A researcher observes a microscopic particle that shows no sign of any metabolic activity when isolated in a test tube, but resumes all metabolic functions as soon as it enters a living host cell. (i) On the basis of this observation, would you classify this particle as living or non-living? Give a reason. (ii) Explain why the presence or absence of metabolic processes is used as a defining criterion for life.
Generated by claude-sonnet-4-6 · 2026-06-26 00:59 · grounding rag
Model Answer
(i) This particle is neither fully living nor non-living — it is best classified like a virus, on the borderline. It shows no metabolic activity outside a host cell but resumes all life functions inside one, making it a non-cellular entity that depends entirely on a host.
(ii) Metabolic processes (nutrition, respiration, excretion, etc.) are the life processes that maintain the organised structure of living organisms. Living structures constantly break down due to environmental effects; continuous molecular movements and chemical reactions are needed to repair and maintain order. Without metabolism, this maintenance stops and the organism ceases to be alive. Hence, metabolism is the defining criterion for life.
Source: Chapter 5 — Introduction and Section 5.1 (What Are Life Processes?)
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Explanation
- For part (i), the passage explicitly mentions viruses showing no molecular movement until they infect a cell, and calls this the reason for the controversy over their living status — use this directly.
- For part (ii), examiners expect the logic chain: living structures break down → need repair → repair requires molecular movement → molecular movement = metabolic processes → no metabolism = not alive.
- Don't write "living" or "non-living" definitively for part (i); the textbook presents it as a borderline/controversial case. That nuance earns marks.