For fertilization to take place and a new human being to be created, some important changes must happen in both the sperm and the ovum. First, they must be able to meet in the right place. The egg must move down into the fallopian tube, and the sperm must make its way up into the fallopian tube.
As the sperm travels up the fallopian tube, it undergoes two processes to help it prepare to fertilize the ovum. One is called capacitation, which changes the sperm's membrane (or outer layer) so that it can enter into the ovum. The other process is called hyperactivation, which allows the sperm's tail to move more efficiently to allow it to penetrate the ovum.
The ovum, too, must experience a change if it is to receive the sperm and be fertilized. Its outer layer of cells, the cumulus oophorus, must be dissolved so that the sperm can enter. The sperm secretes an enzyme that breaks through this layer through a process that is essentially fusion and digestion of the membrane's cells. This is called the acrosome reaction.
Once the sperm has entered the ovum and fertilization occurs, the ovum's membrane changes once again, this time making itself impenetrable by other sperm. This, too, is an enzyme-based reaction that creates a protective layer of coritcal granules that repel other sperm cells and prevent polyspermy, the fertilization of an ovum by more than one sperm.