The cell is the basic unit of life. The cell theory, set forth in the 1850's, states that cells are the fundamental units of life, because a cell is the simplest unit capable of independent existence. All living things are made of cells. They are made up of 90% water. The rest of the present molecules are 50% protein, 15% carbohydrate, 15% nucleic acid, 10% lipids and 10% others.

Two types of cells are recognized in living things. Prokaryotes (literally, "before the nucleus") are cells that have no distinct nucleus that is found to float around the cell. Most prokaryotic organisms are single-celled, such as bacteria and algae. Eucaryotic (literally, "true nucleus") organisms, on the other hand, have a distinct nucleus and a highly organized internal structure. The nucleus directs all cellular activities by controlling the synthesis of proteins. The nucleus contains encoded instructions for the synthesis of proteins in a helical double-stranded molecule...