A<unk>  (plural: flagella) is a long, slender projection from the cell body, composed of microtubules and surrounded by the plasma membrane. In small, single-cell organisms they may function to propel the cell by beating in a whip-like motion; in larger animals, they often serve to move fluids along mucous membranes such as the lining of the trachea.
Eukaryotic flagella are quite different from the flagella of prokaryotes and bacteria. They have an internal structure comprised of nine microtubule doublets, forming a cylinder around a central pair of microtubules. The nine peripheral doublets are linked to each other by proteins such as dynein, a molecular motor which can cause flagella to bend.
A eukaryotic cell usually has only one or two flagella. As in prokaryotes, the eukaryotic<unk>  may be used in locomotion; one well known example of this is the sperm cell, in which the "tail" of the sperm (a<unk>  is used to propel the cell forward. However, all non-dividing eukaryotic cells contain a<unk>  (or cilium), not only sperm cells. Stationary cells (such as kidney, intestine, and nerve cells) also contain flagella (cilia) which project from the cell body out into the extracellular environment. There, these flagella can serve in sensation or in the movement of extracellular fluid.
The main differences among bacterial, archaeal, and eukaryotic flagella are summarized below:
Bacterial flagella are helical filaments that rotate like screws.
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.
Sometimes eukaryotic flagella are called cilia or undulipodia to emphasize their distinctiveness.
Flagellum
Electron shell
Ribozyme
Viral envelope
Viral tegument
Striae
Sarcomere
Ovacome