How are all cells like a unicellular organisms?

Unicellular organisms are made up of only one cell that carries out all of the functions needed by the organism, 

Unicellular organisms are made up of only one cell that carries out all of the functions needed by the organism, while multicellular organisms use many different cells to function. Unicellular organisms include bacteria, protists, and yeast. ... One can easily observe the differences in these cells under a microscope.

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