CAPF AC 2021 Q11: Science & Technology – Biology
Which one of the following is not a breakdown product of glucose during respiration?
- (a) Ethanol
- (b) Lactic acid
- (c) Water
- (d) Methanol
Answer: (d) Methanol
Explanation
Breakdown products of glucose
How to think: recall the NCERT diagram with three pathways for glucose.
Glucose first breaks into pyruvate in the cytoplasm.
Pyruvate then goes one of three ways.
Write down the products and see which option is missing.
| Pathway | Where | Oxygen | Products |
|---|---|---|---|
| Aerobic respiration | Mitochondria | Present | Carbon dioxide + water + a lot of energy |
| Fermentation | Yeast | Absent | Ethanol + carbon dioxide + a little energy |
| Anaerobic, in muscle | Muscle cells | Lacking | Lactic acid + a little energy |
Ethanol and methanol are different:
| Alcohol | Formula | Note |
|---|---|---|
| Ethanol | C₂H₅OH | Made by yeast from glucose |
| Methanol | CH₃OH | Not made in respiration; poisonous |
Answer: (d) Methanol.
Why each option is right or wrong
- (a) Incorrect. Ethanol is formed when yeast breaks down glucose without oxygen.
- (b) Incorrect. Lactic acid is formed in muscle cells when oxygen is lacking.
- (c) Incorrect. Water is formed along with carbon dioxide in aerobic respiration.
- (d) CORRECT. Methanol is not a product of any pathway of glucose breakdown.
Key point
Glucose → pyruvate, then: with oxygen → CO₂ + water; in yeast → ethanol + CO₂; in muscle without enough oxygen → lactic acid.
Remember
Respiration gives ethanol, never methanol.
Common mistakes
- Reading methanol as ethanol; the two names differ by only two letters.
- Forgetting that water is a product of aerobic respiration.
- Thinking lactic acid forms only in curd; it also builds up in tired muscles and causes cramps.
Exam tip
Educated guess: when two options look almost the same, here ethanol and methanol, the answer is usually one of them. Yeast makes ethanol, the alcohol in drinks.
Sources
- NCERT Class X Science — Life Processes (respiration)
- NCERT Class XI Biology — Respiration in Plants (glycolysis and fermentation)