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.
|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; 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
|Animal cell diagram|
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.
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 organum). 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):
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 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:
- 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: porosome, photosystem I, ATP synthase
- large DNA and protein complexes: nucleosome
- centriole and microtubule-organizing center (MTOC)
- stress granule
- germ cell granule
- neuronal transport granule
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). 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.
|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 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 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)|
|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.
|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".||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|
|porosome||secretory portal||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||membraneless
|TIGER domain||mRNA encoding proteins||membraneless||most organisms|
|vesicle||material transport||single-membrane compartment||all eukaryotes|
Other related structures:
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. 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.
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 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.
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). Membrane-bound anammoxosomes 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 structures. Purple bacteria have "chromatophores", which are reaction centers found in invaginations of the cell membrane. Green sulfur bacteria have chlorosomes, which are photosynthetic antenna complexes found bonded to cell membranes. 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.
|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|
|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|
- CoRR hypothesis
- Endosymbiotic theory
- Organelle biogenesis
- Membrane vesicle trafficking
- Host-pathogen interface
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