Living organisms are commonly divided into six kingdoms: archaea, bacteria, fungi, protists, plants, and animals. What’s the biggest problem with classifying organisms in this way?
The biggest problem with classifying organisms into six kingdoms is that single-cell eukaryotes (protists) and algae (plants) do not strictly fall into one of these groups as many members of these groups do not share similarities at biochemical and genetic levels. These are highly diverse groups of organisms that cannot be classified together in one kingdom. The only thing that they share is their unicellular structure typical to eukaryotes.