Q1. [3] deep thorough-understanding
A sealed jar contains a germinating seed respiring aerobically. After 24 hours the oxygen inside is almost used up, but the seed is still alive and releasing CO₂. Which respiratory pathway has the seed most likely switched to, and what waste product, other than CO₂, would you expect to accumulate?
Generated by claude-sonnet-4-6 · 2026-06-26 01:01 · grounding rag
Model Answer
The seed has most likely switched to anaerobic respiration (fermentation).
When oxygen is exhausted, the seed cannot continue aerobic respiration. Instead, pyruvate (formed in the cytoplasm from glucose) is converted without oxygen via anaerobic respiration. In plants and yeast, this pathway converts pyruvate into ethanol (alcohol) and carbon dioxide.
Therefore, the waste product other than CO₂ that would accumulate is ethanol (ethyl alcohol).
Source: Chapter 5, Section 5.3 — Respiration
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Explanation
- Examiners expect you to name the pathway (anaerobic respiration/fermentation) and justify it briefly (oxygen used up → switch from aerobic).
- The key fact from the textbook: in yeast/plants under anaerobic conditions, pyruvate → ethanol + CO₂ (not lactic acid, which is the animal/muscle pathway).
- Do not confuse: lactic acid is produced in muscle cells (humans) during oxygen shortage; ethanol is produced in plants and yeast. Since the question is about a seed, ethanol is the correct answer.
- Mentioning pyruvate and where breakdown occurs (cytoplasm) adds accuracy and can earn full marks.