pyruvate is first decarboxylated (loses a CO₂ molecule) to become acetaldehyde, then acetaldehyde is reduced to ethanol by NADH. The CO₂ released is what makes bread rise and beer fizzy.

\text{Glucose} \rightarrow 2\,\text{Ethanol} + 2\,\text{CO}_2 + 2\,\text{ATP}

Why the Yield Is So Small …

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Step 1. Set up the anaerobic demonstration. A pinch of dry baker's yeast, suspended in a 10% glucose solution, is placed in a test tube whose liquid surface is sealed with a layer of oil to exclude air; the tube is closed tightly with a stopper carrying a delivery tube that leads to a second tube of lime water, so the two tubes are gas-tight but connected.

Step 2. Observe the anaerobic result. The lime water gradually turns milky, showing that CO2 is being evolved from the yeast preparation. The level of lime water in the delivery tube does NOT rise, showing that there is no decline in the gas volume inside the sealed tube - i.e. no oxygen is being taken up by the yeast at all. After the preparation is left for a day or two, opening the stopper reveals a distinct smell of alcohol. Together, these three observations (CO2 evolved, no O2 consumed, ethanol formed) show that the sealed-off yeast is fermenting glucose anaerobically to ethanol and CO2. …

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

CBSE 2026Set ANNUAL1 markMCQ
$Q. Anaerobic respiration in muscles produces$
(a) Ethanol
(b) Lactic acid
(c) Acetic acid
(d) Pyruvate
›Reveal solutionSolution

Anaerobic respiration in muscles produces lactic acid, so the answer is (B).

When oxygen is insufficient (e.g. during strenuous exercise), muscle cells carry out anaerobic respiration (lactic acid fermentation). Pyruvic acid produced in glycolysis is reduced by NADH to lactic acid:

Pyruvic acid + NADH → Lactic acid + NAD+

…

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CBSE 2025Set ANNUAL1 mark
$Q. Choose True or False: In the process of fermentation, glucose is completely oxidized under oxidative conditions.$
›Reveal solutionSolution

The statement is False — fermentation is anaerobic and incomplete; complete oxidation of glucose (to CO2 and H2O) happens only in aerobic respiration.

Fermentation occurs in the absence of oxygen. Pyruvic acid formed from glycolysis is converted into ethanol and CO2 (alcoholic fermentation, as in yeast) or into lactic acid (lactic acid fermentation, as in some bacteria and muscle cells), regenerating NAD+ so glycolysis can continue. Glucose is only partially broken down in this process, releasing much less energy than aerobic respiration, in which glucose is completely oxidized to CO2 and water in the presence of oxygen, rele …

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CBSE 2024Set HALF_YEARLY1 mark
$Q. State whether True or False: Fermentation is a type of aerobic respiration.$
›Reveal solutionSolution

This statement is False: fermentation takes place in the complete absence of oxygen, so it is an anaerobic process, not an aerobic one.

Cellular respiration can proceed along two broad routes depending on oxygen availability. Aerobic respiration uses O2 as the final electron acceptor in the electron transport chain and fully oxidises glucose to CO2 and water via glycolysis, the Krebs cycle and oxidative phosphorylation, yielding a large amount of ATP (about 36-38 ATP per glucose). Fermentation, by contrast, occurs when oxygen is unavailable: after glycolysis converts glucose to pyruvate (net 2 ATP, no O2 required), the pyruvate is converted either to ethanol and CO2 (alcoholic fermentation, e.g. in yeast) or to lactic acid …

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Q.How would you demonstrate that yeast can respire both aerobically and anaerobically?

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