Organelle

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 (also called membrane-bound organelles) or are spatially distinct functional units without a surrounding lipid bilayer (non-membrane bound organelles). Although most organelles are functional units within cells, some functional units that extend outside of cells are often termed organelles, such as cilia, the flagellum and archaellum, and the trichocyst.

Organelle
Details
Pronunciation	/ɔːrɡəˈnɛl/
Part of	Cell
Identifiers
Latin	organella
MeSH	D015388
TH	H1.00.01.0.00009
FMA	63832
Anatomical terms of microanatomy

Organelles are identified by microscopy, and can also be purified by cell fractionation. There are many types of organelles, particularly in eukaryotic cells. They include structures that make up the internal endomembrane system (such as the nuclear envelope, endoplasmic reticulum, and Golgi apparatus), and other structures such as mitochondria and plastids. While prokaryotes do not possess eukaryotic organelles, some do contain protein-shelled bacterial microcompartments, which are thought to act as primitive prokaryotic organelles,[1] and there is also evidence of membrane-bounded structures. 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.

History and terminology

Cell biology
The animal cell
Components of a typical animal cell: Nucleolus Nucleus Ribosome (little dots) Vesicle Rough endoplasmic reticulum Golgi apparatus (or, Golgi body) Cytoskeleton Smooth endoplasmic reticulum Mitochondrion Vacuole Cytosol (fluid that contains organelles, comprising the cytoplasm) Lysosome Centrosome Cell membrane

In biology organs are defined as confined functional units within an organism.[2] 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.[3]

Credited as the first[4][5][6] to use a diminutive of organ (i.e., little organ) for cellular structures was German zoologist Karl August Möbius (1884), who used the term organula (plural of organulum, the diminutive of Latin organum).[7] 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.[7][8]

Types

While most cell biologists consider the term organelle to be synonymous with cell compartment, a space often bound by one or two lipid bilayers, some cell biologists choose to limit the term to include only those cell compartments that contain deoxyribonucleic acid (DNA), having originated from formerly autonomous microscopic organisms acquired via endosymbiosis.[9][10][11]

Under this definition, there would only be two broad classes of organelles (i.e. those that contain their own DNA, and have originated from endosymbiotic bacteria):

• mitochondria (in almost all eukaryotes)
• plastids[12] (e.g. in plants, algae, and some protists).

Other organelles are also suggested to have endosymbiotic origins, but do not contain their own DNA (notably the flagellum – see evolution of flagella).

A second, less restrictive definition of organelles is that they are membrane-bound structures. However, even by using this definition, some parts of the cell that have been shown to be distinct functional units do not qualify as organelles. Therefore, the use of organelle to also refer to non-membrane bound structures such as ribosomes is common and accepted.[13][14][15] This has led many texts to delineate between membrane-bound and non-membrane bound organelles.[16] The non-membrane bound organelles, also called large biomolecular complexes, are large assemblies of macromolecules that carry out particular and specialized functions, but they lack membrane boundaries. Many of these are referred to as "proteinaceous organelles" as there many structure is made of proteins. Such cell structures include:

• large RNA and protein complexes: ribosome, spliceosome, vault
• large protein complexes: proteasome, DNA polymerase III holoenzyme, RNA polymerase II holoenzyme, symmetric viral capsids, complex of GroEL and GroES; membrane protein complexes: photosystem I, ATP synthase
• large DNA and protein complexes: nucleosome
• centriole and microtubule-organizing center (MTOC)
• cytoskeleton
• flagellum
• nucleolus
• stress granule
• germ cell granule
• neuronal transport granule

The mechanisms by which such non-membrane bound organelles form and retain their spatial integrity have been likened to liquid-liquid phase separation.[17]

Eukaryotic organelles

Eukaryotic cells are structurally complex, and by definition are organized, in part, by 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 each of the organelles listed below. Exceptional organisms have cells that do not include some organelles that might otherwise be considered universal to eukaryotes (such as mitochondria).[18] 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.

Major eukaryotic organelles

Organelle	Main function	Structure	Organisms	Notes
cell membrane	separates the interior of all cells from the outside environment (the extracellular space) which protects the cell from its environment.	two-dimensional liquid	all eukaryotes
cell wall	The cell wall is composed of peptidoglycan and is rigid, provides shape to the cell, helps to keeps the organelles inside the cell, and does not let the cell burst due to changes in osmotic pressure.	cellulose	plants, protists, rare kleptoplastic organisms
chloroplast (plastid)	photosynthesis, traps energy from sunlight	double-membrane compartment	plants, protists, rare kleptoplastic organisms	has own DNA; theorized to be engulfed by the ancestral eukaryotic cell (endosymbiosis)
endoplasmic reticulum	translation and folding of new proteins (rough endoplasmic reticulum), expression of lipids (smooth endoplasmic reticulum)	single-membrane compartment	all eukaryotes	rough endoplasmic reticulum is covered with ribosomes, has folds that are flat sacs; smooth endoplasmic reticulum has folds that are tubular
flagellum	locomotion, sensory	protein	some eukaryotes
Golgi apparatus	sorting, packaging, processing and modification of proteins	single-membrane compartment	all eukaryotes	cis-face (convex) nearest to rough endoplasmic reticulum; trans-face (concave) farthest from rough endoplasmic reticulum
mitochondrion	energy production from the oxidation of glucose substances and the release of adenosine triphosphate	double-membrane compartment	most eukaryotes	constituting element of the chondriome; has own DNA; theorized to have been engulfed by an ancestral eukaryotic cell (endosymbiosis)[19]
nucleus	DNA maintenance, controls all activities of the cell, RNA transcription	double-membrane compartment	all eukaryotes	contains bulk of genome
vacuole	storage, transportation, helps maintain homeostasis	single-membrane compartment	eukaryotes

Mitochondria and plastids, including chloroplasts, have double membranes and their own DNA. According to the endosymbiotic theory, they are believed to have originated from incompletely consumed or invading prokaryotic organisms.

Minor eukaryotic organelles and cell components

Organelle/Macromolecule	Main function	Structure	Organisms
acrosome	helps spermatozoa fuse with ovum	single-membrane compartment	most animals
autophagosome	vesicle that sequesters cytoplasmic material and organelles for degradation	double-membrane compartment	all eukaryotes
centriole	anchor for cytoskeleton, organizes cell division by forming spindle fibers	Microtubule protein	animals
cilium	movement in or of external medium; "critical developmental signaling pathway".[20]	Microtubule protein	animals, protists, few plants
cnidocyst	stinging	coiled hollow tubule	cnidarians
eyespot apparatus	detects light, allowing phototaxis to take place		green algae and other unicellular photosynthetic organisms such as euglenids
glycosome	carries out glycolysis	single-membrane compartment	Some protozoa, such as Trypanosomes.
glyoxysome	conversion of fat into sugars	single-membrane compartment	plants
hydrogenosome	energy & hydrogen production	double-membrane compartment	a few unicellular eukaryotes
lysosome	breakdown of large molecules (e.g., proteins + polysaccharides)	single-membrane compartment	animals
melanosome	pigment storage	single-membrane compartment	animals
mitosome	probably plays a role in Iron-sulfur cluster (Fe-S) assembly	double-membrane compartment	a few unicellular eukaryotes that lack mitochondria
myofibril	myocyte contraction	bundled filaments	animals
nucleolus	pre-ribosome production	protein-DNA-RNA	most eukaryotes
ocelloid	detects light and possibly shapes, allowing phototaxis to take place	double-membrane compartment	members of the family Warnowiaceae
parenthesome	not characterized	not characterized	fungi
peroxisome	breakdown of metabolic hydrogen peroxide	single-membrane compartment	all eukaryotes
proteasome	degradation of unneeded or damaged proteins by proteolysis	very large protein complex	all eukaryotes, all archaea, and some bacteria
ribosome (80S)	translation of RNA into proteins	RNA-protein	all eukaryotes
stress granule	mRNA storage[21]	membraneless (mRNP complexes)	most eukaryotes
TIGER domain	mRNA encoding proteins	membraneless	most organisms
vesicle	material transport	single-membrane compartment	all eukaryotes

Other related structures:

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

Prokaryotic organelles

(A) Electron micrograph of Halothiobacillus neapolitanus cells, arrows highlight carboxysomes. (B) Image of intact carboxysomes isolated from H. neapolitanus. Scale bars are 100 nm.[22]

Structure of Candidatus Brocadia anammoxidans, showing an anammoxosome and intracytoplasmic membrane

Prokaryotes are not as structurally 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, and lack cellular compartments; but slowly, details are emerging about prokaryotic internal structures. An early false turn was the idea developed in the 1970s that bacteria might contain cell membrane folds termed mesosomes, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy.[23]

However, there is increasing evidence of compartmentalization in at least some prokaryotes.[24] Recent research has revealed that at least some prokaryotes have microcompartments, such as carboxysomes. These subcellular compartments are 100–200 nm in diameter and are enclosed by a shell of proteins.[1] Even more striking is the description of membrane-bound magnetosomes in bacteria, reported in 2006.[25][26]

In the bacterial phylum Planctomycetes, internal membrane-bound structures have been found. Plancomycetes species have an intracytoplasmic membranes that separates the cytoplasm into paryphoplasm (an outer ribosome-free space) and pirellulosome (or riboplasm, an inner ribosome-containing space).[27] Membrane-bound anammoxosomes have been discovered in five Plancomycetes anammox genera.[28] In the Plancomycetes Gemmata obscuriglobus, a nucleus-like structure surrounded by lipid membranes has been reported.[27][29]

Compartmentalization is a feature of prokaryotic photosynthetic structures.[24] Purple bacteria have "chromatophores", which are reaction centers found in invaginations of the cell membrane.[24] Green sulfur bacteria have chlorosomes, which are photosynthetic antenna complexes found bonded to cell membranes.[24] Cyanobacteria have internal thylakoid membranes for light-dependent photosynthesis; studies have revealed that the cell membrane and the thylakoid membranes are not continuous with each other.[24]

Prokaryotic organelles and cell components

Organelle/macromolecule	Main function	Structure	Organisms
anammoxosome	anaerobic ammonium oxidation	ladderane lipid membrane	"Candidatus" bacteria within Planctomycetes
carboxysome	carbon fixation	protein-shell bacterial microcompartment	some bacteria
chlorosome	photosynthesis	light harvesting complex attached to cell membrane	green sulfur bacteria
flagellum	movement in external medium	protein filament	some prokaryotes and eukaryotes
magnetosome	magnetic orientation	inorganic crystal, lipid membrane	magnetotactic bacteria
nucleoid	DNA maintenance, transcription to RNA	DNA-protein	prokaryotes
pilus	Adhesion to other cells for conjugation or to a solid substrate to create motile forces.	a hair-like appendage sticking out (though partially embedded into) the plasma membrane	prokaryotic cells
plasmid	DNA exchange	circular DNA	some bacteria
ribosome (70S)	translation of RNA into proteins	RNA-protein	bacteria and archaea
thylakoid membranes	photosynthesis	photosystem proteins and pigments	mostly cyanobacteria

See also

• CoRR hypothesis
• Ejectosome
• Endosymbiotic theory
• Organelle biogenesis
• Membrane vesicle trafficking
• Host-pathogen interface

References

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4. Bütschli O (1888). Dr. H. G. Bronn's Klassen u. Ordnungen des Thier-Reichs wissenschaftlich dargestellt in Wort und Bild. Erster Band. Protozoa. Dritte Abtheilung: Infusoria und System der Radiolaria. p. 1412. Die Vacuolen sind demnach in strengem Sinne keine beständigen Organe oder O r g a n u l a (wie Möbius die Organe der Einzelligen im Gegensatz zu denen der Vielzelligen zu nennen vorschlug).
5. Ryder JA, ed. (February 1889). "Embryology: The Structure of the Human Spermatozoon". American Naturalist. 23: 184. It may possibly be of advantage to use the word organula here instead of organ, following a suggestion by Möbius. Functionally differentiated multicellular aggregates in multicellular forms or metazoa are in this sense organs, while, for functionally differentiated portions of unicellular organisms or for such differentiated portions of the unicellular germ-elements of metazoa, the diminutive organula is appropriate.
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External links

• Media related to Organelles at Wikimedia Commons
• Tree of Life project: Eukaryotes
• Organelle Databases