Protists are difficult to characterize because of the great diversity of the kingdom. These organisms vary in body form, nutrition, and reproduction. They may be unicellular, colonial, or multicellular. As eukaryotes, protists can have many different organelles, including a nucleus, mitochondria, contractile vacuoles, food vacuoles, eyespots, plastids, pellicles, and flagella. The nuclei of protists contain chromosomes, with DNA associated with proteins. Protists are also capable of sexual, as well as asexual reproduction, meiosis, and mitosis. Protists can be free-living, or they may live symbiotically with another organism. This symbiosis can be mutualistic, where both partners benefit, or parasitic, where the protist uses its host as a source of food or shelter while providing no advantage to the other organism. Many protists are economically important and beneficial to mankind, while others cause fatal diseases. Protists make up the majority of the plankton in aquatic systems, where they serve as the base of the food chain. Many protists are motile, using structures such as cilia, flagella, or pseudopodia (false feet) to move, while others are sessile. They may be autotrophs, producing their own food from sunlight, or heterotrophs, requiring an outside source of nutrition. Researchers are currently comparing the RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) sequences of the protists with those of plants and animals, but the evidence is inconclusive. It is unknown whether protists were the precursors to plants, animals, or fungi. It is possible that several evolutionary lines of protists developed separately. Biologists consider the protists as a polyphyletic group, meaning they probably do not share a common ancestor. The word protist comes from the Greek word for the very first, which indicates that researchers believe protists may have been the first eukaryotes to evolve on Earth.
Despite the great diversity evident in this kingdom, scientists have been able to classify the protists into several groups. The protists can be classified into one of three main categories, animal-like, plant-like, and fungus-like. Grouping into one of the three categories is based on an organism's mode of reproduction, method of nutrition, and motility. The animal-like protists are known as the protozoa, the plant-like protists are the algae, and the fungus-like protists are the slime molds and water molds.