Unicellular Organism : The organisms made up of single cell are called unicellular organism.
Most of the microorganisms, such as, bacteria is is made up of single cell. The figure given below is of a Escherichia coli bacteria, which is magnified 10,000 times.
Unicellular organism are are divided into two types ;
1. Prokaryotic : The prokaryotic cells are those cells, whose nucleus is not bounded by nuclear membrane. The nucleus and cystoplasm can't be differentiated.
Example - Bacteria, Archae etc...
2. Eukaryotic cell : The Eukaryotic cell are those cell, whose nucleus is bounded by the nuclear membrane. Theses cells are very complex and nucleus can be easily differentiated from the rest of the cell.