What are three main problems with having only two kingdoms

There are three main problems with having only two kingdoms. The first problem concerns the unicellular flagellates such as Euglena and its relatives. These were put   with  protozoa in the animal kingdom. However, some euglenoids for example Euglena itself, contain Chlorophyll and feed autotrophically by photosynthesis. With only two kingdoms we have to content with the embrassing fact that these organisms car, in effect hop from one kingdom to the other.

'The  second problem concerns the fungi. Although fungal hyphae bear a superficial resemblance to the filaments of simple multicellular algae like Spirogyra, fungi are really very different from green plants. Not only do they lack chlorophyll and feed heterotrophically but in several structures differ from those of the plants.

The third problem is posed by bacteria. In electron microscope we can see that bacteria have simple prokaryotic cell structure, and same feature is also found in blue-green algae. They appear to be similar to each other, and markedly different from all other organisms which are eukaryotic. To solve this problem new five kingdom system was proposed.