|The animal cell|
The name organelle comes from the idea that these structures are to cells what an organ is to the body (hence the name 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 organelles per se, some do contain protein-based microcompartments, which are thought to act as primitive 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.
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). From the context, it is clear that he referred to reproduction related structures of protists. 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. Thus, the original definition was limited to structures of unicellular organisms.
It would take several years before organulum, or the later term organelle, became accepted and expanded in meaning to include subcellular structures in multicellular organisms. Books around 1900 from Valentin Häcker, Edmund Wilson and Oscar Hertwig still referred to cellular organs. Later, both terms came to be used side by side: Bengt Lidforss wrote 1915 (in German) about "Organs or Organells".
Around 1920, the term organelle was used to describe propulsion structures ("motor organelle complex", i.e., flagella and their anchoring) and other protist structures, such as ciliates. Alfred Kühn wrote about centrioles as division organelles, although he stated that, for Vahlkampfias, the alternative 'organelle' or 'product of structural build-up' had not yet been decided, without explaining the difference between the alternatives.
Later, the now widely used definition of organelle emerged, after which only cellular structures with surrounding membrane had been considered organelles. However, the more original definition of subcellular functional unit in general still coexists.
In 1978, Albert Frey-Wyssling suggested that the term organelle should refer only to structures that convert energy, such as centrosomes, ribosomes, and nucleoli. This new definition, however, did not win wide recognition.
Types of organelles
While most cell biologists consider the term organelle to be synonymous with "cell compartment", other cell biologists choose to limit the term organelle to include only those that are DNA-containing, having originated from formerly autonomous microscopic organisms acquired via endosymbiosis.
Other organelles are also suggested to have endosymbiotic origins, but do not contain their own DNA (notably the flagellum – see evolution of flagella).
Under the more restricted definition of membrane-bound structures, some parts of the cell do not qualify as organelles. Nevertheless, the use of organelle to refer to non-membrane bound structures such as ribosomes is common. This has led some texts to delineate between membrane-bound and non-membrane bound organelles. These structures are large assemblies of macromolecules that carry out particular and specialized functions, but they lack membrane boundaries. 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)
- cellular structure which is not membrane bound and does not have a well-defined structure: nucleolus
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.
|chloroplast (plastid)||photosynthesis, traps energy from sunlight||double-membrane compartment||plants, protists (rare kleptoplastic organisms)||has some genes; 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|
|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|
|mitochondria||energy production from the oxidation of glucose substances and the release of adenosine triphosphate||double-membrane compartment||most eukaryotes||has some DNA; theorized to be engulfed by an ancestral eukaryotic cell (endosymbiosis)|
|vacuole||storage,transportation, helps maintain homeostasis||single-membrane compartment||eukaryotes|
|nucleus||DNA maintenance, controls all activities of the cell, RNA transcription||double-membrane compartment||all eukaryotes||contains bulk of genome|
Mitochondria and chloroplasts, which have double-membranes and their own DNA, are believed to have originated from incompletely consumed or invading prokaryotic organisms, which were adopted as a part of the invaded cell. This idea is supported in the Endosymbiotic theory.
|acrosome||helps spermatozoa fuse with ovum||single-membrane compartment||many 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|
|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||most eukaryotes|
|melanosome||pigment storage||single-membrane compartment||animals|
|mitosome||probably plays a role in Fe-S cluster 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|
|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, some bacteria|
|ribosome (80S)||translation of RNA into proteins||RNA-protein||all eukaryotes|
|vesicle||material transport||single-membrane compartment||all eukaryotes|
Other related structures:
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; 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, as well as the nucleus-like structures of the Planctomycetes that are surrounded by lipid membranes.
Recently, periplasm of gram negative microbes has been proposed to form physiological extensions, suggested to be named 'periplasmic organelles', as precursors of secretion of bacterial outer membrane bounded vesicles (OMVs). Structure and function of these periplasmic organelles have been discussed in detail 
|carboxysome||carbon fixation||protein-shell compartment||some bacteria|
|chlorosome||photosynthesis||light harvesting complex||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|
|plasmid||DNA exchange||circular DNA||some bacteria|
|ribosome (70S)||translation of RNA into proteins||RNA-protein||bacteria and archaea|
|thylakoid||photosynthesis||photosystem proteins and pigments||mostly cyanobacteria|
|mesosomes||functions of golgi bodies,centrioles etc..||small irregular shaped oraganelle containing ribosomes||present in most of the prokaryotic cells|
Proteins and organelles
The function of a protein is closely correlated with the organelle in which it resides. Some methods were proposed for predicting the organelle in which an uncharacterized protein is located according to its amino acid composition and some methods were based on pseudo amino acid composition.
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