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 function units that extend outside of cells are often termed organelles, such as cilia, the flagellum and archaellum, and the trichocyst.

Part ofCell
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 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 other membrane-bounded structures.[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.

History and terminology

Cell biology
Animal cell diagram
Components of a typical animal cell:
  1. Nucleolus
  2. Nucleus
  3. Ribosome (dots as part of 5)
  4. Vesicle
  5. Rough endoplasmic reticulum
  6. Golgi apparatus (or, Golgi body)
  7. Cytoskeleton
  8. Smooth endoplasmic reticulum
  9. Mitochondrion
  10. Vacuole
  11. Cytosol (fluid that contains organelles; with which, comprises cytoplasm)
  12. Lysosome
  13. Centrosome
  14. Cell membrane

In biology organs are defined as confined functional units within an organism.[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.[4]

Credited as the first[5][6][7] 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).[8] 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.[8][9]


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.[10][11][12]

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):

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.[14][15][16] This has led many texts to delineate between membrane-bound and non-membrane bound organelles.[17] 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 their main structure is made of proteins. Such cell structures include:

The mechanisms by which such non-membrane bound organelles form and retain their spatial integrity have been likened to liquid-liquid phase separation.[18]

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).[19] 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 membraneseparates the interior of all cells from the outside environment (the extracellular space) which protects the cell from its environment.double-layered, fluid sheet of phospholipids all eukaryotes
cell wallThe cell wall is a rigid structure composed of cellulose that provides shape to the cell, helps keep the organelles inside the cell, and does not let the cell burst from osmotic pressure.various plants, protists, rare kleptoplastic organisms
chloroplast (plastid)photosynthesis, traps energy from sunlightdouble-membrane compartmentplants, algae, rare kleptoplastic organismshas own DNA; theorized to be engulfed by the ancestral archaeplastid cell (endosymbiosis)
endoplasmic reticulumtranslation and folding of new proteins (rough endoplasmic reticulum), expression of lipids (smooth endoplasmic reticulum)single-membrane compartmentall eukaryotesrough endoplasmic reticulum is covered with ribosomes, has folds that are flat sacs; smooth endoplasmic reticulum has folds that are tubular
flagellumlocomotion, sensoryprotein some eukaryotes
Golgi apparatussorting, packaging, processing and modification of proteinssingle-membrane compartmentall eukaryotescis-face (convex) nearest to rough endoplasmic reticulum; trans-face (concave) farthest from rough endoplasmic reticulum
mitochondrionenergy production from the oxidation of glucose substances and the release of adenosine triphosphatedouble-membrane compartmentmost eukaryotesconstituting element of the chondriome; has own DNA; theorized to have been engulfed by an ancestral eukaryotic cell (endosymbiosis)[20]
nucleusDNA maintenance, controls all activities of the cell, RNA transcriptiondouble-membrane compartmentall eukaryotescontains bulk of genome
vacuolestorage, transportation, helps maintain homeostasissingle-membrane compartmentall 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
acrosomehelps spermatozoa fuse with ovumsingle-membrane compartmentmost animals (including sponges)
autophagosomevesicle that sequesters cytoplasmic material and organelles for degradationdouble-membrane compartmentall eukaryotes
centrioleanchor for cytoskeleton, organizes cell division by forming spindle fibersMicrotubule proteinanimals
ciliummovement in or of external medium; "critical developmental signaling pathway".[21]Microtubule proteinanimals, protists, few plants
cnidocyststingingcoiled hollow tubulecnidarians
eyespot apparatusdetects light, allowing phototaxis to take placegreen algae and other unicellular photosynthetic organisms such as euglenids
glycosomecarries out glycolysissingle-membrane compartmentSome protozoa, such as Trypanosomes.
glyoxysomeconversion of fat into sugarssingle-membrane compartmentplants
hydrogenosomeenergy & hydrogen productiondouble-membrane compartmenta few unicellular eukaryotes
lysosomebreakdown of large molecules (e.g., proteins + polysaccharides)single-membrane compartmentanimals
melanosomepigment storagesingle-membrane compartmentanimals
mitosomeprobably plays a role in Iron–sulfur cluster (Fe–S) assemblydouble-membrane compartmenta few unicellular eukaryotes that lack mitochondria
myofibrilmyocyte contractionbundled filamentsanimals
nucleoluspre-ribosome productionprotein–DNA–RNAmost eukaryotes
ocelloiddetects light and possibly shapes, allowing phototaxis to take placedouble-membrane compartmentmembers of the family Warnowiaceae
parenthesomenot characterizednot characterizedfungi
peroxisomebreakdown of metabolic hydrogen peroxidesingle-membrane compartmentall eukaryotes
porosomesecretory portalsingle-membrane compartmentall eukaryotes
proteasomedegradation of unneeded or damaged proteins by proteolysisvery large protein complexall eukaryotes, all archaea, and some bacteria
ribosome (80S)translation of RNA into proteinsRNA-proteinall eukaryotes
stress granule mRNA storage[22] membraneless

(mRNP complexes)

most eukaryotes
TIGER domain mRNA encoding proteins membraneless most organisms
vesiclematerial transportsingle-membrane compartmentall eukaryotes

Other related structures:

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.[23]
Structure of Candidatus Brocadia anammoxidans, showing an anammoxosome and intracytoplasmic membrane

Prokaryotes are not as structurally complex as eukaryotes, and were once thought as having little internal organization, and lack cellular compartments and internal membranes; but slowly, details are emerging about prokaryotic internal structures that overturn these assumptions.[2] 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.[24]

However, there is increasing evidence of compartmentalization in at least some prokaryotes.[2] 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]

The bacterial phylum Planctomycetota has revealed a number of compartmentalization features. The Planctomycetota cell plan includes 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 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.[27][29]

Compartmentalization is a feature of prokaryotic photosynthetic structures.[2] Purple bacteria have "chromatophores", which are reaction centers found in invaginations of the cell membrane.[2] Green sulfur bacteria have chlorosomes, which are photosynthetic antenna complexes found bonded to cell membranes.[2] 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.[2]

Prokaryotic organelles and cell components
Organelle/macromolecule Main function Structure Organisms
anammoxosomeanaerobic ammonium oxidationladderane lipid membrane"Candidatus" bacteria within Planctomycetota
carboxysomecarbon fixationprotein-shell bacterial microcompartmentsome bacteria
chlorosomephotosynthesislight harvesting complex attached to cell membranegreen sulfur bacteria
flagellummovement in external mediumprotein filamentsome prokaryotes
magnetosomemagnetic orientationinorganic crystal, lipid membranemagnetotactic bacteria
nucleoidDNA maintenance, transcription to RNADNA-proteinprokaryotes
pilusAdhesion 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 membraneprokaryotic cells
plasmidDNA exchangecircular DNAsome bacteria
ribosome (70S)translation of RNA into proteinsRNA-proteinbacteria and archaea
thylakoid membranesphotosynthesisphotosystem proteins and pigmentsmostly cyanobacteria

See also


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