Organisms that are composed of many cells working together to form tissues, organs, and organ systems are referred to as complex unicellular bicellular multicellular

Multicellular, since they have many cells working together in one organism.

Cell division is important to single celled organisms because it allows the cells to reproduce. through cell division, cells make exact copies of themselves. when single celled organisms go through cell division, they make exact copies of themselves which is how they reproduce.