All living organisms consist of cells. Even a single cell can sustain all basic life processes and live as a unicellular organism [1] . On the other hand, trillions of cells, distinguished by hundreds of different types, join in incredibly complex ways to build and animate a multicellular plant or animal [2] . The size of most cells ranges from 1 to 100 μm, but some bacteria are smaller than 0.5 μm and the axons of spinal nerve cells can exceed the length of 1 m. Some hundred trillion atoms, bound in water molecules, other small molecules, and life's pivotal macromolecules (proteins, nucleid acids, lipids, carbohydrates) build and animate the cell (see also Sheet). In amazing ways, bizarre molecular structures provide machine-like functionality. Cells differ greatly between two basic types: eukaryotic cells, which have a nucleus and organelles; and prokaryotic cells, which have neither a distinct nucleus nor specialized organelles. The distinction defines the fundamental domains [3] of all known life forms and raises intriguing questions about life's origin.
Unicellular organisms include all archaea, most bacteria, some fungi, and even an important type of algae. Unicellular algae in the form of diatoms are a major constituent of phytoplankton, the primary photosynthetic producer of organic compounds for the oceanic food chain (according to NASA, phytoplankton accounts for half of all photosynthetic activity on earth). Unicellular, animal-like amoebae have a very large genome , move by protrusion of their cell plasma, and feed by engulfing microorganisms, such as diatoms.
It is estimated that the human body is composed of several tens of trillion cells made up of more than two hundred cell types derived from endoderm (source of dozens of different types of gland and lining cells), ectoderm (source of dozens of different types of epidermal and nerve cells), and mesoderm (source of dozens of different types of muscle, blood, and internal organ cells).
The superdomain system groups all life into eukaryotes and prokaryotes, while the more recent three-domain system divides prokaryotes into two separate domains: bacteria and archaea, which are similar in size and shape, but differ in genes, metabolism, and basic biochemistry. The origin of these lifeforms possibly dates back more than 3.5 billion years. Today, bacteria and archaea form a very large biomass, about as large as all plants and animals combined.