Protists are eukaryotic organisms that are not plants, fungi, or animals.
Protists are most easily described as eukaryotes that are not plants, fungi, or animals. They are extremely diverse in their structure, reproduction, and modes of gaining energy and carbon. Most protists are unicellular; however, there are a few multicellular groups. Some protists are autotrophic, meaning they make their own food, while others are heterotrophic, or consume other organisms for food. Many reproduce asexually, but others reproduce sexually, and some use spores, similar to fungal reproduction. A spore is a haploid reproductive structure of plants, algae, and fungi that divides by mitosis into a multicellular gametophyte. It can travel long distances or remain dormant for a long time. Many protists are free-living, while others are symbiotic, living in close association with other organisms. Many are pathogenic, causing disease in other organisms. For example, malaria, African sleeping sickness, giardia, and amoebic dysentery are all human illnesses caused by protists. Protists continue to present a taxonomic challenge for biologists because it does not include all descendants of a single common ancestor. Modern biologists have largely abandoned this distinction in favor of eukaryotic supergroups. The term protist is frequently still used for practical purposes. The current classification recognizes four eukaryotic supergroups: Excavata, SAR clade, Archaeplastida, and Unikonta. A supergroup is a recent expansion of the traditional nested taxonomy of the Eukarya.
- Excavata are characterized by their cell shape and their unusual cellular structure for motility, flagella, and energy production, mitochondria. The organism responsible for giardia, an illness caused by drinking contaminated water, belongs to this group.
- The SAR clade is named after three large groups: Stramenopila, Alveolata, and Rhizaria. Many of the members of this supergroup are photosynthetic. However, this group also contains the protist responsible for malaria, Plasmodium, which kills many thousands of people annually.
- Archaeplastida is the eukaryotic supergroup that includes plants and plantlike protists. Red and green algae were once considered to be plants, partially due to their ability to photosynthesize. The slippery green organism commonly called seaweed is one of the few multicellular protists. Some algae in this group possess an alternation of generations life cycle, which is also a characteristic of plants. Alternation of generations is the production of alternating asexual and sexual reproductive stages, consisting of a multicellular gametophyte (haploid, having a single set of chromosomes) generation and a multicellular sporophyte (diploid, having a double set of chromosomes) generation.
- Unikonta, includes animals, fungi, and closely related protists. This group includes amoebas that move with footlike cellular projections. Some amoebic protists were once thought to be fungi because their reproduction uses spores and closely resembles that of certain fungi. One group of protists, the choanoflagellates, is so closely related to animals that they have cells nearly identical in appearance to cells found in sponges, primitive animals.