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Difficulty: Very hardKingdom Protista: Protozoa and Unicellular Algae

A culture of the freshwater protist *Euglena gracilis* is maintained in a nutrient-rich culture medium containing dissolved organic compounds, but is kept in complete darkness for several weeks. Which physiological response will be observed in this organism under these prolonged conditions?

  1. The organism loses its green pigmentation and shifts exclusively to heterotrophic absorption of dissolved nutrients.Answer
  2. B
    The contractile vacuole increases its pulsation rate to drive the dark reactions of carbon fixation.
  3. C
    The flexible pellicle is shed and replaced by a thick chitinous cell wall to absorb solid food holozoically.
  4. D
    The cell undergoes immediate autolysis because it is an obligate autotroph unable to utilize external carbon sources.

Answer

The organism loses its green chloroplast pigmentation and shifts exclusively to heterotrophic absorption of dissolved nutrients.
The correct answer highlights the mixotrophic capability of *Euglena*. In light, it synthesizes food autotrophically using chlorophyll within its chloroplasts. In complete darkness with soluble organic matter present, chlorophyll degrades (bleaching) and the cell absorbs dissolved organic nutrients saprozoically across its membrane, demonstrating nutritional flexibility.

Step-by-Step Solution

1
Identify the mode of nutrition and structural features of *Euglena*
*Euglena* possesses chloroplasts containing chlorophyll for autotrophic nutrition in light, but also has the capacity for heterotrophic (saprozoic) nutrition when organic nutrients are present.
Understanding mixotrophy is essential to predicting organismal adaptation under changing environmental conditions.
2
Analyze the impact of complete darkness on photosynthetic apparatus
In prolonged darkness, chlorophyll synthesis stops and existing chloroplasts regress, causing the cell to lose its green color.
Light is required to maintain functional chloroplast structures and drive photosynthesis.
3
Determine the metabolic pathway utilized in the dark medium
Because dissolved organic nutrients are available in the culture medium, *Euglena* absorbs them directly across its plasma membrane/pellicle, functioning as a heterotroph.
Mixotrophs switch metabolic reliance from autotrophy to heterotrophy when light is absent but organic substrates are abundant.

Key Concept

Mixotrophic Nutrition in Protista (*Euglena*)
Estimated Time:1m 30s
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Topics

Kingdom Protista: Protozoa and Unicellular Algae

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