Unicellular organisms are organisms that are made up of only one cell.
In a unicellular organism, all the important life processes such as respiration, digestion, excretion, and reproduction take place within a single cell.
These organisms cannot be visualized by the naked eye.
Characteristics:
Reproduction in these organisms occurs by asexual means.
Unicellular organisms can be prokaryotes if they do not have a nuclear wall and they can be eukaryotes if they do have a nuclear wall.
They can be found in one of the harshest environments where survival of other multicellular organisms is not possible. For example in hot springs.
They take up their nutrition by the process of diffusion.