Living things that are composed of one cell are known as unicellular organisms. Examples of unicellular organisms include amoebas, bacteria and certain fungi. Unicellular organisms are either prokaryotes or eukaryotes
In a unicellular organism, one cell performs all life-functions for the organism. This includes ingestion of nutrients, digestion and breakdown of nutrients and cellular respiration. Eukaryotic cells have membrane-bound organelles that perform specialized functions in the cell. For example, the nucleus of the cell stores the genetic information in the cell, mitochondria are responsible for cellular respiration and energy production, and ribosomes are responsible for putting together proteins. By contrast, prokaryotic cells do not have membrane-bound organelles.