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In biology, motility is the ability to move spontaneously and actively, consuming energy in the process. Motility is genetically determined (see genetic determinism) but may be affected by environmental factors. For instance, muscles give animals motility but the consumption of hydrogen cyanide (the environmental factor in this case) would adversely affect muscle physiology causing them to stiffen leading to rigor mortis. Most animals are motile but the term applies to unicellular and simple multicellular organisms, as well as to some mechanisms of fluid flow in multicellular organs, in addition to animal locomotion. Motile marine animals are commonly called free-swimming.


Cellular-level motility

At the cellular level, different modes of motility exist:

  • flagellar motility, a swimming-like motion (observed for example in spermatozoa, propelled by the regular beat of their flagellum, or E. coli, which swims by rotating a helical prokaryotic flagellum)
  • amoeboid movement, a crawling-like movement, which also makes swimming possible
  • gliding motility
  • Swarming motility

Many cells are not motile, for example Klebsiella pneumoniae and Shigella, or under specific circumstances such as Yersinia pestis at 37 °C.[citation needed]