A flagellum (plural: flagella) is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and it functions in locomotion. The soma, or cyton or perikaryon, is the bulbous end of a Neuron, containing the Cell nucleus. The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex ^[1][2] An example of a eukaryotic flagellated cell is the sperm cell, which uses its flagellum to propel itself through the female reproductive tract. The term sperm is derived from the Greek word (σπέρμα sperma (meaning "seed" and refers to the male reproductive cells. ^[3] An example of a flagellated bacterium is the ulcer-causing Helicobacter pylori, which uses its multiple flagella to propel iteself through the mucus lining to reach the stomach epithelium. Helicobacter pylori ( is a Gram-negative, microaerophilic Bacterium that inhabits various areas of the stomach and Duodenum. In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body ^[4] Prokaryotic and eukaryotic flagella have some notable differences, such as protein composition, structure, and mechanism of propulsion. Flagella are structurally identical to eukaryotic cilia, although distinctions are sometimes made according to function and/or length. A cilium (plural cilia) is an Organelle found in eukaryotic cells Cilia are tail-like projections extending approximately ^[5] Flagella are cellular structures, not organelles. In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed

Types

Three quite distinct types of flagella have so far been distinguished; bacterial, archaeal and eukaryotic.

The main differences among these three types are summarized below:

• Bacterial flagella are helical filaments that rotate like screws. The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have ^[6][7][8] They provide two of several kinds of bacterial motility. ^[9][10]
• Archaeal flagella are superficially similar to bacterial flagella, but are different in many details and considered non-homologous. In Evolutionary biology, homology has come to mean any similarity between characters that is due to their shared ancestry. ^[11][12]
• Eukaryotic flagella - those of animal, plant, and protist cells - are complex cellular projections that lash back and forth. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex

Sometimes eukaryotic flagella are called cilia or undulipodia to emphasize their distinctiveness. A cilium (plural cilia) is an Organelle found in eukaryotic cells Cilia are tail-like projections extending approximately An undulipodium or 9+2 organelle is an extracellular projection of a Eukaryotic cell containing a Microtubule array

Bacterial

Flagellum of Gram-negative Bacteria

Examples of bacterial flagella arrangement schemes. A-Monotrichous; B-Lophotrichous; C-Amphitrichous; D-Peritrichous;

The bacterial flagellum is made up of the protein flagellin. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Flagellin is a Protein that arranges itself in a hollow cylinder to form the filament in Bacterial Flagellum. Its shape is a 20 nanometer-thick hollow tube. A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a It is helical and has a sharp bend just outside the outer membrane; this "hook" allows the helix to point directly away from the cell. A helix (pl helixes or helices) from the Greek word έλιξ, is a special kind of Space curve, i A shaft runs between the hook and the basal body, passing through protein rings in the cell's membrane that act as bearings. A basal body (sometimes basal granule or kinetosome) is an Organelle formed from a Centriole, a short Cylindrical array Gram-positive organisms have 2 of these basal body rings, one in the peptidoglycan layer and one in the plasma membrane. Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. Not to be confused with Glycoprotein. Peptidoglycan, also known as murein, is a Polymer consisting of sugars and amino The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Gram-negative organisms have 4 such rings: the L ring associates with the lipopolysaccharides, the P ring associates with peptidoglycan layer, the M ring is embedded in the plasma membrane, and the S ring is directly attached to the plasma membrane. Gram-negative bacteria are those Bacteria that do not retain Crystal violet dye in the Gram staining protocol The L-ring of the bacterial Flagellum is the ring in the lipid outer Cell membrane through which the Axial filament (rod hook and flagellum Lipopolysaccharides ( LPS) are large Molecules consisting of a Lipid and a Polysaccharide joined by a Covalent bond; they are found Not to be confused with Glycoprotein. Peptidoglycan, also known as murein, is a Polymer consisting of sugars and amino The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer The filament ends with a capping protein. ^[13][14]

The bacterial flagellum is driven by a rotary engine made up of protein (Mot complex), located at the flagellum's anchor point on the inner cell membrane. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer The engine is powered by proton motive force, i. In Cellular biology, an electrochemical gradient is a spatial variation of both Electrical potential and chemical Concentration across a membrane e. , by the flow of protons (hydrogen ions) across the bacterial cell membrane due to a concentration gradient set up by the cell's metabolism (in Vibrio species there are two kinds of flagella, lateral and polar, and some are driven by a sodium ion pump rather than a proton pump^[15]). The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement Vibrio is a Genus of Gram-negative Bacteria possessing a curved rod shape Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 "Ion pump" redirects here For pumps that reduce pressure see Ion pump (physics. This article is about proton pumps in general For the proton P-type ATPase see Proton ATPase. The rotor transports protons across the membrane, and is turned in the process. The rotor alone can operate at 6,000 to 17,000 rpm, but with the flagellar filament attached usually only reaches 200 to 1000 rpm.

Flagella do not rotate at a constant speed but instead can increase or decrease their rotational speed in relation to the strength of the proton motive force. Flagellar rotation can move bacteria through liquid media at speed of up to 60 cell lengths/second (sec). Although this is only about 0. 00017 km/h, when comparing this speed with that of higher organisms in terms of number of lengths moved per second, it is extremely fast. The fastest land animal, the cheetah, moves at a maximum rate of about 110 km/h, but this represents only about 25 body lengths/sec. The cheetah ( Acinonyx jubatus) is an atypical member of the cat family ( Felidae) that is unique in its speed while lacking climbing abilities Thus, when size is accounted for, prokaryotic cells swimming at 50-60 lengths/sec are actually much faster than larger organisms.

The components of the bacterial flagellum are capable of self-assembly without the aid of enzymes or other factors. Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins Both the basal body and the filament have a hollow core, through which the component proteins of the flagellum are able to move into their respective positions. During assembly, protein components are added at the flagellar tip rather than at the base.

The basal body has several traits in common with some types of secretory pores, such as the hollow rod-like "plug" in their centers extending out through the plasma membrane. Secretion is the process of segregating elaborating and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance Given the structural similarities, it was thought that bacterial flagella may have evolved from such pores; however, it is now known that these pores are derived from flagella.

Flagella arrangement schemes

Different species of bacteria have different numbers and arrangements of flagella. Monotrichous bacteria have a single flagellum (e. g. , Vibrio cholerae). Vibrio cholerae (also Kommabacillus) is a Gram negative curved-rod shaped Bacterium with a polar Flagella that causes Cholera Lophotrichous bacteria have multiple flagella located at the same spot on the bacteria's surfaces which act in concert to drive the bacteria in a single direction. In many cases, the bases of multiple flagella are surrounded a specialized region of the cell membrane; the so-called polar membrane. In the scientific literature polar membrane has acquired two separate meanings Amphitrichous bacteria have a single flagellum on each of two opposite ends (only one flagellum operates at a time, allowing the bacteria to reverse course rapidly by switching which flagellum is active). Peritrichous bacteria have flagella projecting in all directions (e. g. , Escherichia coli).

In some bacteria, such as the larger forms of Selenomonas, the individual flagella are organized outside the cell body, helically twining about each other to form a thick structure called a "fascicle". The genus Selenomonas constitutes a group of motile crescent-shaped bacteria within the Acidaminococcaceae family and include Species living in the Other bacteria, such as Spirochetes, have a specialized type of flagellum called an "axial filament" that is located in the periplasmic space, the rotation of which causes the entire bacterium to move forward in a corkscrew-like motion. Spirochaetes is a phylum of distinctive Gram-negative bacteria, which have long helically coiled cells The periplasmic space or periplasm is a space between the inner Cytoplasmic membrane and external Outer membrane of Gram-negative bacteria

Counterclockwise rotation of monotrichous polar flagella thrust the cell forward with the flagella trailing behind. Periodically, the direction of rotation is briefly reversed, causing what is known as a "tumble" in which the cell seems to thrash about in place. This results in the reorientation of the cell. When moving in a favorable direction, "tumbles" are unlikely; however, when the cell's direction of motion is unfavorable (e. g. , away from a chemical attractant), a tumble may occur, with the chance that the cell will be thus reoriented in the correct direction.

In some Vibrio (particularly Vibrio parahemolyticus^[16]) and related proteobacteria such as Aeromonas, two flagellar systems co-exist, using different sets of genes and different ion gradients for energy. Vibrio is a Genus of Gram-negative Bacteria possessing a curved rod shape Vibrio parahaemolyticus is a curved rod-shaped Gram-negative Bacterium found in Brackish saltwater, which when ingested The Proteobacteria are a major group ( Phylum) of Bacteria. They include a wide variety of Pathogens such as Escherichia, Aeromonas is a Gram-negative, facultative anaerobic rod that morphologically resembles members of the family Enterobacteriaceae. The polar flagella are constitutively expressed and provide motility in bulk fluid, while the lateral flagella are expressed when the polar flagella meets too much resistance to turn. ^{[17][18][19][20][21][22]} These provide swarming motility on surfaces or in viscous fluids.

Archaeal

The archaeal flagellum is superficially similar to the bacterial (or eubacterial) flagellum; in the 1980s they were thought to be homologous on the basis of gross morphology and behavior. ^[23] Both flagella consist of filaments extending outside of the cell, and rotate to propel the cell.

However, discoveries in the 1990s revealed numerous detailed differences between the archaeal and bacterial flagella; these include:

• Bacterial flagella are motorized by a flow of H⁺ ions (or occasionally Na⁺ ions); archaeal flagella are almost certainly powered by ATP. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Adenosine-5'-triphosphate ( ATP) is a multifunctional Nucleotide that is most important as a " molecular currency" of intracellular Energy The torque-generating motor that powers rotation of the archaeal flagellum has not been identified. A torque (τ in Physics, also called a moment (of force is a pseudo- vector that measures the tendency of a force to rotate an object about
• While bacterial cells often have many flagellar filaments, each of which rotates independently, the archaeal flagellum is composed of a bundle of many filaments that rotate as a single assembly.
• Bacterial flagella grow by the addition of flagellin subunits at the tip; archaeal flagella grow by the addition of subunits to the base.
• Bacterial flagella are thicker than archaeal flagella, and the bacterial filament has a large enough hollow "tube" inside that the flagellin subunits can flow up the inside of the filament and get added at the tip; the archaeal flagellum is too thin to allow this.
• Many components of bacterial flagella share sequence similarity to components of the type III secretion systems, but the components of bacterial and archaeal flagella share no sequence similarity. Secretion is the process of segregating elaborating and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance Instead, some components of archaeal flagella share sequence and morphological similarity with components of type IV pili, which are assembled through the action of type II secretion systems (the nomenclature of pili and protein secretion systems is not consistent). Secretion is the process of segregating elaborating and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance

These differences mean that the bacterial and archaeal flagella are a classic case of biological analogy, or convergent evolution, rather than homology. Analogy is both the cognitive process of transferring Information from a particular subject (the analogue or source to another particular subject (the target and Convergent evolution describes the acquisition of the same biological trait in unrelated lineages In Evolutionary biology, homology has come to mean any similarity between characters that is due to their shared ancestry. However, in comparison to the decades of well-publicized study of bacterial flagella (e. g. by Berg), archaeal flagella have only recently begun to get serious scientific attention. Therefore, many assume erroneously that there is only one basic kind of prokaryotic flagellum, and that archaeal flagella are homologous to it. For example, Cavalier-Smith (2002)^[23] is aware of the differences between archaeal and bacterial flagellins, but retains the misconception that the basal bodies are homologous.

Eukaryotic

Difference of beating pattern of flagellum and cilia

The eukaryotic flagellum is completely different from the prokaryote flagellum in both structure and evolutionary origin. The only shared characteristics among bacterial, archaeal, and eukaryotic flagella are their superficial appearance; they are intracellular extensions used in creating movement. Along with cilia, flagella make up a group of organelles known as undulipodia. A cilium (plural cilia) is an Organelle found in eukaryotic cells Cilia are tail-like projections extending approximately An undulipodium or 9+2 organelle is an extracellular projection of a Eukaryotic cell containing a Microtubule array

Structure

A eukaryotic flagellum is a bundle of nine fused pairs of microtubule doublets surrounding two central single microtubules. 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 so-called "9+2" structure is characteristic of the core of the eukaryotic flagellum called an axoneme. Chlamydomonas TEM 17jpg|thumb|Micrograph of thin x-section cut through Chlamydomonas axoneme]]Numerous Eukaryotic cells carry whip-like appendages ( Cilia At the base of a eukaryotic flagellum is a basal body, "blepharoplast" or kinetosome, which is the microtubule organizing center (MTOC) for flagellar microtubules and is about 500 nanometers long. A basal body (sometimes basal granule or kinetosome) is an Organelle formed from a Centriole, a short Cylindrical array The microtubule-organizing center ( MTOC) is a structure found in eukaryotic cells from which Microtubules emerge Basal bodies are structurally identical to centrioles. A Centriole is a barrel shaped Organelle found in most animal Eukaryotic cells though absent in Higher plants and Fungi. The flagellum is encased within the cell's plasma membrane, so that the interior of the flagellum is accessible to the cell's cytoplasm. The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane.

Mechanism

Each of the outer 9 doublet microtubules extends a pair of dynein arms (an "inner" and an "outer" arm) to the adjacent microtubule; these dynein arms are responsible for flagellar beating, as the force produced by the arms causes the microtubule doublets to slide against each other and the flagellum as a whole to bend. Dynein is a Motor protein (also called molecular motor or motor molecule in cells which converts the chemical Energy contained in ATP into the These dynein arms produce force through ATP hydrolysis. Hydrolysis is a Chemical reaction during which one or more water molecules are split into hydrogen and hydroxide ions which may go on to participate in further reactions The flagellar axoneme also contains radial spokes, polypeptide complexes extending from each of the outer 9 mictrotubule doublets towards the central pair, with the "head" of the spoke facing inwards. The radial spoke is a multi-unit Protein structure found in the Axonemes of Eukaryotic Cilia and Flagella. The radial spoke is thought to be involved in the regulation of flagellar motion, although its exact function and method of action are not yet understood.

Flagella vs Cilia

Though eukaryotic flagella and cilia are ultrastructurally identical, the beating pattern of the two organelles can be different. In the case of flagella (e. g. the tail of a sperm) the motion is propeller-like. In contrast, beating of cilia is consists of coordinated back-and-forth cycling of many cilia on the cell surface. Thus, motile flagella serve for the propulsion of single cells (e. g. swimming of protozoa and spermatozoa), and cilia for the transport of fluids (e. g. transport of mucus by stationary flagellated cells in the trachea).

Intraflagellar Transport

Intraflagellar transport (IFT), the process by which axonemal subunits, transmembrane receptors, and other proteins are moved up and down the length of the flagellum, is essential for proper functioning of the flagellum, in both motility and signal transduction. Intraflagellar transport or IFT is the cellular process essential for the formation and maintenance of Eukaryotic Cilia and Flagella. Transmembrane receptors are Integral membrane proteins which reside and operate typically within a cell's Plasma membrane, but also in the membranes of ^[24]

For information on biologists' ideas about how the various flagella may have evolved, see evolution of flagella. The Evolution of Flagella is of great interest to Biologists because the three known varieties of flagella ( eukaryotic, bacterial and

Irreducibly complex?

See also: Evolution of flagella and Irreducible complexity#Flagella

In his 1996 book Darwin's Black Box, microbiologist and intelligent design proponent Michael Behe cited the bacterial flagellum as an example of an irreducibly complex structure that could not have evolved through naturalistic means. The Evolution of Flagella is of great interest to Biologists because the three known varieties of flagella ( eukaryotic, bacterial and This article covers irreducible complexity as used by those who argue for Intelligent design. Darwin's Black Box The Biochemical Challenge to Evolution ( 1996, first edition 2006, second edition is a book written by Michael J Intelligent Michael J Behe (born 1952 is an American Biochemist and Intelligent design advocate This article covers irreducible complexity as used by those who argue for Intelligent design. This article covers metaphysical naturalism as a worldview Naturalism (philosophy discusses methodological naturalism Behe argued that the flagellum becomes useless if any one of its constituent parts is removed, and thus could not have arisen through numerous, successive, slight modifications; therefore, it is hopelessly improbable that the proteins making up the flagellar motor could have come together all at once, by chance. Thus, according to Behe, "Darwinian theory has given no explanation for the cilium or flagellum. The overwhelming complexity of the swimming systems push us to think it may never give an explanation. "^[25]

While Behe discussed the immune system and the blood clotting cascade in greater detail, the bacterial flagellum has become a "poster child" for intelligent design proponents and other creationists. An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor Coagulation is a complex process by which Blood forms Clots It is an important part of Hemostasis (the cessation of blood loss from a damaged vessel whereby "Creationism" can also refer to Creation myths in general or to a concept about the origin of the soul. It is one of only two rotary structures found in nature (the other being ATP synthase)^[26] and it is billions of years older than Behe's other two examples, which exist in many homologous forms, simplifying the explanation of their origin. An ATP synthase ( is a general term for an Enzyme that can synthesize Adenosine triphosphate (ATP from Adenosine diphosphate (ADP and inorganic In Evolutionary biology, homology has come to mean any similarity between characters that is due to their shared ancestry. ^[27]

Potentially viable evolutionary pathways have since been proposed for the bacterial flagellum. ^[28][29][30]

In addition, the Type III secretory system, a molecular syringe which bacteria use to inject toxins into other cells, appears to be a simplified sub-set of the bacterial flagellum's components, meaning that it is much less likely to be irreducibly complex. Secretion is the process of segregating elaborating and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance ^[31][32]

Behe's arguments have been examined and rejected by the scientific community at large. Exaptation explains how systems with multiple parts can evolve through natural means. Exaptation, cooption, and preadaptation are related terms referring to shifts in the function of a trait during Evolution. ^[33]

See also

• Evolution of flagella

References

External links

• Molecular Machines Museum Index. Access Research Network (2001). Retrieved on 2008-05-18. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 1152 - Henry II of England marries Eleanor of Aquitaine.
• Howard Berg (1999). Motile Behavior of Bacteria. Physics Today on the Web. American Institute of Physics. Retrieved on 2008-05-18. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 1152 - Henry II of England marries Eleanor of Aquitaine.
• Charles Lindemann (2008-04-04). 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 1581 - Francis Drake completes a circumnavigation of the world and is knighted by Elizabeth I. Mechansims of sperm motility. Oakland University. Retrieved on 2008-05-18. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 1152 - Henry II of England marries Eleanor of Aquitaine.

This article incorporates content from the 1728 Cyclopaedia, a publication in the public domain. Cyclopaedia or A Universal Dictionary of Arts and Sciences ( folio, 2 vols The public domain is a range of abstract materials &ndash commonly referred to as Intellectual property &ndash which are not owned or controlled by anyone