ABSTRACT
L iving organisms are all composed of cells. M ulticellu lar organisms are composed of several kinds of specialized cells, spatially organized and interacting with each other in com plex ways; unicellular organisms, on the other hand, are composed of just one cell that must accomplish all the necessary functions for its maintenance and reproduction. The cellular nature of all forms of life and the evidence that no life exists at hierarchical levels below the cell, can be seen as the most striking property of living organisms.
