Flagella and cilia have the exact same structures and functions, and the names merely indicate how many are present on a given cell. When found singly or in a pair, these cell protrusions are called flagella. Identical cell protrusions are called cilia if found in groups of three or more.
Each individual cilium or flagellum is constructed of a microtubule core extending from the plasma membrane of the cell. The microtubules are arranged in a specific pattern of nine pairs surrounding two singlet microtubules to allow for sliding. The sliding motion lets the flagellum or cilium bend, facilitating movement.
Movement in flagella and cilia begins at the base next to the cell surface and spreads out along the protrusion to the end in waves.
Flagella are found on sperm cells and on some algae, and they are used to propel the entire cell forward. Cilia are found embedded in the lining of the respiratory tract, where they sweep away debris and germs that enter the nose or mouth, and in the female reproductive tract. Some protozoa are covered in cilia and use them both for locomotion and for pulling in food. The smallest cilia and flagella are each just a few microns long, while the largest stretch over 2 millimeters long. Flagella typically grow longer than cilia.