When yeast cells are O2 starved, fermentation serves as the source of energy. This results in the production of :
- (a)ATP + CO2 + Ethanol
- (b)ATP + O2 + Pyruvate
- (c)ATP + CO2 + Lactic acid
- (d)ATP + O2 + Acetaldehyde
Correct — A, ATP + CO2 + Ethanol. When yeast is deprived of oxygen it switches to alcoholic (anaerobic) fermentation: each glucose is broken down to pyruvate, which is then converted to ethanol and carbon dioxide while releasing a small amount of ATP. The end products are therefore ATP, CO2 and ethanol.
- (b)ATP + O2 + Pyruvate — Fermentation releases no oxygen, and pyruvate is only an intermediate that is further converted to ethanol, so it is not a final product.
- (c)ATP + CO2 + Lactic acid — Lactic-acid fermentation occurs in muscle cells and some bacteria, not in yeast; yeast produces ethanol, not lactic acid.
- (d)ATP + O2 + Acetaldehyde — No oxygen is produced, and acetaldehyde is a transient intermediate that is promptly reduced to ethanol.
Fermentation is a way of extracting energy from glucose without oxygen. In yeast the anaerobic pathway is alcoholic fermentation, converting glucose to ethanol and carbon dioxide with a small net yield of ATP. It is far less efficient than aerobic respiration, which fully oxidises glucose to CO2 and water.
The reliable way to answer is to remember the two anaerobic outcomes — yeast makes ethanol and CO2, whereas overworked muscle makes lactic acid. Options naming O2 as a product are automatically wrong because fermentation is oxygen-free.
- In alcoholic fermentation glucose becomes 2 ethanol + 2 CO2 + 2 ATP (net).
- Fermentation is anaerobic, taking place without oxygen.
- Yeast fermentation underlies brewing, wine-making and bread-making, where the CO2 leavens the dough.
- Muscle cells and lactobacilli instead carry out lactic-acid fermentation.
- Assuming oxygen is a product of fermentation.
- Confusing yeast (ethanol) with muscle (lactic acid) fermentation.
NDA/UPSC ask for the products of yeast fermentation or contrast it with aerobic respiration — remember yeast gives ethanol + CO2 + a little ATP.
No directly related past PYQ was found.
- practice — not a real PYQ
During vigorous exercise, the anaerobic breakdown of glucose in human muscle produces
- (a)ethanol and CO2
- (b)lactic acid
- (c)oxygen and water
- (d)acetic acid
Answer(b) lactic acid — muscle cells carry out lactic-acid fermentation, unlike yeast which makes ethanol.
- practice — not a real PYQ
The gas responsible for making bread dough rise during baking is produced by
- (a)yeast fermentation
- (b)baking soda only
- (c)evaporation of water
- (d)oxidation of flour
Answer(a) yeast fermentation — yeast releases carbon dioxide, which leavens the dough.