By Ellie Gambrel | Updated March 24, 2022
Scientists sometimes refer to the Protista kingdom as the “catch‑all” group because its members don’t fit neatly into the other three classical kingdoms. Protists are defined simply by their exclusion from the kingdoms Animalia, Plantae, and Fungi, and are further classified by their closest resemblance to animals, plants, or fungi.
All protists are eukaryotes with a true nucleus and mitochondria, enabling aerobic respiration. Most are unicellular—though a few, such as certain algal species, are multicellular. Their diminutive size allows them to rely on diffusion for gas exchange and waste removal. Protists are predominantly aquatic, but many thrive in moist soils, decaying organic matter, or even the human body.
Locomotion varies widely: flagella and cilia provide rapid swimming, while some use pseudopodia—dynamic “false feet”—to glide or engulf food.
Protozoa, literally “first animals,” are heterotrophic protists that capture prey through phagocytosis, enveloping it in a vacuole. Examples include the amoeba, paramecium, and the parasitic plasmodium responsible for malaria.
Algae are autotrophic protists that harness photosynthesis. This group spans red, brown, and green algae, as well as diatoms, dinoflagellates, and euglena. Many algal species exhibit complex life cycles, and green algae are widely regarded as the ancestral lineage of land plants. Unlike vascular plants, algae possess only thin blades and lack a system for long‑distance transport of water and nutrients.
Slime molds are often bright yellow or orange and absorb nutrients from their environment rather than producing them. Found in decayed wood, they form multinucleate masses that can move via pseudopodia. Despite their fungal resemblance, slime molds use phagocytosis to ingest bacteria and other small organisms.
