Protozoa are single‑celled eukaryotic organisms that lack a rigid cell wall and feed on organic matter. They are typically motile and are distinguished by their varied mechanisms of locomotion and complex life cycles.
Classification and cellular features
Protozoa comprise several groups of heterotrophic protists, traditionally classified by their mode of movement and other ultrastructural features. Amoebae such as Entamoeba and Acanthamoeba move using extensions of the cytoplasm called pseudopodia and engulf food by phagocytosis. Flagellated protozoa, including Giardia lamblia and Trypanosoma brucei, possess one or more whip‑like flagella for propulsion. Ciliates such as Paramecium are covered with rows of cilia that beat in coordinated waves to enable swimming and feeding. The Apicomplexa are non‑motile in their mature forms and have an apical complex of organelles for host cell invasion; this group includes Plasmodium, Toxoplasma and Cryptosporidium. All protozoa have a membrane‑bound nucleus, mitochondria (or related organelles), Golgi apparatus and endoplasmic reticulum. Many can form hardy cysts that resist environmental stresses and facilitate transmission between hosts. A trophozoite stage typically carries out growth and feeding.
Ecological roles and significance in disease
Protozoa occupy diverse habitats, from freshwater and marine environments to soil and the gastrointestinal tracts of animals. Free‑living species are important consumers of bacteria and algae, forming a key link in microbial food webs and recycling nutrients. Some protozoa live in mutualistic associations, such as those in termite guts that digest cellulose. Others are opportunistic or obligate parasites of humans and animals. Plasmodium species cause malaria, characterised by cyclical fevers and anaemia; Trypanosoma brucei causes African trypanosomiasis; Leishmania spp. cause cutaneous and visceral leishmaniasis; Entamoeba histolytica is the agent of amoebic dysentery; and Giardia lamblia causes diarrhoeal disease. Control measures include improved water sanitation, vector control and antiparasitic drugs. Molecular techniques are reshaping protozoan taxonomy and revealing evolutionary relationships with other eukaryotes.
The term protozoa encompasses a broad array of unicellular eukaryotes that are united by their lack of a cell wall and heterotrophic nutrition. Their diversity underscores the evolutionary experimentation within the eukaryotic domain.
Related Terms: Eukaryote, Amoeba, Flagellate, Sporozoan, Cyst