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Unicellular organisms are composed of a single cell. Multicellular organisms are composed of many specialized cells. Specialized cells differ in structure (size, shape...) and function (the role they perform in the organism).
Multicellular organisms need specialised organ systems, whereas all the life processes in a unicellular organism take place in that one cell. Multicellular organisms need organ systems to carry out functions such as:1  Communication between cells, eg the nervous system and circulatory system.ulticellular organisms need specialised organ systems, whereas all the life processes in a unicellular organism take place in that one cell. Multicellular organisms need organ systems to carry out functions such as: