By Dr Ananya Mandal, MD
A sperm is the male reproductive cell. The full name for a sperm that has a single flagellum (or "tail") to propel it along, is a spermatozoon. Unlike this motile sperm, when a sperm lacks a flagellum and is non-motile, it is termed a spermatium.
The spermatozoon requires a liquid medium that it can move through in order to reach and fertilize the female reproductive cell, the egg.
Motile sperm are not unique to animals. They are also produced by some plants such as ferns and the seed producing conifer and ginkgo. The sperm are the only flagellated cell in many of these plants and frequently, the sperm have more than one flagellum and are termed multi-flagellated.
Movement of the spermatozoa
The flagellum of the sperm provides a whip like movement in order to propel it forwards. The sperm cannot swim backwards due to the nature of the flagellar movement and propulsion. The sperm consists of a head that is 5 µm by 3 µm in size and a flagellum of around 50 µm in length.
There is little cytoplasm within the sperm and much of it is the DNA. The head contains tightly packed DNA while the neck of the sperm contains mitochondria to provide the sperm with energy. Nearly 1000 sperm are made in the human male testes per second. Once released, the sperm can survive for approximately 48 hours.
The fluid medium into which sperm are released is called semen. Semen is made up of several fluids and enzymes secreted by the male reproductive tract. It provides nutrition to the spermatozoa and prevents the sperm from being motile due to its thick and alkaline nature. The sperm achieve complete motility once they reach the vagina of the female.
The vaginal pH is typically acidic and neutralizes the alkalinity of the semen, allowing them to travel up towards the ovaries where the egg is presented.
Reviewed by Sally Robertson, BSc