Flagella (singular, flagellum) and cilia (singular, cilium) are microtubular structures that extend outside some cells and that assist in moving the cell or the cell's surroundings. Flagella are much longer than cilia. When a cell has cilia, their number is large, whereas a cell will have few flagella, or a single flagellum. Many protists have cilia, and the sperm of many plants and animals have flagella. Flagella and cilia are assembled from tubulin subunits organized to form a circular arrangement of nine pairs of microtubules attached to a central pair. Flagella and cilia bend to cause movement of either the cell or the surroundings. The bending uses energy derived from the hydrolysis of ATP.
