A beaker of pond water containing few specimens of Euglena was placed in a dark room for two weeks. At the end…

Analyze the question: Euglena are placed in darkness for two weeks and still survive. We need to find why they stay alive when light is not available.

Concept: Euglena is a unicellular protist with a mixed (mixotrophic) mode of nutrition:

• In light: it is autotrophic and makes food by photosynthesis using its chloroplasts.
• In darkness: photosynthesis stops, so it switches to heterotrophic feeding. It can absorb dissolved organic matter (saprophytic/osmotrophic nutrition) and, in some descriptions, ingest tiny food particles (holozoic nutrition).

Step-by-step:

1. Option B: photosynthesis needs light, so it cannot happen in a dark room. This is wrong.
2. Option C: Euglena is not better adapted to darkness. It does best in light, and it just survives in the dark. This is wrong.
3. Option D: overcrowding has no bearing on survival without light. Wrong.
4. Option A: in the dark, Euglena feeds on organic matter in the pond water, which is a heterotrophic mode of nutrition. Of the options given, this is the only one that explains survival in darkness. JAMB treats it as holozoic nutrition.

Note on accuracy: Strictly speaking, Euglena in the dark mostly absorbs dissolved nutrients (saprophytic nutrition) rather than truly ingesting food particles.

Answer: A

Exam tip: Remember "Euglena = both feeders." Light means photosynthesis; dark means feeding on organic matter. If a question mentions darkness and survival, eliminate any option involving photosynthesis straight away.