The smaller, usually motile male reproductive cell of most organisms that reproduce sexually. Sperm cells are haploid (they have half the number of chromosomes as the other cells in the organism's body). Sperm often have at least one flagellum. During fertilization, the nucleus of a sperm fuses with the nucleus of the much larger egg cell (the female reproductive cell) to form a new organism. In male animals, sperm are normally produced by the testes in extremely large numbers in order to increase the chances of fertilizing an egg. Motile sperm cells produced by some multicellular protist groups (such as the algae),
the bryophyte plants, and the seedless vascular plants, require water to swim to the egg cell. In gymnosperms and angiosperms, sperm do not need water for mobility but are carried to the female reproductive organs in the pollen grain. In the cycads and the gingko (both gymnosperms),
the sperm are motile and propel themselves down the pollen tube to reach the egg cell. In the conifers and angiosperms, the sperm are not themselves motile but are conveyed to the ovule by the growing pollen tube.
A Closer Look The human sperm cell is divided into a head that contains the nucleus, a mid-section that contains mitochondria to provide energy for the sperm, and a flagellum that allows the sperm to move. When fertilization occurs, the nucleus and other contents from the sperm cells are drawn into the cytoplasm of the egg, but the mitochondria in the sperm are destroyed and do not survive in the zygote. Since mitochondria contain their own DNA (thought to be a relic from an existence as separate symbiotic organisms),
all of the mitochrondrial DNA in humans is thus inherited from the female. The semen produced by the male reproductive tract as a medium for sperm typically contains over 100 million sperm cells, all of which have but one purpose: to fertilize the single available egg.