<P> Motile sperm cells typically move via flagella and require a water medium in order to swim toward the egg for fertilization . In animals most of the energy for sperm motility is derived from the metabolism of fructose carried in the seminal fluid . This takes place in the mitochondria located in the sperm 's midpiece ( at the base of the sperm head ) . These cells can not swim backwards due to the nature of their propulsion . The uniflagellated sperm cells ( with one flagellum ) of animals are referred to as spermatozoa , and are known to vary in size . </P>
where are the mitochondria located in the sperm
midpiece