A sperm cell has enzymes in its head to help it penetrate the zona pellucida of the egg. The zona pellucida is a thick coat that surrounds the eggs of mammals.
Before a sperm can fertilize the egg, it must penetrate the egg's protective coating. The enzymes act as a sort of drill, making it easier for the sperm to penetrate the zona pellucida. These enzymes work with the sperm's tail, which flagellates to propel the sperm through the cervix. As the sperm passes through the zona pellucida, its enzymes and plasma membrane break down. If a sperm's enzymes break down before it reaches the egg, fertilization does not occur.