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Difficulty: EasyKingdom Protista: Protozoa and Unicellular Algae

Unicellular organisms such as *Chlamydomonas* require light to synthesize organic compounds. Which specialized subcellular organelle functions as a light-sensitive region that enables the organism to perceive light and exhibit positive phototaxis?

  1. StigmaAnswer
  2. B
    Pyrenoid
  3. C
    Contractile vacuole
  4. D
    Pellicle

Answer

The stigma (also known as the eyespot) is the organelle that perceives light and enables directional movement toward light sources.
The stigma, also referred to as the eyespot, is a pigmented organelle found in unicellular photosynthetic protists like *Chlamydomonas* and *Euglena*. It filters light and allows the cell to perceive illumination intensity and direction, driving positive phototaxis.

Step-by-Step Solution

1
Identify the primary physiological requirement mentioned in the stem.
The organism needs to sense light direction to perform phototaxis toward light for photosynthesis.
Phototaxis relies on a specialized photoreceptor structure.
2
Evaluate the organelle associated with light perception in unicellular algae like *Chlamydomonas*.
The stigma contains carotenoid pigments that filter light and help the cell orient its flagellar swimming toward illumination.
This differentiates the light-perceiving organelle from metabolic or osmoregulatory structures.

Key Concept

Function of the Stigma (Eyespot) in Phototaxis
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Topics

Kingdom Protista: Protozoa and Unicellular Algae

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