Flagellerade

Flagellerade, or flagellated in English, is a biological descriptor for cells or organisms that bear one or more flagella, whip-like organelles used for locomotion, feeding, or sensing their environment. Flagella occur across all domains of life, but their structure and origin differ among bacteria, archaea, and eukaryotes.

In bacteria, flagella are helical protein filaments driven by a rotary motor in the cell envelope. They rotate to propel the cell through liquids and can enable chemotaxis. Eukaryotic flagella, by contrast, are longer, composed of an axoneme with a 9+2 arrangement of microtubules, and move by dynein-driven bending. Archaean flagella resemble bacterial propellers in function but are structurally distinct and assembled differently.

Flagellated cells include many bacteria (for example Escherichia coli), and a wide range of protists and algae, such as Euglena, Giardia, Trypanosoma, Chlamydomonas, and Volvox. In some organisms flagella are used for swimming, in others to create feeding currents or to sense the surrounding milieu.

Historically, flagellates was used as a taxonomic grouping for motile protists that rely on flagella. Modern classifications place these organisms in several clades within the broader eukaryotic diversity, and the term is now more descriptive than taxonomic.

Medical and ecological relevance: flagellated protists include both benign species and pathogens, such as Giardia lamblia and Trypanosoma spp., which affect humans and animals. Studying flagellar structure and function provides insights into cell motility, evolution, and host–pathogen interactions.