A flagellate is a cell or organism with one or more
whip-like
appendages called
flagella. The word flagellate also describes a particular construction (or level of organization) characteristic of many
prokaryotes and
eukaryotes and their means of motion. The term presently does not imply any specific relationship or classification of the organisms that possess flagella. However, several
derivations of the term "flagellate" (such as "
dinoflagellate" and "
choanoflagellata") are more formally characterized.[1]
Form and behavior
Flagella in eukaryotes are supported by
microtubules in a characteristic arrangement, with nine fused pairs surrounding two central singlets. These arise from a
basal body. In some flagellates, flagella direct food into a
cytostome or mouth, where food is
ingested. Flagella often support hairs, called
mastigonemes, or contain rods. Their ultrastructure plays an important role in classifying
eukaryotes.
Among
protoctists and
microscopic animals, a flagellate is an organism with one or more flagella. Some cells in other
animals may be flagellate, for instance the
spermatozoa of most animal phyla. Flowering plants do not produce flagellate cells, but
ferns,
mosses,
green algae, and some
gymnosperms and closely related plants do so.[2] Likewise, most fungi do not produce cells with flagellae, but the primitive fungal
chytrids do. [3] Many protists take the form of single-celled flagellates.
Flagella are generally used for
propulsion. They may also be used to create a current that brings in food. In most such organisms, one or more flagella are located at or near the anterior of the cell (e.g, Euglena). Often there is one directed forwards and one trailing behind. Many parasites that affect human health or economy are flagellates in at least one stage of life cycle, such as Naegleria, Trichomonas and Plasmodium.[4][5] Flagellates are the major consumers of primary and secondary production in
aquatic ecosystems - consuming bacteria and other protists.[citation needed]
Flagellates as specialized cells or life cycle stages
An overview of the occurrence of flagellated cells in eukaryote groups, as specialized cells of multicellular organisms or as life cycle stages, is given below (see also the article
flagellum):[6][7][8]
In older classifications, flagellated
protozoa were grouped in Flagellata (= Mastigophora), sometimes divided into Phytoflagellata (= Phytomastigina, mostly autotrophic) and Zooflagellata (= Zoomastigina, heterotrophic). They were sometimes grouped with
Sarcodina (ameboids) in the group
Sarcomastigophora.
The autotrophic flagellates were grouped similarly to the botanical schemes used for the corresponding algae groups. The
colourless flagellates were customarily grouped in three groups, highly artificial:[11]
Protomastigineae, in which absorption of food-particles in holozoic nutrition occurs at a localised point of the cell surface, often at a cytostome, although many groups were merely saprophytes; it included the majority of colourless flagellates, and even many "apochlorotic" algae;
Pantostomatineae (or Rhizomastigineae), in which the absorption takes place at any point on the cell surface; roughly corresponds to "amoeboflagellates";
Distomatineae, a group of binucleate "double individuals" with symmetrically distributed flagella and, in many species, two symmetrical mouths; roughly corresponds to current
Diplomonadida.
Presently, these groups are known to be highly
polyphyletic. In modern classifications of the protists, the principal flagellated taxa are placed in the following eukaryote groups, which include also non-flagellated forms (where "A", "F", "P" and "S" stands for autotrophic, free-living heterotrophic, parasitic and symbiotic, respectively):[12][13]
Although the taxonomic group Flagellata was abandoned, the term "flagellate" is still used as the description of a
level of organization and also as an
ecological functional group. Another term used is "monadoid", from
monad.[15] as in Monas, and Cryptomonas and in the groups as listed above.
^Raven, J.A. 2000. The flagellate condition. In: (B.S.C. Leadbeater and J.C. Green, eds) The flagellates. Unity, diversity and evolution. The Systematics Association Special Volume 59. Taylor and Francis, London. pp. 269–287.
^Webster, J & Weber, R (2007). Introduction to Fungi (3rd ed.). Cambridge: Cambridge University Press. pp. 23–24,
[1]
^Fritsch, F.E. The Structure and Reproduction of the Algae. Vol. I. Introduction, Chlorophyceae. Xanthophyceae, Chrysophyceae, Bacillariophyceae, Cryptophyceae, Dinophyceae, Chloromonadineae, Euglenineae, Colourless Flagellata. 1935. Cambridge, England: Cambridge University Press,
[2].