Biology · Respiration in Plants · NEET
| End product(s) | Ethanol + CO2 | Lactic acid only |
| Enzymes | Pyruvic acid decarboxylase + alcohol dehydrogenase | Lactate dehydrogenase |
| CO2 released | Yes | No |
| Where it happens | Yeast, some microbes | Some bacteria, human muscle in low oxygen |
Yeast converts one molecule of pyruvic acid into ethanol (ethyl alcohol) and CO2. So the two end products are ethanol and carbon dioxide. This is why yeast is used to make bread (CO2 makes it rise) and alcoholic drinks (ethanol is the alcohol).
Two enzymes are needed. First, pyruvic acid decarboxylase removes CO2 from pyruvic acid to form acetaldehyde. Then alcohol dehydrogenase reduces acetaldehyde to ethanol. NCERT names both, so remember the order: decarboxylase first (releases CO2), dehydrogenase second (makes alcohol).
Both start from pyruvic acid and both are anaerobic with low ATP. But alcoholic fermentation (in yeast) gives ethanol + CO2 and uses two enzymes. Lactic acid fermentation (in some bacteria and human muscle) gives lactic acid, releases NO CO2, and uses only one enzyme, lactate dehydrogenase.
Glycolysis produces NADH. To keep glycolysis running, NAD+ must be regenerated. In yeast, NADH gives its electrons to acetaldehyde, turning it into ethanol and becoming NAD+ again. This recycled NAD+ lets glycolysis continue making ATP without oxygen.
Only a net 2 ATP per glucose, all made in glycolysis. Fermentation itself makes no extra ATP - it only regenerates NAD+. NCERT says less than 7% of glucose energy is released, so fermentation is very inefficient compared with aerobic respiration.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Ethanol (ethyl alcohol) and carbon dioxide (CO2), formed from pyruvic acid under anaerobic conditions in yeast.
Pyruvic acid decarboxylase and alcohol dehydrogenase.
When alcohol concentration reaches about 13 per cent, the ethanol becomes toxic and yeast poison themselves to death. This limits natural fermented drinks to about 13% alcohol.
No. Fermentation is anaerobic, meaning it happens without oxygen. This is why yeast can survive in oxygen-poor conditions.
It is a favourite MCQ topic. NEET often asks the exact enzymes, the end products (ethanol + CO2), the net ATP (2), and how it differs from lactic acid fermentation.
