There are organisms which are composed of a single cell and this single cell performs all the vital functions of that organism. Such an organism is called___.

A unicellular organism also known as a single-celled organism is an organism that consists of only one cell and this cell performs all the vital functions of that organism. A multicellular organism that consists of more than one cell. The main groups of unicellular organisms are bacteria, archaea, protozoa, unicellular algae and unicellular fungi.