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 organ
s 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 the trichocyst
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
s. While prokaryote
s do not possess eukaryotic organelles, some do contain protein
-shelled bacterial microcompartment
s, which are thought to act as primitive prokaryotic organelles
and there is also evidence of other 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
In biology ''organ
s'' are defined as confined functional units within an organism
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
theory which said that microorganisms have the same organs of multicellular animals, only minor.
Credited as the first 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
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.
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
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
(in almost all eukaryotes)
s (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. This has led many texts to delineate between membrane-bound and non-membrane bound organelles. The non-membrane bound organelles, also called large biomolecular complex
es, are large assemblies of macromolecule
s that carry out particular and specialized functions, but they lack membrane boundaries. Many of these are referred to as "proteinaceous organelles" as their main structure is made of proteins. Such cell structures include:
* large RNA and protein complexes: ribosome
* large protein complexes
, DNA polymerase III holoenzyme
, RNA polymerase II holoenzyme
, symmetric viral capsid
s, complex of GroEL
; membrane protein complexes: porosome
, photosystem I
, ATP synthase
* large DNA and protein complexes: nucleosome
and microtubule-organizing center
* stress granule
* germ cell
* 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
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
, 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). 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 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
Other related structures:
* endomembrane system
300px|Structure of ''Candidatus'' Brocadia anammoxidans
, showing an anammoxosome
and intracytoplasmic membrane
s are not as structurally complex as eukaryotes, and were once thought as having little internal organization, and lack cellular compartment
s and internal membranes
; but slowly, details are emerging about prokaryotic internal structures that overturn these assumptions.
An early false turn was the idea developed in the 1970s that bacteria might contain cell membrane
folds termed mesosome
s, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy
However, there is increasing evidence of compartmentalization in at least some prokaryotes.
Recent research has revealed that at least some prokaryotes have microcompartments
, such as carboxysome
s. These subcellular compartments are 100–200 nm in diameter and are enclosed by a shell of proteins.
Even more striking is the description of membrane-bound magnetosome
s in bacteria, reported in 2006.
The bacterial phylum Planctomycetes
has revealed a number of compartmentalization features. The Planctomycetes cell plan includes a intracytoplasmic membranes that separates the cytoplasm into paryphoplasm (an outer ribosome-free space) and pirellulosome (or riboplasm, an inner ribosome-containing space).
s have been discovered in five Planctomycetes "anammox" genera, which perform anaerobic ammonium oxidation
. In the Planctomycetes species ''Gemmata obscuriglobus
'', a nucleus-like structure surrounded by lipid membranes has been reported.
Compartmentalization is a feature of prokaryotic photosynthetic
, which are reaction centers found in invagination
s of the cell membrane.
Green sulfur bacteria
s, which are photosynthetic antenna complex
es found bonded to cell membranes.
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.
* CoRR hypothesis
* Endosymbiotic theory
* Organelle biogenesis
* Membrane vesicle trafficking
* Host-pathogen interface
*Tree of Life project: EukaryotesOrganelle Databases