Organelle

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 (these could be referred to as membrane bound in the sense that they are attached to (or bound to) the membrane).

[2] Also, the prokaryotic flagellum which protrudes outside the cell, and its motor, as well as the largely extracellular pilus, are often spoken of as organelles.

[3] The analogy of bodily organs to microscopic cellular substructures is obvious, as from even early works, authors of respective textbooks rarely elaborate on the distinction between the two.

In the 1830s, Félix Dujardin refuted Ehrenberg theory which said that microorganisms have the same organs of multicellular animals, only minor.

Such cell structures include: The mechanisms by which such non-membrane bounded organelles form and retain their spatial integrity have been likened to liquid-liquid phase separation.

[15] The second, more restrictive definition of organelle includes only those cell compartments that contain deoxyribonucleic acid (DNA), having originated from formerly autonomous microscopic organisms acquired via endosymbiosis.

[27] Membrane-bounded anammoxosomes have been discovered in five Planctomycetota "anammox" genera, which perform anaerobic ammonium oxidation.

[28] In the Planctomycetota species Gemmata obscuriglobus, a nucleus-like structure surrounded by lipid membranes has been reported.

[2] Green sulfur bacteria have chlorosomes, which are photosynthetic antenna complexes found bonded to cell membranes.