Respuesta :
They are considered complex because they are ever changing. Something is constantly happening in the cell, whether it be protein creations, Mitosis, or Meosis, or even just the regular transportation of proteins throughout the cell.
Mostly because they can function independently. For example, microorganisms, more especifically unicellular organisms, are composed by one cell, meaning they're a cell, but even so can function like a normal living organism.
Hope it helped,
BioTeacher101