Examkin

Question

Difficulty: Very hardKingdom Protista: Protozoa and Unicellular Algae

An aquatic unicellular protist possessing a rigid cellulose cell wall, two equal anterior flagella, a cup-shaped chloroplast with a pyrenoid, and a prominent eyespot is transferred from freshwater to a hypertonic saline medium under continuous illumination. Which of the following responses describes the physiological and subcellular changes occurring in this organism?

  1. Contractile vacuole discharge rate decreases while water leaves the cytoplasm by osmosis.Answer
  2. B
    Contractile vacuole discharge rate increases to pump out excess salt while photolysis of water produces oxygen gas during the Calvin cycle.
  3. C
    The organism, classified under binomial nomenclature as chlamydomonas Reinhardtii, increases contractile vacuole discharge to maintain cell turgidity.
  4. D
    The cellulose cell wall dissolves completely and the organism switches immediately to saprophytic extracellular digestion of dissolved mineral salts.

Answer

Contractile vacuole discharge rate decreases while water leaves the cytoplasm by osmosis.
Freshwater unicellular protists rely on contractile vacuoles to pump out excess water that enters by endosmosis. When transferred to a hypertonic saline medium, exosmosis occurs, drawing water out of the cell and eliminating the osmotic influx, which causes the contractile vacuole pulsation rate to decline sharply.

Step-by-Step Solution

1
Identify the unicellular protist from diagnostic structural traits
The organism with a cellulose cell wall, two equal anterior flagella, cup-shaped chloroplast, pyrenoid, and eyespot is Chlamydomonas.
These combined morphological traits uniquely define Chlamydomonas within unicellular chlorophyte algae.
2
Evaluate the osmotic movement of water in a hypertonic environment
Water moves passively out of the cell across the semi-permeable membrane down a water potential gradient.
A hypertonic medium has a lower water potential compared to the internal cytoplasm of a freshwater organism.
3
Determine the functional response of the contractile vacuole
The rate of contractile vacuole pulsation decreases or stops.
Contractile vacuoles actively eliminate excess water gained by freshwater organisms; when water loss occurs osmotic gain stops, rendering rapid vacuolar pumping unnecessary.

Key Concept

Osmoregulation and diagnostic cell structure in Chlamydomonas (Kingdom Protista)
Rate this question

Topics

Kingdom Protista: Protozoa and Unicellular Algae

More questions from this topic