Question

A sea urchin egg is treated with an inhibitor that prevents the rise in intracellular Ca2+ right before fertilization. Which of the following events would be blocked by addition of this inhibitor? Choose one or more:

A. formation of vesicles under the plasma membrane

B. exocytosis of hydrolytic enzymes

C. fertilization envelope rising from the cell surface

D. entry of multiple sperm into the cell

Answer 1

Option D is correct as it is done to prevent polyspermy as it is important that only one sperm fuses with egg.Certain changes happen at tip of sperm, Acrosome which help in fertilisation, this is best studied in sea urchins. there are two mechanism to prevent polyspermy, fast block and slow block. In this case of slow block to polyspermy, after few seconds of fusion of egg and sperm plasma membrane, Inositol phosphate releases calcium ions from intracellular space in egg endoplasmic reticulum.A wave of calcium ions result in fusion of cortical vesicles with egg plasma membrane which release their contents into space surrounding eggs ie., in perivitelline space.This raises vitelline membrane and inactivates binding receptors on it.So, any additional sperm present releases from there and no more binding happens.

Thus this is a mechanism to prevent polyspermy well studied in Sea urchins.