Re: What are single celled organisms?

Amber, 
A cell is the unit of life. A cell (with a few variations) contains one full 
copy (or more) of genetic material (DNA) and mechanisms to read this genetic 
material for making protein machines necessary for survival (respiration, 
nutrient intake, waste disposal, etc.), and of course to produce more cells 
(reproduction). With time, cells have evolved a variety of lifestyles, 
including living in clonal groups (all cells coming from one cell) to make 
fungi, plants and animals (including humans). Some types of cells though 
have evolved to live independently, such as bacteria, protozoa and yeast. 
They can absorb foods, extract nutrients, produce energy, dispose of waste, 
reproduce... all in all, live as a single cell. These single celled 
organisms are very important for the planet (e.g. as decomposers) and for 
the multicellular organisms like us (e.g. as disease producing "pathogens").

For reference, its hard to find a book or a review article that would cover 
all kinds of unicellular organisms, but you can use any of the biology 
textbooks that describe the cell and unicellular organisms (look under 
eukaryotes, bacteria, protozoa, prokaryotes, and yeast).