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Difficulty: Very hardKingdom Protista: Protozoa and Unicellular Algae

A microscopic examination of two unicellular protists isolated from different environments shows distinct cellular adaptations. Organism X is a freshwater autotroph containing a cup-shaped chloroplast with a pyrenoid, an eyespot (stigma), and contractile vacuoles. Organism Y is an obligate intraerythrocytic parasite that lacks chloroplasts, a cell wall, and contractile vacuoles. Which of the following correctly identifies Organism X and provides the true physiological explanation for why Organism Y does not require a contractile vacuole?

  1. Organism X is *Chlamydomonas*, and Organism Y lacks a contractile vacuole because host blood plasma is isotonic to its cytoplasm, preventing excessive water influx by osmosis.Answer
  2. B
    Organism X is *chlamydomonas*, and Organism Y lacks a contractile vacuole because it produces glucose through light-independent photosynthetic reactions inside red blood cells.
  3. C
    Organism X is *Chlamydomonas*, and Organism Y lacks a contractile vacuole because oxygen gas released during dark photosynthetic reactions oxidizes intracellular waste.
  4. D
    Organism X is *Chlamydomonas*, and Organism Y lacks a contractile vacuole because it excretes metabolic wastes using flame cells and Malpighian tubules directly into plasma.

Answer

Organism X is *Chlamydomonas*, and Organism Y lacks a contractile vacuole because host blood plasma is isotonic to its cytoplasm, preventing excessive water influx by osmosis.
The correct response accurately identifies *Chlamydomonas* by its diagnostic cup-shaped chloroplast, pyrenoid, eyespot, and flagellar contractile vacuoles. It also correctly states that parasitic protists like *Plasmodium* residing in human erythrocytes do not need contractile vacuoles because host blood plasma is isotonic to their cytoplasm, eliminating osmotic water influx.

Step-by-Step Solution

1
Identify Organism X based on subcellular characteristics.
Unicellular protists with cup-shaped chloroplasts, starch-synthesizing pyrenoids, phototactic eyespots, and contractile vacuoles belong to the green algal genus *Chlamydomonas*.
*Chlamydomonas* utilizes its chloroplast for photosynthesis and contractile vacuoles to pump out excess water gained hypotonically from freshwater.
2
Analyze the osmoregulatory requirements of parasitic Organism Y (*Plasmodium*).
Because blood plasma is isotonic to the parasite's cytoplasm, there is no net osmotic movement of water into the cell.
Contractile vacuoles are essential only in hypotonic freshwater habitats to prevent osmotic lysis; marine and endoparasitic protists in isotonic media do not need them.
3
Evaluate distractor misconceptions.
Scientific names require genus capitalization (*Chlamydomonas*); oxygen is produced during light-dependent photolysis, not dark reactions; and flame cells/Malpighian tubules belong to multicellular animals, not protozoa.
Eliminating invalid biological claims confirms the correct identification and osmoregulatory mechanism.

Key Concept

Organelle functions, nutritional modes, and environmental osmoregulatory adaptations across Kingdom Protista
Estimated Time:1m 30s
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Topics

Kingdom Protista: Protozoa and Unicellular Algae

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