A cell is the smallest individual unit of any living organism. All living organisms are composed of one or more cells. Although individual cells may perform different functions and have slightly different structures, they are for the most part made of the same structures, undergo the same chemical reactions within, and can perform the same basic functions. A human being is made up of about ten trillion (10,000,000,000,000) cells.
Antoine van Leeuwenhook was the first person to see invidual cells through his microscope. Cells in multicellular organisms were first identified in cork in 1665 by Robert Hooke. However, it was not until the 20th century that cell metabolism first became understood.