Examkin

Question

Difficulty: MediumKingdom Protista: Protozoa and Unicellular Algae

A freshwater organism such as *Amoeba proteus* relies on its contractile vacuole to eliminate excess water that enters the cell across a concentration gradient. If a healthy specimen of *Amoeba* is transferred from its natural freshwater habitat into a hypertonic salt solution, which of the following physiological changes will occur in the activity of its contractile vacuole?

  1. The contractile vacuole decreases its rate of pulsation or ceases to function entirely.Answer
  2. B
    The contractile vacuole increases its rate of contraction to actively pump salt ions out of the cytoplasm.
  3. C
    The contractile vacuole expands continuously without discharging until the cell membrane ruptures.
  4. D
    The contractile vacuole converts into a chloroplast to produce osmotically active sugars.

Answer

The contractile vacuole decreases its rate of pulsation or ceases to function entirely.
In fresh water, *Amoeba* is hypertonic to its environment, causing water to constantly enter by osmosis and requiring the contractile vacuole to pump it out. When placed in a hypertonic salt solution, the external water potential becomes lower than that inside the cell. Consequently, water flows out of the organism, eliminating water buildup inside the cell, which causes the contractile vacuole to slow down its pulsation rate or stop working.

Step-by-Step Solution

1
Analyze the osmotic gradient between the cytoplasm of the protozoan and the surrounding medium.
The hypertonic salt solution has a lower water potential than the cytoplasm of *Amoeba*.
Water molecules move by osmosis from a region of higher water potential (inside the cell) to a region of lower water potential (outside the cell).
2
Determine the primary biological function of the contractile vacuole in freshwater protists.
The contractile vacuole collects and expels excess water that continuously diffuses into the cell under hypotonic conditions.
Fresh water is hypotonic to protozoan cytoplasm, necessitating active water expulsion to prevent osmotic lysis.
3
Evaluate the vacuolar response when the surrounding medium becomes hypertonic.
Since water no longer diffuses into the cell, water accumulation stops, leading to reduced or suspended vacuolar contractions.
Without an influx of excess water, the contractile vacuole does not fill or need to contract.

Key Concept

Osmoregulation via Contractile Vacuoles in Unicellular Protists
Estimated Time:1m 0s
Rate this question

Topics

Kingdom Protista: Protozoa and Unicellular Algae

More questions from this topic