Unicellular organisms are known as single-celled organisms. They are made up of a single cell. Organisms like the amoeba, Paramecium are single-celled organisms, they are the oldest forms of life, they existed about 3.8 million years ago. Bacteria, archaea, protozoa, unicellular algae and unicellular fungi are the main groups of unicellular organisms. The single-cell regulates all the activity of the organism. Unicellular organisms are small are mostly invisible to the naked eye.
There are two general categories of unicellular organisms: prokaryotic and eukaryotic organisms. Prokaryotic unicellular organisms are protists and some fungi. Some of these unicellular prokaryotes live in colonies. They live together and all the cells of the colony is the same. All the process of life is carried out in each cell in order for the cell to survive. Simplest multicellular organisms are made of cells that dependent on each other for their survival. Most of the multicellular organism are microscopic and are known as microscopic organisms.
Unicellular organisms vary in size. The smallest organism a bacteria is only 300 nanometers and range up to 20cm. These organisms usually posses cilia, flagella or pseudopodia that help them in locomotion. They have simple body with basic features. Reproduction is both by asexual and sexual means. Nutrition is usually by the process of phagocytosis, where the food particle is engulfed and stored in vacuoles present in the organism.
