Movement

Euglena gracilis

Amoeba

Paramecium caudatum

Many unicellular organisms swim, glide, thrash, or crawl to search for food and escape enemies. Swimming organisms often move by means of a flagellum, a long tail-like structure made of protein. Many bacteria, for example, have one, two, or many flagella that rotate like propellers to drive the organism along. Some single-celled eukaryotic organisms, such as euglena, also have a flagellum, but it is longer and thicker than the prokaryotic flagellum. The eukaryotic flagellum works by waving up and down like a whip. In higher animals, the sperm cell uses a flagellum to swim toward the female egg for fertilization. Movement in eukaryotes is also accomplished with cilia, short, hairlike proteins built by centrioles, which are barrel-shaped structures located in the cytoplasm that assemble and break down protein filaments. Typically, thousands of cilia extend through the plasma membrane and cover the surface of the cell, giving it a dense, hairy appearance. By beating its cilia as if they were oars, an organism such as the paramecium propels itself through its watery environment. In cells that do not move, cilia are used for other purposes. In the respiratory tract of humans, for example, millions of ciliated cells prevent inhaled dust, smog, and microorganisms from entering the lungs by sweeping them up on a current of mucus into the throat, where they are swallowed. Eukaryotic flagella and cilia are formed from basal bodies, small protein structures located just inside the plasma membrane. Basal bodies also help to anchor flagella and cilia.Still other eukaryotic cells, such as amoebas and white blood cells, move by amoeboid motion, or crawling. They extrude their cytoplasm to form temporary pseudopodia, or false feet, which actually are placed in front of the cell, rather like extended arms. They then drag the trailing end of their cytoplasm up to the pseudopodia. A cell using amoeboid motion would lose a race to a euglena or paramecium. But while it is slow, amoeboid motion is strong enough to move cells against a current, enabling water-dwelling organisms to pursue and devour prey, for example, or white blood cells roaming the blood stream to stalk and engulf a bacterium or virus.