Organelle


In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. Organelles are either separately enclosed within their own lipid bilayers or are spatially distinct functional units without a surrounding lipid bilayer. 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.
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 identified by microscopy, and can also be purified by cell fractionation. There are many types of organelles, particularly in eukaryotic cells. While prokaryotes do not possess intracellular organelles per se, some do contain protein-based bacterial microcompartments, which are thought to act as primitive [|prokaryotic organelles]. 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 organs are defined as confined functional units 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.
In the 1830s, Félix Dujardin refuted Ehrenberg theory which said that microorganisms have the same organs of multicellular animals, only minor.
Credited as the first to use a diminutive of organ for cellular structures was German zoologist Karl August Möbius, who used the term organula. 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.

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, having originated from formerly autonomous microscopic organisms acquired via endosymbiosis.
Under this definition, there would only be two broad classes of organelles :
Other organelles are also suggested to have endosymbiotic origins, but do not contain their own DNA.
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 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:
The mechanisms by which such non-membrane bound organelles form and retain their spatial integrity have been likened to liquid-liquid phase separation.

Eukaryotic organelles

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. There are also occasional exceptions to the number of membranes surrounding organelles, listed in the tables below. In addition, the number of individual organelles of each type found in a given cell varies depending upon the function of that cell.
OrganelleMain functionStructureOrganismsNotes
cell membraneseparates the interior of all cells from the outside environment which protects the cell from its environment.two-dimensional liquidall eukaryotes
cell wallThe 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.celluloseplants, protists, rare kleptoplastic organisms
chloroplast photosynthesis, traps energy from sunlightdouble-membrane compartmentplants, protists, rare kleptoplastic organismshas own DNA; theorized to be engulfed by the ancestral eukaryotic cell
endoplasmic reticulumtranslation and folding of new proteins, expression of lipids 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, sensoryproteinsome eukaryotes
Golgi apparatussorting, packaging, processing and modification of proteinssingle-membrane compartmentall eukaryotescis-face nearest to rough endoplasmic reticulum; trans-face 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
nucleusDNA maintenance, controls all activities of the cell, RNA transcriptiondouble-membrane compartmentall eukaryotescontains bulk of genome
vacuolestorage, transportation, helps maintain homeostasissingle-membrane compartmenteukaryotes

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.
Organelle/MacromoleculeMain functionStructureOrganisms
acrosomehelps spermatozoa fuse with ovumsingle-membrane compartmentmost animals
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".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 single-membrane compartmentanimals
melanosomepigment storagesingle-membrane compartmentanimals
mitosomeprobably plays a role in Iron-sulfur cluster 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
proteasomedegradation of unneeded or damaged proteins by proteolysisvery large protein complexall eukaryotes, all archaea, and some bacteria
ribosome translation of RNA into proteinsRNA-proteinall eukaryotes
stress granulemRNA storagemembraneless
most eukaryotes
TIGER domainmRNA encoding proteinsmembranelessmost organisms
vesiclematerial transportsingle-membrane compartmentall eukaryotes

Other related structures:
s. Image of intact carboxysomes isolated from H. neapolitanus. Scale bars are 100 nm.

Prokaryotic organelles

s 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, but slowly, details are emerging about prokaryotic internal structures. An early false turn was the idea developed in the 1970s that bacteria might contain membrane folds termed mesosomes, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy.
However, more 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. Even more striking is the description of membrane-bound magnetosomes in bacteria, reported in 2006, as well as the nucleus-like structures of the Planctomycetes that are surrounded by lipid membranes, reported in 2005.
Organelle/macromoleculeMain functionStructureOrganisms
anammoxosomeanaerobic ammonium oxidationladderane membrane"Candidatus" bacteria within Planctomycetes
carboxysomecarbon fixationprotein-shell compartmentsome bacteria
chlorosomephotosynthesislight harvesting complexgreen sulfur bacteria
flagellummovement in external mediumprotein filamentsome prokaryotes and eukaryotes
magnetosomemagnetic orientationinorganic crystal, lipid membranemagnetotactic bacteria
mesosomesfunctions of Golgi bodies, centrioles, etc.small irregular shaped organelle containing ribosomespresent in most prokaryotic cells
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 the plasma membraneprokaryotic cells
plasmidDNA exchangecircular DNAsome bacteria
ribosome translation of RNA into proteinsRNA-proteinbacteria and archaea
thylakoidphotosynthesisphotosystem proteins and pigmentsmostly cyanobacteria