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Difficulty: MediumExcretion and Excretory Organs

A freshwater unicellular protist such as Amoeba continuously absorbs water from its hypotonic environment while producing metabolic nitrogenous waste. Which of the following correctly identifies the organelle responsible for maintaining osmotic balance and the primary mechanism by which its nitrogenous waste is eliminated?

  1. Contractile vacuole for osmoregulation, and diffusion across the cell surface membrane for nitrogenous waste excretionAnswer
  2. B
    Flame cells for osmoregulation, and nephridia for nitrogenous waste excretion
  3. C
    Contractile vacuole for nitrogenous waste excretion, and diffusion across the cell surface membrane for osmoregulation
  4. D
    Malpighian tubules for osmoregulation, and contractile vacuole for nitrogenous waste excretion

Answer

The contractile vacuole functions in osmoregulation, while nitrogenous wastes are excreted via diffusion across the general cell surface membrane.
In freshwater protozoans like Amoeba, the contractile vacuole serves specifically for osmoregulation by collecting and pumping out excess water that constantly diffuses into the hypertonic cytoplasm. Nitrogenous waste, mainly ammonia, is small, soluble, and removed primarily by simple diffusion across the cell surface membrane into the aquatic environment.

Step-by-Step Solution

1
Identify the osmoregulatory challenge of freshwater single-celled organisms
Because the interior cytoplasm of Amoeba is hypertonic to freshwater, water constantly enters the cell via osmosis.
To prevent bursting (osmotic lysis), excess water must be continuously gathered and discharged.
2
Determine the organelle responsible for water balance
The contractile vacuole collects excess intracellular fluid, migrates to the plasma membrane, and fuses with it to expel water externally.
This establishes the contractile vacuole as the dedicated osmoregulatory organelle.
3
Identify how metabolic waste products are discharged
Ammonia (NH3NH_3) is highly soluble and diffuses readily down its concentration gradient across the thin cell surface membrane.
Unicellular organisms lack specialized organ systems, relying on a high surface area-to-volume ratio for waste removal via simple diffusion.

Key Concept

Excretory mechanisms and osmoregulation in single-celled organisms
Estimated Time:1m 0s
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Topics

Excretion and Excretory Organs

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