In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is separately enclosed within its own lipid membrane. See also List of basic cell biology topics. Cell biology (also called cellular biology or formerly cytology, from the The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called Lipids are broadly defined as any fat- Soluble ( lipophilic) naturally-occurring Molecule, such as fats oils waxes cholesterol sterols fat-soluble

A typical animal cell. Within the cytoplasm, the major organelles and cellular structures include: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (6) Golgi apparatus (7) cytoskeleton (8) smooth endoplasmic reticulum (9) mitochondria (10) vacuole (11) cytosol (12) lysosome (13) centriole. The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. The nucleolus (also called nucleole) is a structure found within the nucleus in which Ribosomal RNA is transcribed. In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that A vesicle is a small bubble of liquid within a cell A more formal definition in Cell biology, would be that a vesicle is a relatively small intracellular membrane-enclosed The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle The Golgi apparatus (also called the cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. In general vacuole functions include Removing unwanted structural debris Isolating materials that might be harmful or a threat to the cell Containing The cytosol or intracellular fluid (or cytoplasmic matrix) is the liquid found inside cells. Lysosomes are Organelles that contain Digestive enzymes (acid Hydrolases. A Centriole is a barrel shaped Organelle found in most animal Eukaryotic cells though absent in Higher plants and Fungi.

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). In Biology, an organ ( Latin: organum, "instrument tool" from Greek όργανον - organon "organ instrument With regard to living things, a body is the integral physical material of an individual A diminutive is a formation of a Word used to convey a slight degree of the root meaning smallness of the object or quality named encapsulation intimacy or endearment Organelles are identified by microscopy, and can also be purified by cell fractionation. Microscopy is the technical field of using microscopes to view samples or objects Cell fractionation is the separation of homogeneous sets usually Organelles from a heterogeneous population of cells There are many types of organelles, particularly in the eukaryotic cells of higher organisms. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex Prokaryotes were once thought not to have organelles, but some examples have now been identified. The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other ^[1]

History and Terminology

In biology, an organ is defined as a confined functional unit within an organism. In Biology, an organ ( Latin: organum, "instrument tool" from Greek όργανον - organon "organ instrument 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. Analogy is both the cognitive process of transferring Information from a particular subject (the analogue or source to another particular subject (the target and

Credited as the first^[2][3][4] to use a diminutive of organ for respective cellular structures was German zoologist Karl August Möbius (1884), who used the term "organula" ^[5] (plural form of organulum, the diminutive of latin organum). A diminutive is a formation of a Word used to convey a slight degree of the root meaning smallness of the object or quality named encapsulation intimacy or endearment Karl August Möbius ( 7 February 1825 in Eilenburg &ndash 26 April 1908 in Berlin) was a German Zoologist Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. From the context, it is clear that he referred to reproduction related structures of protists. Protists (ˈproʊtɨst are a diverse group of eukaryotic Microorganisms Historically protists were treated as the kingdom Protista but this 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,^[6] Edmund Wilson^[7] and Oscar Hertwig^[8] still referred to cellular organs. Edmund Beecher Wilson ( October 19, 1856 – March 3, 1939) was a pioneering American Zoologist and Geneticist Oscar Hertwig ( April 21, 1849, Friedberg, Hesse - October 25, 1922, Berlin) was a German Zoologist Later, both terms came to be used side by side: Bengt Lidforss wrote 1915 (in German) about "Organs or Organells". Bengt Lidforss (1868 - 1913 was a Swedish Socialist, natural scientist and writer ^[9]

Around 1920, the term organelle was used to describe propulsion structures ("motor organelle complex", i. e. , flagella and their anchoring)^[10] and other protist structures, such as ciliates. A flagellum ( plural flagella) is a tail-like structure that projects from the Cell body of certain Prokaryotic and Eukaryotic cells and it The ciliates are one of the most important groups of Protists common almost everywhere there is water — lakes ponds oceans rivers and soils with many ectosymbiotic ^[11] 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. A Centriole is a barrel shaped Organelle found in most animal Eukaryotic cells though absent in Higher plants and Fungi. ^[12]

In his 1953 textbook, Max Hartmann used the term for extracellular (pellicula, shells, cell walls) and intracellular skeletons of protists. ^[13]

Later, the now-widely-used^{[14][15][16][17]} definition of organelle emerged, after which only cellular structures with surrounding membrane had been considered organelles. MembraneA biological membrane or biomembrane is an enclosing or separating Amphipathic layer that acts as a barrier within or around a cell. However, the more original definition of subcellular functional unit in general still coexists. ^[18][19]

In 1978, Albert Frey-Wyssling suggested that the term organelle should refer only to structures that convert energy, such as centrosomes, ribosomes, and nucleoli. Albert Friedrich Frey-Wyssling ( November 8, 1900 &ndash August 30, 1988) was a Swiss botanist who pioneered submicroscopic morphology and helped ^[20][21] This new definition, however, did not win wide recognition.

Examples

Whereas 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. Cellular compartments in Cell biology comprise all closed parts within a cell whose lumen is usually surrounded by a single or double lipid layer An endosymbiont is any Organism that lives within the body or cells of another organism i

The most notable of these organelles having originated from endosymbiont bacteria are:

• mitochondria (in almost all eukaryotes)
• chloroplasts (in plants, algae and protists). An endosymbiont is any Organism that lives within the body or cells of another organism i The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. Chloroplasts are Organelles found in Plant cells and eukaryotic Algae that conduct Photosynthesis.

Other organelles are also suggested to have endosymbiotic origins, (notably the flagellum - see evolution of flagella). The Evolution of Flagella is of great interest to Biologists because the three known varieties of flagella ( eukaryotic, bacterial and

Not all parts of the cell qualify as organelles, and the use of the term to refer to some structures is disputed. These structures are large assemblies of macromolecules that carry out particular and specialized functions, but they lack membrane boundaries. The term macromolecule by definition implies "large Molecule " Such cell structures, which are not formally organelles, include:

• ribosome
• cytoskeleton
• flagellum. Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. A flagellum ( plural flagella) is a tail-like structure that projects from the Cell body of certain Prokaryotic and Eukaryotic cells and it

Eukaryotic organelles

Eukaryotes are the most structurally complex cell type, and by definition are in part organized by smaller interior compartments, that are themselves enclosed by lipid membranes that resemble the outermost cell membrane. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer The larger organelles, such as the nucleus and vacuoles, are easily visible with the light microscope. In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed In general vacuole functions include Removing unwanted structural debris Isolating materials that might be harmful or a threat to the cell Containing The optical microscope, often referred to as the "light microscope" is a type of Microscope which uses Visible light and a system of lenses to They were among the first biological discoveries made after the invention of the microscope. A microscope ( Greek: ( micron) = small + ( skopein) = to look or see is an instrument for viewing objects that are

Not all eukaryotic cells have every one of the organelles listed below. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex Exceptional organisms have cells which do not include some organelles that might otherwise be considered universal to eukaryotes (such as mitochondria). ^[22] 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.

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. Chloroplasts are Organelles found in Plant cells and eukaryotic Algae that conduct Photosynthesis. Plastids are major Organelles found in plants and algae Plastids often contain pigments used in photosynthesis and the types of pigments present can change Photosynthesis is a Metabolic pathway that converts Light Energy into Chemical energy. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle The Golgi apparatus (also called the In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. In general vacuole functions include Removing unwanted structural debris Isolating materials that might be harmful or a threat to the cell Containing Homeostasis (from Greek: ὅμος hómos, "equal" and ιστημι istēmi, "to stand" lit In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed Transcription is the synthesis of RNA under the direction of DNA In classical genetics the genome of a Diploid Organism including Eukarya refers to a full set of chromosomes or genes in a Gamete, thereby Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other This idea is supported in the Endosymbiotic theory. The endosymbiotic theory concerns the origins of mitochondria and Plastids (e

Other related structures:

• cytosol
• endomembrane system
• nucleosome
• microtubule
• cell membrane

Prokaryotic organelles

Prokaryotes are not as structurally or metabolically complex as eukaryotes, and were once thought not to have any internal structures enclosed by lipid membranes. In a biological cell, a melanosome is an Organelle containing Melanin, the most common light-absorbing pigment found in the animal kingdom A mitosome is an Organelle found in some unicellular eukaryotic organisms Myofibrils (obsolete term sarcostyles) are cylindrical Organelles found within Muscle cells They are bundles of actomyosin filaments The nucleolus (also called nucleole) is a structure found within the nucleus in which Ribosomal RNA is transcribed. Within the cells of Basidiomycete fungi are found microscopic structures called parenthesomes or septal pore caps. Peroxisomes are Ubiquitous Organelles in Eukaryotes that participate in the metabolism of Fatty acids and other metabolites Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that Translation is the first stage of Protein biosynthesis (part of the overall process of Gene expression) A vesicle is a small bubble of liquid within a cell A more formal definition in Cell biology, would be that a vesicle is a relatively small intracellular membrane-enclosed The cytosol or intracellular fluid (or cytoplasmic matrix) is the liquid found inside cells. The endomembrane system is composed of the different membranes that are suspended in the Cytoplasm within a Eukaryotic cell. Nucleosomes form the fundamental repeating units of eukaryotic Chromatin, which is used to pack the large eukaryotic genomes into the nucleus while still ensuring Microtubules are one of the components of the Cytoskeleton. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other A lipid bilayer or bilayer lipid membrane ( BLM) is a membrane composed of Lipid molecules (usually Phospholipids. 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 1970's 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. Mesosomes are folded Invaginations in the Plasma membrane of Bacteria that are produced by the chemical fixation techniques used to prepare samples An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image ^[23] However, more recent research has revealed that at least some prokaryotes have microcompartments, which are compartments enclosed by proteins. ^[1] Even more striking is the description of magnetosomes,^[24][25] as well as the nucleus-like structures of the Planctomycetes that are surrounded by lipid membranes. A lipid bilayer or bilayer lipid membrane ( BLM) is a membrane composed of Lipid molecules (usually Phospholipids. ^[26]

See also

• Cell
• Endosymbiotic theory

References

• Alberts, Bruce et al. (2003). Essential Cell Biology, 2nd ed. , Garland Science, 2003, ISBN 081533480X.
• Alberts, Bruce et al. (2002). The Molecular Biology of the Cell, 4th ed. , Garland Science, 2002, ISBN 0-8153-3218-1.