Final answer:
Unicellular organisms consist of a single cell, while multicellular organisms are made up of multiple cells.
Step-by-step explanation:
The difference between a unicellular organism and a multicellular organism lies in their structure. Unicellular organisms are made up of a single cell that performs all the necessary functions required for survival. On the other hand, multicellular organisms are composed of multiple cells that are specialized to carry out different tasks.
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