Final answer:
Polyspermy is prevented by the fast block and slow block mechanisms. The fast block involves a change in ion permeability, while the slow block is triggered by calcium ions and involves the cortical reaction. Both mechanisms modify the molecular state of the oocyte.
Step-by-step explanation:
The first mechanism is the fast block, which involves a near instantaneous change in sodium ion permeability upon binding of the first sperm, depolarizing the oocyte plasma membrane and preventing the fusion of additional sperm cells. The fast block sets in almost immediately and lasts for about a minute. The second mechanism is the slow block, which is triggered by an influx of calcium ions following sperm penetration. In this process, known as the cortical reaction, cortical granules fuse with the oocyte plasma membrane, releasing zonal inhibiting proteins and mucopolysaccharides into the space between the plasma membrane and the zona pellucida. The zonal inhibiting proteins release any other attached sperm and destroy the oocyte's sperm receptors, preventing more sperm from binding. The mucopolysaccharides then coat the nascent zygote in an impenetrable barrier, together with the hardened zona pellucida, called the fertilization membrane.