Q. 102 Entamoeba histolytica is an intestinal parasite that causes dysentery in humans. This parasite
resides in the isotonic environment of intestine and other tissues in the human body and does not possess
contractile vacuoles. If this parasite is placed in fresh water, it will
(A) survive for long time, until they re-enter the host environment.
(B) die due to hypoosmotic shock.
(C) not survive in water as they require high salt content.
(D) die due to hyperosmotic shock
Here’s an SEO-friendly article tailored to your request, optimized for educational searches in biology and parasitology. I’ve incorporated the correct answer with a detailed explanation of osmoregulation in Entamoeba histolytica, while analyzing all options for clarity.
Understanding Osmoregulation in Entamoeba histolytica
Entamoeba histolytica, the protozoan parasite behind amoebic dysentery, thrives in the human intestine’s isotonic environment—matching the host’s osmolarity around 300 mOsm/L. Unlike freshwater protists like Paramecium, it lacks contractile vacuoles for expelling excess water. This adaptation suits its parasitic lifestyle in balanced fluids like blood or gut contents.
When placed in freshwater (hypotonic, near 0 mOsm/L), water rushes in via osmosis, causing cytolysis. The question tests this: “If this parasite is placed in fresh water, it will…”
Correct Answer: (B) die due to hypoosmotic shock.
Why Hypoosmotic Shock Kills It: Step-by-Step Explanation
Hypoosmotic shock occurs in hypotonic environments where external solute concentration is lower, driving water influx.
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Intestine vs. Freshwater Gradient: Human fluids are isotonic to the parasite; freshwater has far fewer solutes.
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No Contractile Vacuole: Lacks this organelle, so it can’t pump out water—unlike Amoeba proteus in ponds.
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Cell Bursting (Cytolysis): Influx swells the cell until the membrane ruptures, leading to death.
Studies confirm E. histolytica trophozoites lyse rapidly in distilled water, highlighting its osmoregulatory limits outside the host.
Breaking Down All Options
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(A) survive for long time, until they re-enter the host environment.
Incorrect. Without osmoregulatory mechanisms, it can’t endure prolonged hypotonic stress; death is swift, not delayed. -
(B) die due to hypoosmotic shock.
Correct. Hypotonic water causes unstoppable influx, bursting the cell—classic for parasites adapted to isotonic hosts. -
(C) not survive in water as they require high salt content.
Incorrect and misleading. It doesn’t “require high salt” (hypertonic); it matches isotonic host fluids. Freshwater is hypotonic, not salty. -
(D) die due to hyperosmotic shock.
Incorrect. Hyperosmotic shock is water loss in hypertonic media (high solute outside). Freshwater is hypotonic, causing gain, not loss.
| Option | Key Concept | Why Right/Wrong |
|---|---|---|
| (A) | Long-term survival | Wrong: No adaptation for hypotonic survival |
| (B) | Hypoosmotic shock | Correct: Water influx → cytolysis |
| (C) | High salt need | Wrong: Isotonic, not hypertonic preference |
| (D) | Hyperosmotic shock | Wrong: Opposite of freshwater conditions |
Key Takeaways for NEET/Exam Prep
This MCQ highlights osmotic pressure differences:
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Hypotonic → lysis (no vacuole).
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Isotonic → thrives (host).
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Hypertonic → crenation (not relevant here).
Relate to human cells too—they’d burst in pure water without cell walls or pumps.
