Final answer:
In the course of double fertilization within angiosperms, one sperm cell fuses with the egg to form a zygote, while the second one fuses with the two polar nuclei to form the endosperm.
Step-by-step explanation:
In the process of double fertilization in angiosperms, one of the crucial steps involves two sperm cells that have unique roles after a pollen tube delivers them to the embryo sac. One sperm cell fuses with the egg cell to form a diploid zygote, which will develop into the future plant embryo. The other sperm cell fuses with the two polar nuclei located in the central cell of the embryo sac. This fusion results in the formation of a unique triploid cell, which develops into the endosperm. The endosperm serves as a nutritive tissue, providing a food reserve for the developing embryo within the seed.