Many unicellular organisms can move about in water while certain stationary cells of multicellular organisms can cause water waters currents that move food and other substances over their surfaces. The structures that bring about these movements are cilia and flagella.
Cilia and flagella are very similar structures. They are composed mainly of microtubules and are powered by ATP.
Cilia are short, hair like structures that project out of the cell's surface. They are usually numerous and packed closely together. They move in co-ordinated way to bring about movement. In the paramecium, the action of the cilia moves the whole organism; at the same time it also directs a current of water containing food into the paramecium's gullet. In the cells lining the human windpipe, the beating of the cilia causes mucus to move up towards the throat.
Flagella are at least ten times longer than cilia, and are fewer in number. A flagellum is like a whip and can send waves of movement along its length. Flagella help to move organisms like euglena and chlamydomonas, and motile parts of organisms like the human sperms
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