13. Consider the following events which occur during fertilization of sea urchin eggs.
    (A) Resact/Speract are peptides released from the egg jelly and help in sperm attraction.
    (B) Bindin, an acrosomal protein interacts in a species specific manner, with eggs.
    (C) A “respiratory burst” occurs during cross-linking of the fertilization envelope, where a calcium- dependent increase in oxygen level is observed.
    (D) lP₃, which is formed at site of sperm entry, releases sequestered calcium leading to cortical granule exocytosis.
    Which of the above statement(s) is NOT true?
    (1) Only C (2) A and C
    (3) Only D (4) B and D

The statements and correctness regarding sea urchin fertilization are as follows:

(A) Resact/Speract are peptides released from the egg jelly and help in sperm attraction.
True. Resact and Speract are chemoattractant peptides released by the egg jelly that guide sperm towards the egg through chemotaxis.

(B) Bindin, an acrosomal protein interacts in a species-specific manner with eggs.
True. Bindin is a sperm acrosomal protein which mediates species-specific binding to receptors on the egg’s vitelline membrane.

(C) A “respiratory burst” occurs during cross-linking of the fertilization envelope, where a calcium-dependent increase in oxygen level is observed.
Not True. While cross-linking of the fertilization envelope involves peroxidase-mediated tyrosine cross-linking requiring hydrogen peroxide, it is not characterized as a “respiratory burst” with a calcium-dependent increase in oxygen consumption. The fertilization envelope hardening involves oxygen but not a respiratory burst in the usual sense.

(D) IP3, which is formed at the site of sperm entry, releases sequestered calcium leading to cortical granule exocytosis.
True. The inositol trisphosphate (IP3) produced at sperm entry triggers the release of Ca2+ from intracellular stores within the egg, initiating cortical granule exocytosis which contributes to the slow block to polyspermy.

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Summary Table of Statements:

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Thus, the incorrect statement is (C) alone.

Therefore, the correct answer is:
(1) Only C

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Detailed Explanation

Sea urchin fertilization is a well-studied process involving multiple coordinated biochemical and cellular events. The egg jelly releases peptides such as Resact/Speract that serve as chemical attractants guiding sperm by chemotaxis. Bindin is released from the sperm acrosomal vesicle and facilitates specific binding to receptors on the egg vitelline membrane, ensuring species-specific fertilization.

After sperm-egg binding and fusion, the egg undergoes calcium signaling driven by IP3 production, which releases calcium stored within the egg, triggering exocytosis of cortical granules. These granules’ contents lead to reinforcement of the fertilization envelope, blocking further sperm entry.

While oxygen consumption and enzyme-mediated cross-linking do occur during fertilization envelope formation, the term “respiratory burst”—a sudden, calcium-dependent rise in oxygen usage typically describing immune responses—is inaccurately applied here, making statement (C) incorrect.

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This article clears up misconceptions and accurately describes key molecular interactions and biochemical processes underlying sea urchin fertilization, providing a nuanced understanding of this crucial biological event.