Final answer:
Sperm undergo capacitation to become capable of fertilizing the egg, which involves increasing motility and altering the acrosomal membrane to penetrate the oocyte's outer layers. This process is essential for the sperm to fuse with the oocyte and form a zygote.
Step-by-step explanation:
Sperm are unable to fertilize the egg upon entry into the vagina and must undergo capacitation, allowing them to penetrate the oocyte's thick outer layer of cells, which facilitates fertilization. During the journey through the female reproductive tract, fluids prepare the sperm for this critical process. Through capacitation, the fluids improve sperm motility, as well as alter the membrane surrounding the acrosome, a cap-like structure in the head of the sperm containing digestive enzymes essential for the sperm to attach to and penetrate the oocyte.
Upon ovulation, the oocyte is surrounded by two layers: the corona radiata and the zona pellucida, both of which the sperm must navigate. Once capacitated, sperm can then break through these barriers and contact the oocyte plasma membrane, allowing one sperm to fuse with the oocyte and mark the moment of fertilization. After fertilization, the zygote, a diploid cell with genetic material from both parents, begins developing into a new individual.