A flagellum (plural: flagella) is a long, whip-like structure that helps some single celled organisms move. It is composed of microtubules. They help propel cells and organisms in a whip-like motion. The flagellum of eukaryotes usually moves with an “S” motion, and is surrounded by cell membrane.
Flagella are structurally almost identical with the much smaller Cilia. So much so that it has been proposed protists bearing either should be unified in the Phylum Undulipodia. Previously, Margulis had proposed that the Ciliates alone should be placed in a Phylum Ciliophora. Admittedly, the Protista is a collection of disparate single-celled forms, but while a more sophisticated taxonomy is in flux (changing), Protista is still a useful term.
Cilia and flagella are cell organelles, specialised units which carry out well-defined functions, like mitochondria and plastids. It is fairly clear now that all or most of these organelles have their origin in once-independent prokaryotes (bacteria or archaea), and that the eukaryote cell is a 'community of micro-organisms' working together in 'a marriage of convenience'.
Three types of flagella have so far been distinguished; bacterial, archaeal and eukaryotic. The main differences among these three types are summarized below:
- Bacterial flagella are helical filaments that rotate like screws. They provide two of several kinds of bacterial motility.
- Archaeal flagella are superficially similar to bacterial flagella, but are different in many details and considered non-homologous.
- Eukaryotic flagella – those of animal, plant, and protist cells – are complex cellular projections that lash back and forth. Eukaryotic flagella are classed along with eukaryotic motile cilia as undulipodia to emphasize their distinctive wavy appendage role in cellular function or motility. Primary cilia are immotile, and are not undulipodia; they have a structurally different 9+0 axoneme rather than the 9+2 axoneme found in both flagella and motile cilia undulopodia.
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- Margulis L. Schwartz K.V. & Dolan M. 1999. Diversity of life: the illustrated guide to the five kingdoms. Jones & Bartlett, Boston, p94. In this work the authors propose 19 phyla for the Protista, and call this 'Kingdom' the 'Protoctista', a term which is unfortunately almost unpronounceable.
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- Molecular Machines Index of Illustrations, Graphics, and Animations
- Physics Today introduction to the bacterial flagellum by Howard Berg