All cells are bound by a plasma membrane that regulates the composition of their contents. The cells of plants and some other organisms have a rigid, protective wall around the membrane. The resolving power of a microscope is limited by the wavelength of the radiation it uses. The distinction between procaryotic and eucaryotic cells forms a significant dividing line in the evolutionary history of life on Earth. By focusing on particular cellular structures, one can synthesize a functional view of the organisms that relates its functions to its evolutionary history and ecological role.