Paramecio, example of an organism formed by a single cell.
A unicellular organism is formed by a single cell. Examples of unicellular organisms are bacteria and algae and some fungi, protozoa. Although it is surprising, the unicellular beings represent the immense majority of the alive beings that populate the Earth at the moment; in number they surpass by far the rest of the living beings of the planet. However, the living beings that are familiar to us are constituted by a set of cells with differentiated functions; they are multicellular organisms. However, it should not be forgotten that these multicellular organisms come from a single cell at the origin of their life. All organisms pass in an initial moment of their existence to be a single cell (zygote).
Most unicellular beings are prokaryotes, like bacteria, but there are some unicellular eukaryotic beings, like protozoa.
Unicellular beings are considered more primitive than multicellular beings, because of their simple system. The unicellular organisms are constituted by a single cell, whereas the multicellular organisms are formed by many cells together specialized in certain functions. Together they make fabrics; those tissues unite and form organs, and a set of organs form a system of organs, and finally, a grouping of these forms a complex organism.
Circulation in unicellular organisms is carried out by the movement of the cytoplasm of the cell, which is called cyclosis.
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