Unicellular and multicellular organisms are different according to cells functions. A unicellular organism depends upon just one for all of its functions. A multicellular organism has cells specialized to perform different functions that collectively support the organism.
In every organism, each cell has specialized cell structures or organelles which are responsible for various cellular functions such as obtaining nutrients, producing energy, and making protein. Unicellular organisms carries out all the functions needed by the organism while multicellular organisms use many different cells to function.