Final answer:
Sperm penetrate the zona pellucida through the acrosomal reaction, in which enzymes released by the acrosome clear a path through the zona pellucida. The winning sperm then fuses with the oocyte's plasma membrane and enters the oocyte's interior.
Step-by-step explanation:
When the sperm cell encounters the oocyte, it first burrows through the corona radiata, a layer of cells surrounding the oocyte. Then, it binds to receptors in the zona pellucida, which is a protective glycoprotein membrane surrounding the oocyte. This binding triggers the acrosomal reaction, where the enzyme-filled acrosome of the sperm releases its digestive enzymes. These enzymes clear a path through the zona pellucida, allowing the sperm to reach the oocyte. Finally, the winning sperm makes contact with the oocyte's plasma membrane, fuses with it, and enters the oocyte's interior.