To stay alive, cells must be able to carry out a selection of functions. A few cells must be able to move, and most cells must be able to divide. All cells must preserve the right absorption of chemicals in their cytoplasm, consume food and use it for energy, reuse molecules, push out wastes, and make proteins. Cells must also be able to react to changes in their environment. The cell functions can be categorized as movement, nutrition, energy, protein synthesis and the cell division.
A lot of unicellular organisms whirl, slither, thrash, or creep to look for food and escape enemies. Swimming organisms habitually move by means of a flagellum, a long tail-like formation made of protein. A lot of 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, in addition 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 moreover 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 additional purposes. In the respiratory tract of humans, for example, millions of ciliated cells prevent inhaled dust, smog, and microorganisms from ingoing the lungs by sweeping them up on a current of mucus into the throat, where they are swallowed..........
