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Protists are a diverse group of mostly unicellular eukaryotic organisms that are not animals, plants, or fungi. They form a broad assemblage rather than a single clade, and modern taxonomy treats them as a collection of multiple supergroups rather than a formal kingdom. Members range from tiny flagellates to large, colonial algae and some multicellular forms.

Diversity within the group includes photosynthetic algae (which possess chloroplasts) and non-photosynthetic heterotrophs (protozoa), as well as mixotrophs that combine photosynthesis and ingestion. They inhabit almost every moist environment: fresh and marine waters, soils, and the bodies of other organisms. They play key roles in food webs, nutrient cycles, and as primary producers in some ecosystems.

Structure and movement: protists vary in size and form; many are unicellular with organelles such as nuclei, mitochondria, and sometimes chloroplasts. Modes of locomotion include flagella, cilia, and amoeboid pseudopods. Some protists form colonies or simple multicellular structures.

Life cycles: reproduction is diverse—binary fission, budding, autosporulation, or sexual processes such as gamete fusion and zygotic meiosis. Some exhibit life cycle stages alternating between haploid and diploid phases.

Impact on humans: several protists are pathogens (for example, Plasmodium, Giardia, Entamoeba) causing disease; others are important model organisms in biology (Dictyostelium) or are studied for their basic cellular processes. Protists remain an active area of research in ecology, evolution, and cell biology.