16. Polyspermy results when two or more sperms fertilize an egg. It is usually lethal since it results in blastomeres with different numbers and types of chromosomes. Many species therefore, have two blocks to polyspermy: the fast block and the slow block.
    In the case of sea urchins:
    (A) the fast block is immediate and causes the egg membrane resting potential’ to rise which does not allow the sperm to fuse with the egg and is mediated by an influx of sodium ions.
    (B) the fast block is immediate and causes the egg membrane resting potential to rise which does not allow the sperm to fuse with the egg and is mediated by an efflux of sodium ions.
    (C) the slow block or cortical granule reaction is mediated by calcium ions
    (D) the slow block or cortical granule reaction is mediated by potassium ions.
    Which of the above statements are true?
    (1) (A) and (C) (2) (A) and (D)
    (3) (B) and (C) (4) (B) and (D)

The true statements about polyspermy blocks in sea urchin fertilization are:

(A) The fast block is immediate and causes the egg membrane resting potential to rise, which prevents sperm fusion, and it is mediated by an influx of sodium ions.
(C) The slow block, or cortical granule reaction, is mediated by calcium ions.

Statements (B) and (D) are incorrect because the fast block involves an influx (not efflux) of sodium ions, and the slow block is mediated by calcium ions, not potassium.

Therefore, the correct answer is:
(1) (A) and (C)

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Introduction

Polyspermy—fertilization of an egg by more than one sperm—is typically lethal due to abnormal chromosome numbers in resultant cells. To prevent this outcome, sea urchin eggs employ two primary defenses: the fast block and the slow block to polyspermy. These mechanisms ensure monospermic fertilization and viable embryonic development.

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The Fast Block to Polyspermy

The fast block is a rapid electrical change at the egg’s plasma membrane occurring within seconds after the first sperm binds. This process involves:

• A sudden influx of sodium ions (Na⁺) into the egg.

• A depolarization that changes the egg’s membrane potential from approximately -70 mV to about +20 mV.

• This positive shift in membrane potential prevents additional sperm from fusing with the egg membrane because sperm fusion requires the egg membrane’s resting negative potential.

This electrical block is transient, lasting roughly one minute, buying time for the slow block to activate.

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The Slow Block to Polyspermy

The slow block occurs minutes after fertilization and involves:

• Calcium ions (Ca²⁺) released from intracellular stores, triggered by sperm entry.

• The cortical granule reaction, where cortical granules beneath the egg membrane fuse with the plasma membrane and release their contents.

• These contents modify the vitelline envelope, hardening it and physically blocking further sperm penetration.

• The cortical reaction creates a durable, mechanical barrier ensuring monospermy.

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Why Other Options Are Incorrect

• The fast block is mediated by an influx (not efflux) of sodium ions; the egg’s depolarization depends on sodium flowing into the egg.

• The slow block is mediated by calcium ions, not potassium.

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Summary Table

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Conclusion

Sea urchin eggs prevent polyspermy through two mechanisms: a fast electrical block mediated by sodium influx depolarizing the membrane, and a slow mechanical block mediated by calcium-triggered cortical granule exocytosis. Thus, statements (A) and (C) are correct.

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This detailed explanation clarifies the biological processes preventing polyspermy in sea urchin fertilization, highlighting the importance of ion fluxes and membrane dynamics in successful reproduction.