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Difficulty: HardKingdom Protista: Protozoa and Unicellular Algae

An experimental culture of the freshwater flagellate Euglena is treated with a respiratory inhibitor that completely stops mitochondrial ATP synthesis. Assuming the organism remains in a hypotonic pond water environment, what immediate physiological impact will this metabolic blockade have on its osmoregulatory function?

  1. The contractile vacuole will cease pulsating, causing the cell to swell and potentially lyse due to unchecked water influx.Answer
  2. B
    The contractile vacuole will increase its rate of discharge to compensate for reduced metabolic water production.
  3. C
    The cell will immediately switch to saprophytic ingestion of dissolved nutrients across the pellicle to maintain turgor pressure.
  4. D
    The pyrenoids will continuously generate starch to lower internal solute concentration and reduce osmotic water entry.

Answer

The contractile vacuole will cease pulsating, causing the cell to swell and potentially lyse due to unchecked water influx.
Freshwater protists continuously gain water by osmosis because their cytoplasm is hypertonic relative to their environment. Expelling this surplus water through contractile vacuole pulsation is an active process requiring ATP energy. When mitochondrial ATP synthesis is inhibited, the contractile vacuole fails to fill and discharge, resulting in water retention, cellular swelling, and potential lysis.

Step-by-Step Solution

1
Analyze the environment and organism osmotic relationship.
Freshwater flagellates live in a hypotonic medium, resulting in continuous passive water entry into the cell across the plasma membrane.
The internal solute concentration of the protist is higher than that of the surrounding pond water.
2
Identify the mechanism of contractile vacuole operation.
Contractile vacuoles pump excess water out of the cytoplasm via energy-dependent active transport mechanisms.
Osmoregulation requires ATP to pump ions into vacuolar canals, drawing water into the vacuole before it fuses with the cell membrane to discharge.
3
Evaluate the effect of inhibiting ATP synthesis.
Without ATP, active solute transport stops, vacuolar pulsation ceases, and water accumulates in the cytoplasm.
Blocking mitochondrial respiration cuts off the ATP supply needed for contractile vacuole discharge, leading to cellular swelling.

Key Concept

Osmoregulation and energy dependence of contractile vacuoles in freshwater protists
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Topics

Kingdom Protista: Protozoa and Unicellular Algae

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