In general, cells (which include prokaryotic and eukaryotic cells) have a main purpose: to survive. In eukaryotic cells, their shapes often reflect their functions. For example, nerve cells have long extensions that reach out in various directions in order to allow the cell to send and receive nerve impulses. Another example is skin cells. Since skin cells are flat and plate-like, they help cover and protect the surface of the body. As organisms evolve into more advanced organisms, their cells become more specialized and eventually were unable to survive independently. Groups of cells that carry out a similar function is called a tissue. Groups of tissues that perform a particular job form an organ. Groups of organs that perform related tasks form organ systems. Finally, groups of organ systems combine to form an organism. In other words, prokaryotic and eukaryotic cells try to keep the organism alive.
