The cilium is a membrane-bound organelle found on most types of eukaryotic cell. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projection that extends from the surface of the much larger cell body. Eukaryotic flagella found on sperm cells and many protozoans have a similar structure to motile cilia that enables swimming through liquids; they are longer than cilia and have a different undulating motion.
SEM micrograph of motile cilia projecting from respiratory epithelium in the trachea
Tracheal respiratory epithelium showing cilia and much smaller microvilli on non-ciliated cells in scanning electron micrograph.
Scanning electron micrograph of nodal cilia on a mouse embryo
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name organelle comes from the idea that these structures are parts of cells, as organs are to the body, hence organelle, the suffix -elle being a diminutive. Organelles are either separately enclosed within their own lipid bilayers or are spatially distinct functional units without a surrounding lipid bilayer. Although most organelles are functional units within cells, some function units that extend outside of cells are often termed organelles, such as cilia, the flagellum and archaellum, and the trichocyst.
(A) Electron micrograph of Halothiobacillus neapolitanus cells, arrows highlight carboxysomes. (B) Image of intact carboxysomes isolated from H. neapolitanus. Scale bars are 100 nm.
Structure of Candidatus Brocadia anammoxidans, showing an anammoxosome and intracytoplasmic membrane