ICSE Class 10 Answered

Binding of the sperm to the egg induces depolarisation of the egg plasma membrane. Depolarisation causes fast block to polyspermy that checks additional sperms from joining to the egg’s plasma membrane. Immediately after the fusion of the sperm and egg plasma membranes, the egg shows a cortical reaction to further check the entry of more sperms. In this reaction, the cortical granules present beneath the egg’s plasma membrane fuse with the plasma membrane and release their contents (enzymes) between the plasma membrane and the zona pellucida. These enzymes harden the zona pellucida, which now functions as a sure block to polyspermy.