Flexi Says: Humans are multicellular eukaryotes. Eukaryotic cells are a type of complex cell that contains a nucleus and other specialized structures called organelles. Bacteria are single-celled prokaryotes. Prokaryotic cells are a type of simple cell that does not contain a nucleus or membrane-bound organelles. Single-celled (unicellular) organisms like bacteria are obviously composed of just one cell, whereas multicellular organisms can be composed of trillions of cells. In humans and other animals, cells are organized into tissues, organs, and organ systems that provide specific functions.