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Organelle (6997 views - Medicine & Health)

In cell biology, an organelle () is a specialized subunit within a cell that has a specific function. Individual organelles are usually separately enclosed within their own lipid bilayers. 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 organelles per se, some do contain protein-based bacterial microcompartments, which are thought to act as primitive organelles.
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Organelle

Organelle

Organelle
Details
Identifiers
Latin organella
Code TH H1.00.01.0.00009
TH H1.00.01.0.00009
FMA 63832
Anatomical terminology

In cell biology, an organelle (/ɔːrɡəˈnɛl/) is a specialized subunit within a cell that has a specific function. Individual organelles are usually separately enclosed within their own lipid bilayers.

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 organelles per se, some do contain protein-based bacterial microcompartments, which are thought to act as primitive organelles.[1]

History and terminology

Cell biology
The animal cell

In biology organs are defined as confined functional units within an organism.[2] 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.[3]

Credited as the first[4][5][6] 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).[7] 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.[citation needed]

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 containing deoxyribonucleic acid (DNA), having originated from formerly autonomous microscopic organisms acquired via endosymbiosis.[8][9][10]

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).

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.[12] This has led some texts to delineate between membrane-bound and non-membrane bound organelles.[13] 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. Such cell structures include:

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).[14] 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
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 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 be 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 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.

Minor eukaryotic organelles and cell components
Organelle/Macromolecule Main function Structure Organisms
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".[15] 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
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
stress granule mRNA storage[16] membraneless

(mRNP complexes)

Most eukaryotes
vesicle material transport single-membrane compartment all eukaryotes

Other related structures:

Prokaryotic organelles

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.[18]

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.[1] Even more striking is the description of membrane-bound magnetosomes in bacteria, reported in 2006,[19][20] as well as the nucleus-like structures of the Planctomycetes that are surrounded by lipid membranes, reported in 2005.[21]

Prokaryotic organelles and cell components
Organelle/Macromolecule Main function Structure Organisms
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
mesosomes functions of Golgi bodies, centrioles, etc. small irregular shaped organelle containing ribosomes present in most prokaryotic cells
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 photosynthesis photosystem proteins and pigments mostly cyanobacteria

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[22][23] and some methods were based on pseudo amino acid composition.[24][25][26][27]

See also



This article uses material from the Wikipedia article "Organelle", which is released under the Creative Commons Attribution-Share-Alike License 3.0. There is a list of all authors in Wikipedia

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