Algae (Euglena) do photosynthesis as plants do. They also move around and eat, as do animals. But they are unicellular. In order to be classified as a plant or animal, an organism has to be multicellular, made of more than one cell. Since it is a unicellular organism with some plant and animal characteristics, it is called a protist. Plant cells have walls while algae does't have one, so it is a protozoan. Algae resemble the protozoa, so they are put into the Protist Kingdom.