Organelle

In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is separately enclosed within its own lipid membrane.  

The name organelle comes from the idea that these structures are to cells what an organ is to the body (hence the name organelle, the suffix -elle being a diminutive). Organelles are identified by microscopy, and can also be purified by cell fractionation. There are many types of organelles, particularly in the eukaryotic cells of higher organisms. Prokaryotes were once thought not to have organelles, but some examples have now been identified.^[1]

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History and Terminology

In biology, an organ is defined as a confined functional unit within an organism. 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.

Credited as the first^[2][3][4] to use a diminutive of organ for respective cellular structures was German zoologist Karl August Möbius (1884), who used the term "organula" ^[5] (plural from of organulum, the diminutive of latin organum). From the context, it is clear that he referred to reproduction related structures of protists. In a footnote, which was published as a correction in the next issue of the journal, he justified his suggestion to call organs of unicellular organisms "organella" since they are only differently formed parts of one cell, in contrast to multicellular organs of multicellular organisms. Thus, the original definition was limited to structures of unicellular organisms.

It would take several years before organulum, or the later term organelle, became accepted and expanded in meaning to include subcellular structures in multicellular organisms. Books around 1900 from Valentin Häcker,^[6] Edmund Wilson^[7] and Oscar Hertwig^[8] still referred to cellular organs. Later, both terms came to be used side by side: Bengt Lidforss wrote 1915 (in German) about "Organs or Organells".^[9]

Around 1920, the term organelle was used to describe propulsion structures ("motor organelle complex", i.e., flagella and their anchoring)^[10] and other protist structures, such as ciliates.^[11] Alfred Kühn wrote about centrioles as division organelles, although he stated that, for Vahlkampfias, the alternative 'organelle' or 'product of structural build-up' had not yet been decided, without explaining the difference between the alternatives.^[12]

In his 1953 textbook, Max Hartmann used the term for extracellular (pellicula, shells, cell walls) and intracellular skeletons of protists.^[13]

Later, the now-widely-used^{[14][15][16][17]} definition of organelle emerged, after which only cellular structures with surrounding membrane had been considered organelles. However, the more original definition of subcellular functional unit in general still coexists.^[18][19]

In 1978, Albert Frey-Wyssling suggested that the term organelle should refer only to structures that convert energy, such as centrosomes, ribosomes, and nucleoli.^[20][21] This new definition, however, did not win wide recognition.

Examples

Whereas most cell biologists consider the term organelle to be synonymous with "cell compartment," other cell biologists choose to limit the term organelle to include only those that are DNA-containing, having originated from formerly-autonomous microscopic organisms acquired via endosymbiosis.

The most notable of these organelles having originated from endosymbiont bacteria are:

• mitochondria (in almost all eukaryotes)
• chloroplasts (in plants, algae and protists).

Other organelles are also suggested to have endosymbiotic origins, (notably the flagellum - see evolution of flagella).

Not all parts of the cell qualify as organelles, and the use of the term to refer to some structures is disputed. These structures are large assemblies of macromolecules that carry out particular and specialized functions, but they lack membrane boundaries. Such cell structures, which are not formally organelles, include:

• ribosome
• cytoskeleton
• flagellum.

Eukaryotic organelles

Eukaryotes are the most structurally complex cell type, and by definition are in part organized by smaller interior compartments, that are themselves enclosed by lipid membranes that resemble the outermost cell membrane. The larger organelles, such as the nucleus and vacuoles, are easily visible with the light microscope. They were among the first biological discoveries made after the invention of the microscope.

Not all eukaryotic cells have every one of the organelles listed below. Exceptional species of cells do not have some organelles that might otherwise be considered universal to eukaryotes (such as mitochondria^[22]). There are also occasional exceptions to the number of membranes surrounding organelles, listed in the tables below (e.g., some that are listed as double-membrane are sometimes found with single or triple membranes). In addition, the number of individual organelles of each type found in a given cell varies depending upon the function of that cell.

Mitochondria and chloroplasts, which have double-membranes and their own DNA, are believed to have originated from incompletely consumed or invading prokaryotic organisms, which were adopted as a part of the invaded cell. This idea is supported in the Endosymbiotic theory.

Other related structures:

• cytosol
• endomembrane system
• nucleosome
• microtubule
• cell membrane

Prokaryotic organelles

Prokaryotes are not as structurally or metabolically complex as eukaryotes, and were once thought not to have any internal structures enclosed by lipid membranes. In the past, they were often viewed as having little internal organization; but, slowly, details are emerging about prokaryotic internal structures. One contributing discovery revealed that at least some prokaryotes have microcompartments, which are compartments enclosed by proteins.^[1] Even more striking is the description of magnetosomes,^[23][24] as well as the nucleus-like structures of the Planctomycetes that are surrounded by lipid membranes.^[25]

See also

• Cell
• Endosymbiotic theory

References

• Alberts, Bruce et al. (2003). Essential Cell Biology, 2nd ed., Garland Science, 2003, ISBN 081533480X.
• Alberts, Bruce et al. (2002). The Molecular Biology of the Cell, 4th ed., Garland Science, 2002, ISBN 0-8153-3218-1.