An organism can exist as a single cell, or as many cells working together. The human body is made of some 10 to 100 trillion cells.

Cells are found with many different forms and functions, from tiny E.coli bacteria that live in the stomach to tree-like nerve cells that can extend to 3 or 4 feet in length. Plant cells have rigid walls and photosynthesize. Blood cells cycle in less than a minute through the body carrying oxygen and nutrients. Cells can even exist in extreme environments, as astrobiologists are discovering, from deep-sea vents that are far from light, highly pressurized and very hot, to the vacuum of space.

"Every animal appears as the sum of vital units, each of which bares in itself the complete characteristics of life."—Rudolph Virchow