Organelle | |
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Details | |
Pronunciation | /ɔːrɡəˈnɛl/ |
Part of | Cell |
Identifiers | |
Latin | organella |
MeSH | D015388 |
TH | H1.00.01.0.00009 |
FMA | 63832 |
Anatomical terms of microanatomy |
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-bounded organelles) or are spatially distinct functional units without a surrounding lipid bilayer (non-membrane bounded organelles). Although most organelles are functional units within cells, some function units that extend outside of cells are often termed organelles, such as cilia, the flagellum and archaellum, and the trichocyst (these could be referred to as membrane bound in the sense that they are attached to (or bound to) the membrane).
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 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; [1] and there is also evidence of other membrane-bounded structures. [2] 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.
Cell biology | |
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Animal cell diagram | |
In biology, organs are defined as confined functional units within an organism. [3] 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. [4]
Credited as the first [5] [6] [7] 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). [8] 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. [8] [9]
While most cell biologists consider the term organelle to be synonymous with cell compartment, a space often bounded 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. [10]
The first, broader conception of organelles is that they are membrane-bounded 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 bounded structures such as ribosomes is common and accepted. [11] [ verification needed ] [12] [13] This has led many texts to delineate between membrane-bounded and non-membrane bounded organelles. [14] The non-membrane bounded 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:
The mechanisms by which such non-membrane bounded organelles form and retain their spatial integrity have been likened to liquid-liquid phase separation. [15]
The second, more restrictive definition of organelle includes only those cell compartments that contain deoxyribonucleic acid (DNA), having originated from formerly autonomous microscopic organisms acquired via endosymbiosis. [10] [16] [17]
Using 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[ by whom? ] to have endosymbiotic origins, but do not contain their own DNA[ citation needed ] (notably the flagellum – see evolution of flagella).
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 (such as mitochondria) that might otherwise be considered universal to eukaryotes. [19] The several plastids including chloroplasts are distributed among some but not all eukaryotes.
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.The cell membrane and cell wall are not organelles.
Organelle | Main function | Structure | Organisms | Notes |
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chloroplast (plastid) | photosynthesis, traps energy from sunlight | double-membrane compartment | plants, algae, rare kleptoplastic organisms | has own DNA; theorized to be engulfed by the ancestral archaeplastid 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 (which are bound to the ribosome membrane), 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) [20] |
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 | all eukaryotes | |
Organelle/Macromolecule | Main function | Structure | Organisms |
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acrosome | helps spermatozoa fuse with ovum | single-membrane compartment | most animals (including sponges) |
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". [21] | 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 [22] | membraneless (mRNP complexes) | most eukaryotes |
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 to have little internal organization, and lack cellular compartments and internal membranes; but slowly, details are emerging about prokaryotic internal structures that overturn these assumptions. [2] 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. [24]
However, there is increasing evidence of compartmentalization in at least some prokaryotes. [2] 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-bounded magnetosomes in bacteria, reported in 2006. [25] [26]
The bacterial phylum Planctomycetota has revealed a number of compartmentalization features. The Planctomycetota cell plan includes intracytoplasmic membranes that separates the cytoplasm into paryphoplasm (an outer ribosome-free space) and pirellulosome (or riboplasm, an inner ribosome-containing space). [27] Membrane-bounded anammoxosomes have been discovered in five Planctomycetota "anammox" genera, which perform anaerobic ammonium oxidation. [28] In the Planctomycetota species Gemmata obscuriglobus , a nucleus-like structure surrounded by lipid membranes has been reported. [27] [29]
Compartmentalization is a feature of prokaryotic photosynthetic structures. [2] Purple bacteria have "chromatophores", which are reaction centers found in invaginations of the cell membrane. [2] Green sulfur bacteria have chlorosomes, which are photosynthetic antenna complexes found bonded to cell membranes. [2] 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. [2]
Organelle/macromolecule | Main function | Structure | Organisms |
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anammoxosome | anaerobic ammonium oxidation | ladderane lipid membrane | " Candidatus " bacteria within Planctomycetota |
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 |
The cell is the basic structural and functional unit of all forms of life. Every cell consists of cytoplasm enclosed within a membrane; many cells contain organelles, each with a specific function. The term comes from the Latin word cellula meaning 'small room'. Most cells are only visible under a microscope. Cells emerged on Earth about 4 billion years ago. All cells are capable of replication, protein synthesis, and motility.
In cell biology, the cytoplasm describes all material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The main components of the cytoplasm are the cytosol, the organelles, and various cytoplasmic inclusions. The cytoplasm is about 80% water and is usually colorless.
Cell biology is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living and functioning of organisms. Cell biology is the study of the structural and functional units of cells. Cell biology encompasses both prokaryotic and eukaryotic cells and has many subtopics which may include the study of cell metabolism, cell communication, cell cycle, biochemistry, and cell composition. The study of cells is performed using several microscopy techniques, cell culture, and cell fractionation. These have allowed for and are currently being used for discoveries and research pertaining to how cells function, ultimately giving insight into understanding larger organisms. Knowing the components of cells and how cells work is fundamental to all biological sciences while also being essential for research in biomedical fields such as cancer, and other diseases. Research in cell biology is interconnected to other fields such as genetics, molecular genetics, molecular biology, medical microbiology, immunology, and cytochemistry.
The endomembrane system is composed of the different membranes (endomembranes) that are suspended in the cytoplasm within a eukaryotic cell. These membranes divide the cell into functional and structural compartments, or organelles. In eukaryotes the organelles of the endomembrane system include: the nuclear membrane, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, endosomes, and plasma (cell) membrane among others. The system is defined more accurately as the set of membranes that forms a single functional and developmental unit, either being connected directly, or exchanging material through vesicle transport. Importantly, the endomembrane system does not include the membranes of plastids or mitochondria, but might have evolved partially from the actions of the latter.
A flagellum is a hairlike appendage that protrudes from certain plant and animal sperm cells, from fungal spores (zoospores), and from a wide range of microorganisms to provide motility. Many protists with flagella are known as flagellates.
Symbiogenesis is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibly other organelles of eukaryotic cells are descended from formerly free-living prokaryotes taken one inside the other in endosymbiosis. Mitochondria appear to be phylogenetically related to Rickettsiales bacteria, while chloroplasts are thought to be related to cyanobacteria.
In biological taxonomy, a domain, also dominion, superkingdom, realm, or empire, is the highest taxonomic rank of all organisms taken together. It was introduced in the three-domain system of taxonomy devised by Carl Woese, Otto Kandler and Mark Wheelis in 1990.
The evolution of flagella is of great interest to biologists because the three known varieties of flagella – each represent a sophisticated cellular structure that requires the interaction of many different systems.
The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is composed of similar proteins in the various organisms. It is composed of three main components: microfilaments, intermediate filaments, and microtubules, and these are all capable of rapid growth or disassembly depending on the cell's requirements.
The three-domain system is a taxonomic classification system that groups all cellular life into three domains, namely Archaea, Bacteria and Eukarya, introduced by Carl Woese, Otto Kandler and Mark Wheelis in 1990. The key difference from earlier classifications such as the two-empire system and the five-kingdom classification is the splitting of Archaea from Bacteria as completely different organisms. It has been challenged by the two-domain system that divides organisms into Bacteria and Archaea only, as Eukaryotes are considered as a clade of Archaea.
A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Most prokaryotes are unicellular and are classified into bacteria and archaea. Many eukaryotes are multicellular, but some are unicellular such as protozoa, unicellular algae, and unicellular fungi. Unicellular organisms are thought to be the oldest form of life, with early protocells possibly emerging 3.5–4.1 billion years ago.
The periplasm is a concentrated gel-like matrix in the space between the inner cytoplasmic membrane and the bacterial outer membrane called the periplasmic space in Gram-negative bacteria. Using cryo-electron microscopy it has been found that a much smaller periplasmic space is also present in Gram-positive bacteria, between cell wall and the plasma membrane. The periplasm may constitute up to 40% of the total cell volume of gram-negative bacteria, but is a much smaller percentage in gram-positive bacteria.
The Planctomycetota are a phylum of widely distributed bacteria, occurring in both aquatic and terrestrial habitats. They play a considerable role in global carbon and nitrogen cycles, with many species of this phylum capable of anaerobic ammonium oxidation, also known as anammox. Many Planctomycetota occur in relatively high abundance as biofilms, often associating with other organisms such as macroalgae and marine sponges.
A bacterium, despite its simplicity, contains a well-developed cell structure which is responsible for some of its unique biological structures and pathogenicity. Many structural features are unique to bacteria and are not found among archaea or eukaryotes. Because of the simplicity of bacteria relative to larger organisms and the ease with which they can be manipulated experimentally, the cell structure of bacteria has been well studied, revealing many biochemical principles that have been subsequently applied to other organisms.
Cellular compartments in cell biology comprise all of the closed parts within the cytosol of a eukaryotic cell, usually surrounded by a single or double lipid layer membrane. These compartments are often, but not always, defined as membrane-bound organelles. The formation of cellular compartments is called compartmentalization.
The following outline is provided as an overview of and topical guide to cell biology:
A prokaryote is a single-cell organism whose cell lacks a nucleus and other membrane-bound organelles. The word prokaryote comes from the Ancient Greek πρό 'before' and κάρυον 'nut, kernel'. In the two-empire system arising from the work of Édouard Chatton, prokaryotes were classified within the empire Prokaryota. However in the three-domain system, based upon molecular analysis, prokaryotes are divided into two domains: Bacteria and Archaea. Organisms with nuclei are placed in a third domain, Eukaryota.
The eukaryotes constitute the domain of Eukarya or Eukaryota, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, and many unicellular organisms are eukaryotes. They constitute a major group of life forms alongside the two groups of prokaryotes: the Bacteria and the Archaea. Eukaryotes represent a small minority of the number of organisms, but given their generally much larger size, their collective global biomass is much larger than that of prokaryotes.
The cell membrane is a biological membrane that separates and protects the interior of a cell from the outside environment. The cell membrane consists of a lipid bilayer, made up of two layers of phospholipids with cholesterols interspersed between them, maintaining appropriate membrane fluidity at various temperatures. The membrane also contains membrane proteins, including integral proteins that span the membrane and serve as membrane transporters, and peripheral proteins that loosely attach to the outer (peripheral) side of the cell membrane, acting as enzymes to facilitate interaction with the cell's environment. Glycolipids embedded in the outer lipid layer serve a similar purpose.
Gemmata obscuriglobus is a species of Gram-negative, aerobic, heterotrophic bacteria of the phylum Planctomycetota. G. obscuriglobus occur in freshwater habitats and was first described in 1984, and is the only described species in its genus.
Die Vacuolen sind demnach in strengem Sinne keine beständigen Organe oder O r g a n u l a (wie Möbius die Organe der Einzelligen im Gegensatz zu denen der Vielzelligen zu nennen vorschlug).
It may possibly be of advantage to use the word organula here instead of organ, following a suggestion by Möbius. Functionally differentiated multicellular aggregates in multicellular forms or metazoa are in this sense organs, while, for functionally differentiated portions of unicellular organisms or for such differentiated portions of the unicellular germ-elements of metazoa, the diminutive organula is appropriate.
Während die Fortpflanzungszellen der vielzelligen Tiere unthätig fortleben bis sie sich loslösen, wandern und entwickeln, treten die einzelligen Tiere auch durch die an der Fortpflanzung beteiligten Leibesmasse in Verkehr mit der Außenwelt und viele bilden sich dafür auch besondere Organula". Footnote on p. 448: "Die Organe der Heteroplastiden bestehen aus vereinigten Zellen. Da die Organe der Monoplastiden nur verschieden ausgebildete Teile e i n e r Zelle sind schlage ich vor, sie „Organula" zu nennen