Of all the kingdoms of life, Kingdom Protista is perhaps one of the most diverse and interesting. Kingdom Protista spans from single-celled organisms with various means of locomotion and obtaining energy, to huge kelp, hundreds of feet in length. that create expansive underwater forests that are home to a wide variety of marine life. But what do these diverse forms of life have in common? First, the cells of all protists have a nucleus and other complex organelles.
unlike the prokaryotes of the domains archaea and bacteria, which have neither nuclei or complex organelles. Secondly, all the members of kingdom protista have one or more characteristics that prevent them from being classified in kingdom fungi, animalia, or plantae. For example, though there are protists that bear a striking resemblance to fungi, molecular analysis reveals that the cell walls of these protists are made up of cellulose, not the chitin found in all fungi. Another example is that, though a form of green algae now classified in kingdom protista is believed to have given rise to the members of kingdom plantae, the method of reproduction nevertheless differs significantly between algae and plants. Finally, though there are animal-like protists which are believed to have given rise to all the larger organisms found in kingdom animalia, these single-celled protists aren't classified as animals simply because, by definition, animals are multicellular.
Various modes of nutrition are represented in Kingdom Protista. The algae of Kingdom Protista are autotrophs that trap solar energy and convert it to chemical energy through photosynthesis. Predatory protists like amoeba and parrots are heterotrophs that obtain energy and nutrients by capturing and ingesting prey. Many fungi-like protists are saprotrophs, which absorb nutrients from dead or decaying organic matter, while a variety of protists are parasites.
As alluded to earlier, the ancestors of modern protists are believed to have given rise to fungi, plants, and animals. To the confusion of taxonomists, many protists, such as Euglena, fit equally well into animalism. like or plant like categories.
Euglena, for example, have photoreceptors and can swim toward a stimulus, features commonly associated with animals. But they use these abilities to seek light levels appropriate for photosynthesis, a process associated with plants. Thus, protistin taxonomy is still the subject of revision and controversy. Here we group members of the kingdom protista into three categories, the fungus-like slime and water molds, animal-like protozoa, and the plant-like unicellular and multicellular algae.