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Carbohydrate metabolism
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Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown, and interconversion of carbohydrates in living organisms.
Carbohydrates are central to many essential metabolic pathways.[1] Plants synthesize carbohydrates from carbon dioxide and water through photosynthesis, allowing them to store energy absorbed from sunlight internally.[2] When animals and fungi consume plants, they use cellular respiration to break down these stored carbohydrates to make energy available to cells.[2] Both animals and plants temporarily store the released energy in the form of high energy molecules, such as ATP, for use in various cellular processes.[3]
Although humans consume a variety of carbohydrates, digestion breaks down complex carbohydrates into a few simple monomers (monosaccharides) for metabolism: glucose, fructose, and galactose.[4] Glucose constitutes about 80% of the products, and is the primary structure that is distributed to cells in the tissues, where it is broken down or stored as glycogen.[3][4] In aerobic respiration, the main form of cellular respiration used by humans, glucose and oxygen are metabolized to release energy, with carbon dioxide and water as byproducts.[2] Most of the fructose and galactose travel to the liver, where they can be converted to glucose.[4]
Some simple carbohydrates have their own enzymatic oxidation pathways, as do only a few of the more complex carbohydrates. The disaccharide lactose, for instance, requires the enzyme lactase to be broken into its monosaccharide components, glucose and galactose.[5]
Contents
[hide]
1 Metabolic pathways
1.1 Glycolysis
1.2 Gluconeogenesis
1.3 Glycogenolysis
1.4 Glycogenesis
1.5 Pentose phosphate pathway
1.6 Fructose metabolism
1.7 Galactose metabolism
2 Energy production
3 Hormonal regulation
4 Carbohydrates as storage
5 Human diseases
6 References
7 External links
Metabolic pathways[edit]
Overview of connections between metabolic processes.
Glycolysis[edit]
Glycolysis is the process of breaking down a glucose molecule into two pyruvate molecules, while storing energy released during this process as ATP and NADH.[2] Nearly all organisms that break down glucose utilize glycolysis.[2] Glucose regulation and product use are the primary categories in which these pathways differ between organisms.[2] In some tissues and organisms, glycolysis is the sole method of energy production.[2] This pathway is anaerobic, because it doesn’t require oxygen.[1]
Glycolysis consists of ten steps, split into two phases.[2] During the first phase, it requires the breakdown of two ATP molecules.[1] During the second phase, chemical energy from the intermediates is transferred into ATP and NADH.[2] The breakdown of one molecule of glucose results in two molecules of pyruvate, which can be further oxidized to access more energy in later processes.[1]
Gluconeogenesis[edit]
Gluconeogenesis is the reverse process of glycolysis.[6] It involves the conversion of non-carbohydrate molecules into glucose.[6] The non-carbohydrate molecules that are converted in this pathway include pyruvate, lactate, glycerol, alanine, and glutamine.[6] This process occurs when there are lowered amounts of glucose.[6] The production of glucose by this pathway is important to tissues that cannot use any other fuels, such as the brain.[1] The liver is the primary location of gluconeogenesis, but some also occurs in the kidney.[1]
This pathway is regulated by multiple different molecules.[6] Glucagon, adrenocorticotropic hormone, and ATP encourage gluconeogenesis.[6] Gluconeogenesis is inhibited by AMP, ADP, and insulin.[6]
Glycogenolysis[edit]
Glycogenolysis refers to the breakdown of glycogen.[6] In the liver, muscles, and the kidney, this process occurs to provide glucose when necessary.[6] A single glucose molecule is cleaved from a branch of glycogen, and is transformed into glucose-1-phosphate during this process.[1] This molecule can then be converted to glucose-6-phosphate, an intermediate in the glycolysis pathway.[1]
Glucose-6-phosphate can then progress through glycolysis.[1] Glycolysis only requires the input of one molecule of ATP when the glucose originates in glycogen.[1] Alternatively, glucose-6-phosphate can be converted back into glucose in the liver and the kidneys, allowing it to raise blood glucose levels if necessary.[2]
Glucagon in the liver stimulates glycogenolysis when the blood glucose is lowered, known as hypoglycemia.[6] The glycogen in the liver can function as a backup source of glucose between meals.[2] Adrenaline stimulates the breakdown of glycogen in the skeletal muscle during exercise.[6] In the muscles, glycogen ensures a rapidly accessible energy source for movement.[2]
Glycogenesis[edit]
Glycogenesis refers to the process of synthesizing glycogen.[6] In humans, excess glucose is converted to glycogen via this process.[2] Glycogen is a highly branched structure, consisting of glucose, in the form of glucose-6-phosphate, linked together.[2][6] The branching of glycogen increases its solubility, and allows for a higher number of glucose molecules to be accessible for breakdown.[2] Glycogenesis occurs primarily in the liver, skeletal muscles, and kidney.[2]
Pentose phosphate pathway[edit]
The pentose phosphate pathway is an alternative method of oxidizing glucose.[6] It occurs in the liver, adipose tissue, adrenal cortex, testis, milk glands, phagocyte cells, and red blood cells.[6] It produces products that are used in other cell processes, while reducing NADP to NADPH.[6][7] This pathway is regulated through changes in the activity of glucose-6-phosphate dehydrogenase.[7]
Fructose metabolism[edit]
Fructose must undergo certain extra steps in order to enter the glycolysis pathway.[2] Enzymes located in certain tissues can add a phosphate group to fructose.[6] This phosphorylation creates fructose-6-phosphate, an intermediate in the glycolysis pathway that can be broken down directly in those tissues.[6] This pathway occurs in the muscles, adipose tissue, and kidney.[6] In the liver, enzymes produce fructose-1-phosphate, which enters the glycolysis pathway and is later cleaved into glyceraldehyde and dihydroxyacetone phosphate.[2]
Galactose metabolism[edit]
Lactose, or milk sugar, consists of one molecule of glucose and one molecule of galactose.[6] After separation from glucose, galactose travels to the liver for conversion to glucose.[6] Galactokinase uses one molecule of ATP to phosphorylate galactose.[2] The phosphorylated galactose is then converted to glucose-1-phosphate, and then eventually glucose-6-phosphate, which can be broken down in glycolysis.[2]
Energy production[edit]
Many steps of carbohydrate metabolism allow the cells to access energy, and store it more transiently in ATP.[8] The cofactors NAD+ and FAD are sometimes reduced during this process to form NADH and FADH2, which drive the creation of ATP in other processes.[8] A molecule of NADH can produce 1.5–2.5 molecules of ATP, whereas a molecule of FADH2 will yield 1.5 molecules of ATP.[9]
Energy produced during metabolism of one glucose molecule
Pathway
ATP Input
ATP Output
Net ATP
NADH Output
FADH2 Output
ATP Final Yield
Glycolysis (aerobic) 2 4 2 2 0 5-7
Citric Acid Cycle 0 2 2 8 2 17-25
Typically, the complete breakdown of one molecule of glucose via aerobic respiration (i.e. involving both glycolysis and the Citric Acid Cycle) is usually about 30–32 molecules of ATP.[9] Oxidation of one gram of carbohydrate yields approximately 4 kcal of energy.[3]
Hormonal regulation[edit]
Glucoregulation is the maintenance of steady levels of glucose in the body.
Hormones released from the pancreas regulate the overall metabolism of glucose.[10] Insulin and glucagon are the primary hormones involved in maintaining a steady level of glucose in the blood, and the release of each is controlled by the amount of nutrients currently available.[10] The amount of insulin released in the blood and sensitivity of the cells to the insulin both determine the amount of glucose that cells break down.[4] Increased levels of glucagon activates the enzymes that catalyze glycogenolysis, and inhibits the enzymes that catalyze glycogenesis.[8] Conversely, glycogenesis is enhanced and glycogenolysis inhibited when there are high levels of insulin in the blood.[8]
The level of circulatory glucose (known informally as "blood sugar") is the most important factor determining the amount of glucagon or insulin produced. The release of glucagon is precipitated by low levels of blood glucose, whereas high levels of blood glucose stimulates cells to produce insulin. Because the level of circulatory glucose is largely determined by the intake of dietary carbohydrates, diet controls major aspects of metabolism via insulin.[11] In humans, insulin is made by beta cells in the pancreas, fat is stored in adipose tissue cells, and glycogen is both stored and released as needed by liver cells. Regardless of insulin levels, no glucose is released to the blood from internal glycogen stores from muscle cells.
Carbohydrates as storage[edit]
Carbohydrates are typically stored as long polymers of glucose molecules with glycosidic bonds for structural support (e.g. chitin, cellulose) or for energy storage (e.g. glycogen, starch). However, the strong affinity of most carbohydrates for water makes storage of large quantities of carbohydrates inefficient due to the large molecular weight of the solvated water-carbohydrate complex. In most organisms, excess carbohydrates are regularly catabolised to form acetyl-CoA, which is a feed stock for the fatty acid synthesis pathway; fatty acids, triglycerides, and other lipids are commonly used for long-term energy storage. The hydrophobic character of lipids makes them a much more compact form of energy storage than hydrophilic carbohydrates. However, animals, including humans, lack the necessary enzymatic machinery and so do not synthesize glucose from lipids (with a few exceptions, e.g. glycerol).[12]
In some animals (such as termites[13]) and some microorganisms (such as protists and bacteria), cellulose can be disassembled during digestion and absorbed as glucose.[14]
Human diseases[edit]
Diabetes mellitus
Lactose intolerance
Fructose malabsorption
Galactosemia
Glycogen storage disease
References[edit]
^ Jump up to: a b c d e f g h i j Maughan, Ron (2009). "Carbohydrate metabolism". Surgery (Oxford). 27 (1): 6–10. doi:10.1016/j.mpsur.2008.12.002.
^ Jump up to: a b c d e f g h i j k l m n o p q r s t 1942-, Nelson, David L. (David Lee), (2013). Lehninger principles of biochemistry. Cox, Michael M.,, Lehninger, Albert L. (6th ed.). New York: W.H. Freeman and Company. ISBN 1429234148. OCLC 824794893.
^ Jump up to: a b c Sanders, L.M. Carbohydrate: Digestion, Absorption and Metabolism. pp. 643–50. doi:10.1016/b978-0-12-384947-2.00114-8.
^ Jump up to: a b c d Hall, John E. (2015). Guyton and Hall Textbook of Medical Physiology E-Book (13 ed.). Elsevier Health Sciences. ISBN 0323389309.
Jump up ^ Hansen, R. Gaurth; Gitzelmann, Richard (1975-06-01). Physiological Effects of Food Carbohydrates. ACS Symposium Series. 15. American Chemical Society. pp. 100–22. doi:10.1021/bk-1975-0015.ch006. ISBN 084120246X.
^ Jump up to: a b c d e f g h i j k l m n o p q r s t u Dashty, Monireh (2013). "A quick look at biochemistry: Carbohydrate metabolism". Clinical Biochemistry. 46 (15): 1339–52. doi:10.1016/j.clinbiochem.2013.04.027.
^ Jump up to: a b Ramos-Martinez, Juan Ignacio (2017-01-15). "The regulation of the pentose phosphate pathway: Remember Krebs". Archives of Biochemistry and Biophysics. 614: 50–52. doi:10.1016/j.abb.2016.12.012. ISSN 0003-9861.
^ Jump up to: a b c d Ahern, Kevin; Rajagopal, Indira; Tan, Taralyn (2017). Biochemistry Free for All. Oregon State University.
^ Jump up to: a b Energetics of Cellular Respiration (Glucose Metabolism)
^ Jump up to: a b Lebovitz, Harold E. Hyperglycemia Secondary to Nondiabetic Conditions and Therapies. pp. 737–51. doi:10.1016/b978-0-323-18907-1.00042-1.
Jump up ^ Brockman, R P (March 1978). "Roles of glucagon and insulin in the regulation of metabolism in ruminants. A review". The Canadian Veterinary Journal. 19 (3): 55–62. ISSN 0008-5286. PMC 1789349  . PMID 647618.
Jump up ^ G Cooper, The Cell, American Society of Microbiology, p. 72
Jump up ^ Watanabe, Hirofumi; Hiroaki Noda; Gaku Tokuda; Nathan Lo (23 July 1998). "A cellulase gene of termite origin". Nature. 394 (6691): 330–31. doi:10.1038/28527. PMID 9690469. Retrieved 16 May 2017.
Jump up ^ Coleman, Geoffrey (8 February 1978). "The Metabolism of Cellulose, Glucose, and Starch by the Rumen Ciliate Protozoon Eudiplodinium Magii" (PDF). Journal of General Microbiology. 107: 359–66. doi:10.1099/00221287-107-2-359.
External links[edit]
Carbohydrate metabolism at the US National Library of Medicine Medical Subject Headings (MeSH)
BBC - GCSE Bitesize - Biology | Humans | Glucoregulation
Sugar4Kids
[show]
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t
e
Metabolism, catabolism, anabolism
General
Metabolic pathway
Metabolic network
Primary nutritional groups
Energy
metabolism
Aerobic respiration
Glycolysis → Pyruvate decarboxylation → Citric acid cycle → Oxidative phosphorylation (electron transport chain + ATP synthase)
Anaerobic respiration
Electron acceptors are other than oxygen
Fermentation
Glycolysis →
Substrate-level phosphorylation
ABE
Ethanol
Lactic acid
Specific
paths
Protein metabolism
Protein synthesis
Catabolism
Carbohydrate metabolism
(carbohydrate catabolism
and anabolism)
Human
Glycolysis ⇄ Gluconeogenesis
Glycogenolysis ⇄ Glycogenesis
Pentose phosphate pathway
Fructolysis
Galactolysis
Glycosylation
N-linked
O-linked
Nonhuman
Photosynthesis
Anoxygenic photosynthesis
Chemosynthesis
Carbon fixation
Xylose metabolism
Radiotrophism
Lipid metabolism
(lipolysis, lipogenesis)
Fatty acid metabolism
Fatty acid degradation (Beta oxidation)
Fatty acid synthesis
Other
Steroid metabolism
Sphingolipid metabolism
Eicosanoid metabolism
Ketosis
Reverse cholesterol transport
Amino acid
Amino acid synthesis
Urea cycle
Nucleotide
metabolism
Purine metabolism
Nucleotide salvage
Pyrimidine metabolism
Other
Metal metabolism
Iron metabolism
Ethanol metabolism
[show]
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Metabolism map
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Pentose
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Citric
Acid Cycle
Glyoxylate
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Urea
Cycle
Fatty
Acid
Synthesis
Fatty
Acid
Elongation
Beta
Oxidation
Peroxisomal
Beta
Oxidation
Glyco-
genolysis
Glyco-
genesis
Glyco-
lysis
Gluconeo-
genesis
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oxylation
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Keto-
lysis
Keto-
genesis
feeders to
Gluconeo-
genesis
Direct / C4 / CAM
Carbon Intake
Light Reaction
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Deamination
Citrate
Shuttle
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-CoA
Succinate
Acetyl
-CoA
Pentose-P
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Glyoxylate
Photosystems
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-CoA
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Amino Acids
Shikimate
Aromatic Amino
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Ascorbate
(Vitamin C)
δ-ALA
Bile
Pigments
Hemes
Cobalamins (Vitamin B12)
Various
Vitamin B's
Calciferols
(Vitamin D)
Retinoids
(Vitamin A)
Quinones (Vitamin K)
& Carotenoids (Vitamin E)
Cofactors
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& Minerals
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Major metabolic pathways in metro-style map. Click any text (name of pathway or metabolites) to link to the corresponding article.
Single lines: pathways common to most lifeforms. Double lines: pathways not in humans (occurs in e.g. plants, fungi, prokaryotes). Orange nodes: carbohydrate metabolism. Violet nodes: photosynthesis. Red nodes: cellular respiration. Pink nodes: cell signaling. Blue nodes: amino acid metabolism. Grey nodes: vitamin and cofactor metabolism. Brown nodes: nucleotide and protein metabolism. Green nodes: lipid metabolism.
[show]
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Metabolism: carbohydrate metabolism, glycogenesis and glycogenolysis enzymes
Glycogenesis
Phosphoglucomutase
UDP-glucose pyrophosphorylase
Glycogen synthase
Glycogen branching enzyme
Glycogenin
Glycogenolysis
extralysosomal:
Glycogen phosphorylase
Debranching enzyme
Phosphoglucomutase
lysosomal:
Alpha-glucosidase (Acid)
Regulation
Phosphorylase kinase
Protein phosphatase
[show]
v
t
e
Metabolism: carbohydrate metabolism: glycolysis/gluconeogenesis enzymes
Glycolysis
Hexokinase (HK1, HK2, HK3, Glucokinase)→/Glucose 6-phosphatase←
Glucose isomerase
Phosphofructokinase 1 (Liver, Muscle, Platelet)→/Fructose 1,6-bisphosphatase←
Fructose-bisphosphate aldolase (Aldolase A, B, C)
Triosephosphate isomerase
Glyceraldehyde 3-phosphate dehydrogenase
Phosphoglycerate kinase
Phosphoglycerate mutase
Enolase
Pyruvate kinase (PKLR, PKM2)
Gluconeogenesis only
to oxaloacetate:
Pyruvate carboxylase
Phosphoenolpyruvate carboxykinase
from lactate (Cori cycle):
Lactate dehydrogenase
from alanine (Alanine cycle):
Alanine transaminase
from glycerol:
Glycerol kinase
Glycerol dehydrogenase
Regulatory
Fructose 6-P,2-kinase:fructose 2,6-bisphosphatase
PFKFB1, PFKFB2, PFKFB3, PFKFB4
Bisphosphoglycerate mutase
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Metabolism: carbohydrate metabolism
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Triokinase
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Sorbitol dehydrogenase
Aldose reductase
Galactose / Galactolysis
Galactokinase
Galactose-1-phosphate uridylyltransferase/UDP-glucose 4-epimerase
Aldose reductase
Lactose
Lactose synthase
Lactase
Mannose
Mannose phosphate isomerase
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Metabolism: carbohydrate metabolism
proteoglycan enzymes
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Sulfotransferase
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EXT1
EXT2
Chondroitin sulfate
PAPSS1
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glycosaminoglycan catabolism
Hunter, Hurler
Iduronate-2-sulfatase
Iduronidase
Sanfilippo, Sly
Heparan sulfamidase
N-acetyltransferase
Alpha-N-acetylglucosaminidase
Glucuronidase
N-acetylglucosamine-6-sulfatase
Morquio/Maroteaux-Lamy
Arylsulfatase B
Galactosamine-6 sulfatase
Beta-galactosidase (GLB1)
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Metabolism: carbohydrate metabolism · glycoprotein enzymes
Anabolism
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mannosidase
alpha-Mannosidase
beta-mannosidase
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X
From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article is about the letter. For the multiplication sign, see Multiplication sign. For other uses, see X (disambiguation).
ISO basic
Latin alphabet
Aa Bb Cc Dd
Ee Ff Gg Hh
Ii Jj Kk Ll
Mm Nn Oo Pp
Qq Rr Ss Tt
Uu Vv Ww Xx
Yy Zz
v
t
e
Writing cursive forms of X
X (named ex /ɛks/, plural exes[1]) is the 24th and antepenultimate letter in the modern English alphabet and the ISO basic Latin alphabet.
Contents
[hide]
1 History
2 Use in writing systems
2.1 English
2.2 Other languages
2.3 Other systems
3 Other uses
4 Related characters
4.1 Descendants and related characters in the Latin alphabet
4.2 Ancestors and siblings in other alphabets
5 Computing codes
6 Other representations
7 See also
8 References
9 External links
History[edit]
Greek Chi
Etruscan
:X
In Ancient Greek, 'Χ' and 'Ψ' were among several variants of the same letter, used originally for /kʰ/ and later, in western areas such as Arcadia, as a simplification of the digraph 'ΧΣ' for /ks/. In the end, more conservative eastern forms became the standard of Classical Greek, and thus 'Χ' (Chi) stands for /kʰ/ (later /x/). However, the Etruscans had taken over 'Χ' from western Greek, and it therefore stands for /ks/ in Etruscan and Latin.[citation needed]
The letter 'Χ' ~ 'Ψ' for /kʰ/ was a Greek addition to the alphabet, placed after the Semitic letters along with phi 'Φ' for /pʰ/. (The variant 'Ψ' later replaced the digraph 'ΦΣ' for /ps/; omega was a later addition).[citation needed]
Use in writing systems[edit]
English[edit]
In English orthography, ⟨x⟩ is typically pronounced as the voiceless consonant cluster /ks/ when it follows the stressed vowel (e.g. ox), and the voiced consonant /ɡz/ when it precedes the stressed vowel (e.g. exam). It is also pronounced /ɡz/ when it precedes a silent ⟨h⟩ and a stressed vowel (e.g. exhaust).[2] Before ⟨i⟩ or ⟨u⟩, it can be pronounced /kʃ/ or /ɡʒ/ (e.g. sexual and luxury); these result from earlier /ksj/ and /ɡzj/. It also makes the sound /kʃ/ in words ending in -xion (typically used only in British-based spellings of the language; American spellings tend to use -ction). When ⟨x⟩ ends a word, it is always /ks/ (e.g. ax), except in loan words such as faux (see French, below).
There are very few English words that start with ⟨x⟩ (the fewest number of any letter). When ⟨x⟩ does start a word, it is usually pronounced /z/ (e.g. xylophone, xenophobia, and xanthan); in rare recent loanwords or foreign proper names, it can also be pronounced /s/ (e.g. the obsolete Vietnamese monetary unit xu) or /ʃ/ (e.g. Chinese names starting with Xi like Xiaomi or Xinjiang). Many of the words that start with ⟨x⟩ are of Greek origin, or standardized trademarks (Xerox) or acronyms (XC). In abbreviations, it can represent "trans-" (e.g. XMIT for transmit, XFER for transfer), "cross-" (e.g. X-ing for crossing, XREF for cross-reference), "Christ-" as shorthand for the labarum (e.g. Xmas for Christmas, Xian for Christian), the "crys-" in crystal (XTAL), or various words starting with "ex-" (e.g. XL for extra large, XOR for exclusive-or).
X is the third least frequently used letter in English (after ⟨q⟩ and ⟨z⟩), with a frequency of about 0.15% in words.[3]
Other languages[edit]
In Latin, ⟨x⟩ stood for [ks]. In some languages, as a result of assorted phonetic changes, handwriting adaptations or simply spelling convention, ⟨x⟩ has other pronunciations:
Basque: as a spelling for[ʃ]. Additionally there is the digraph ⟨tx⟩ [tʃ].
Dutch: ⟨x⟩ usually represents [ks], except in the name of the island of Texel, which is pronounced Tessel. This is because of historical sound-changes in Dutch, where all /ks/ sounds have been replaced by /s/ sounds. Words with an ⟨x⟩ in the Dutch language are nowadays usually loanwords. In the Dutch-speaking part of Belgium, family names with ⟨x⟩ are not uncommon (e.g. Dierckx, Hendrickx, Koninckx, Sterckx, Vranckx).
In Norwegian, ⟨x⟩ is generally pronounced [ks], but since the 19th century, there has been a tendency to spell it out as 'ks'; it may still be retained in personal names, though it is fairly rare, and occurs mostly in foreign words and SMS language. Usage in Danish, German and Finnish is similar.
French: at the ends of words, silent (or [z] in liaison if the next word starts with a vowel). Three exceptions are pronounced [s]: six ("six"), dix ("ten") and in some city names such as Bruxelles (although some people still pronounce it 'ks') or Auxerre; it is fully pronounced [ks] in Aix-la-Chapelle. It is pronounced [z] in sixième and dixième.
In Italian, ⟨x⟩ is either pronounced [ks], as in extra, uxorio, xilofono,[4] or [ɡz], as exogamia, when it is preceded by ⟨e⟩ and followed by a vowel. In several related languages, notably Venetian, it represents the voiced sibilant [z]. It is also used, mainly amongst the young people, as a short written form for "per", meaning "for": for example, "x sempre" ("forever"). This is because in Italian the multiplication sign (similar to ⟨x⟩) is called "per". However, ⟨x⟩ is found only in loanwords, as it is not part of the standard Italian alphabet; in most words with ⟨x⟩, this letter may be replaced with 's' or 'ss' (with different pronunciation: xilofono/silofono, taxi/tassì) or, rarely, by 'cs' (with the same pronunciation: claxon/clacson).
In Old Spanish, ⟨x⟩ was pronounced [ʃ], as it is still currently in other Iberian Romance languages. Later, the sound evolved to a hard [x] sound. In modern Spanish, due to a spelling reform, whenever ⟨x⟩ is used for the [x] sound it has been replaced with ⟨j⟩, including in words that originally had ⟨x⟩ such as ejemplo or ejercicio, though ⟨x⟩ is still retained for some names (notably 'México', even though 'Méjico' may sometimes be used in Spain). Presently, ⟨x⟩ represents the sound [s] (word-initially), or the consonant cluster [ks] (e.g. oxígeno, examen). Rarely, it can be pronounced [ʃ] as in Old Spanish in some proper nouns such as 'Raxel' (a variant of Rachel) and Uxmal.
In Galician and Leonese, ⟨x⟩ is pronounced [ʃ] in most cases. In learned words, such as 'taxativo' (taxing), it is pronounced [ks]. However, Galician speakers tend to pronounce it [s], especially when it appears before plosives, such as in 'externo' (external).
In Catalan, ⟨x⟩ has three sounds; the most common is [ʃ]; as in 'xarop' (syrup). Other sounds are: [ks]; 'fixar' (to fix), [ɡz]; 'examen'. In addition, [ʃ] gets voiced to [ʒ] before voiced consonants; 'caixmir'. Catalan also has the digraph ⟨tx⟩, pronounced [tʃ].
In Portuguese, ⟨x⟩ has four main sounds; the most common is [ʃ], as in 'xícara' (cup). The other sounds are: [ks] as in 'flexão' (flexion); [s], when preceded by E and followed by a consonant, as in 'contexto' ([ʃ] in European Portuguese), and in a small number of other words, such as 'próximo' (close/next); and (the rarest) [z], which occurs in the prefix 'ex-' before a vowel, as in 'exagerado' (exaggerated). A rare fifth sound is [ɡz], coexisting with [z] and [ks] as acceptable pronunciations in exantema and in words with the Greek prefix 'hexa-'.
In Venetian, it represents the voiced alveolar sibilant [z] much like in Portuguese 'exagerado', English 'xylophone' or in the French 'sixième'. Examples from medieval texts include raxon (reason), prexon (prison), dexerto (desert), chaxa/caxa (home). Nowadays, the best-known word is xe (is/are). The most notable exception to this rule is the name Venexia [veˈnɛsja] in which ⟨x⟩ has evolved from the initial voiced sibilant [z] to the present day voiceless sibilant.
In Albanian, ⟨x⟩ represents [dz], while the digraph ⟨xh⟩ represents [dʒ].
In Maltese, ⟨x⟩ is pronounced [ʃ] or, in some cases, [ʒ] (only in loanwords such as 'televixin', and not for all speakers).
In Polish, ⟨x⟩ was used prior to 19th century both in loanwords and native words and was pronounced [ks] or [gz], e.g. xiążę, xięstwo (now książę, księstwo). Later was replaced by ⟨ks⟩ and ⟨gz⟩ in all words and remained only in surnames as Axentowicz, Jaxa, Koxowski, Mixtacki, Rexemowski, Xiężopolski, names as Xawery, Xymena and abbreviations.
Additionally, in languages for which the Latin alphabet has been adapted only recently, ⟨x⟩ has been used for various sounds, in some cases inspired by European usage, but in others, for consonants uncommon in Europe. For these no Latin letter stands out as an obvious choice, and since most of the various European pronunciations of ⟨x⟩ can be written by other means, the letter becomes available for more unusual sounds.
⟨x⟩ represents [x] (voiceless velar fricative) or [ꭓ] (voiceless uvular fricative) in e.g. Azerbaijani, Kurdish, Lojban, Tatar, Uzbek, Pashto and Uyghur (Latin script).
Esperanto: The x-convention replaces ⟨ĉ⟩, ⟨ĝ⟩, ⟨ĥ⟩, ⟨ĵ⟩, ⟨ŝ⟩, and ⟨ŭ⟩ with x-suffixes: ⟨cx⟩, ⟨gx⟩, ⟨hx⟩, ⟨jx⟩, ⟨sx⟩, and ⟨ux⟩.
In transliteration of Indian languages, ⟨x⟩ represents the consonant cluster [kʃ] in alternate spellings of words containing 'क्ष' (kṣ), especially names such as Laxmi and Dixit. Less frequently, ⟨x⟩ is used to represent 'ख़' [x].
In Apache ⟨x⟩ represents [x]
In Nahuatl, ⟨x⟩ represents [ʃ].
In Nguni languages, ⟨x⟩ represents the alveolar lateral click [ǁ].
In Pirahã, ⟨x⟩ symbolizes the glottal stop [ʔ].
An illustrating example of "x" as a "leftover" letter is differing usage in three different Cushitic languages:
Afar: voiced alveolar implosive [ɗ]
Oromo: alveolar ejective [tʼ]
Somali: voiceless pharyngeal fricative [ħ]
In Southeast Asia:
In Lao, based on romanization of Lao consonants, ⟨x⟩ may represent [ɕ], e.g. in Lan Xang.
In Vietnamese, ⟨x⟩ is pronounced like English ⟨s⟩ (at the beginning of a word, e.g. "sing"). This sound was [ɕ] in Middle Vietnamese, resembling the Portuguese sound /ʃ/, spelled ⟨x⟩.
In Hanyu Pinyin, Standard Chinese's official transcription system in China, Malaysia, Singapore, and Taiwan, the letter ⟨x⟩ represents the voiceless alveolo-palatal fricative /ɕ/, for instance in 'Xi', [ɕi]. This sound somewhat resembles [ʃ].
Other systems[edit]
In the International Phonetic Alphabet, ⟨x⟩ represents a voiceless velar fricative.
Other uses[edit]
In mathematics, x is commonly used as the name for an independent variable or unknown value. The modern tradition of using x to represent an unknown was introduced by René Descartes in La Géométrie (1637).[5] As a result of its use in algebra, X is often used to represent unknowns in other circumstances (e.g. X-rays, Generation X, The X-Files, and The Man from Planet X; see also Malcolm X).
In the Cartesian coordinate system, x is used to refer to the horizontal axis.
It may also be used as a typographic approximation for the multiplication sign. In mathematical typesetting, x meaning an algebraic variable is normally in italic type ( x {\displaystyle x\!} ), partly to avoid confusion with the multiplication symbol. In fonts containing both x (the letter) and × (the multiplication sign), the two glyphs are dissimilar.
It can be used as an abbreviation for 'between' in the context of historical dating; e.g., '1483 x 1485'.
Maps and other images sometimes use an X to label a specific location, leading to the expression "X marks the spot".[6]
The Roman numeral Ⅹ represents the number 10.[7][8]
In art or fashion, the use of X indicates a collaboration by two or more artists, e.g. Aaron Koblin x Takashi Kawashima. This application, which originated in Japan, now extends to other kinds of collaboration outside the art world.[9]
Related characters[edit]
Descendants and related characters in the Latin alphabet[edit]
X with diacritics: Ẍ ẍ Ẋ ẋ ᶍ[10]
IPA-specific symbols related to X: χ
Teuthonista phonetic transcription-specific symbols related to X:[11]
U+AB56 ꭖ LATIN SMALL LETTER X WITH LOW RIGHT RING
U+AB57 ꭗ LATIN SMALL LETTER X WITH LONG LEFT LEG
U+AB58 ꭘ LATIN SMALL LETTER X WITH LONG LEFT LEG AND LOW RIGHT RING
U+AB59 ꭙ LATIN SMALL LETTER X WITH LONG LEFT LEG WITH SERIF
ˣ : Modifier letter small x is used for phonetic transcription
ₓ : Subscript small x is used in Indo-European studies[12]
Ancestors and siblings in other alphabets[edit]
Χ χ : Greek letter Chi, from which the following derive:
Х х : Cyrillic letter Kha
Ⲭ ⲭ : Coptic letter Khe, which derives from Greek Chi
𐍇 : Gothic letter enguz, which derives from Greek Chi
𐌗 : Old Italic X, which derives from Greek Chi, and is the ancestor of modern Latin X
ᚷ : Runic letter Gyfu, which may derive from old Italic X
Ξ ξ : Greek letter Xi, which was used in place of Chi in the Eastern (and the modern) Greek alphabets
Computing codes[edit]
Character
X
x
Unicode name LATIN CAPITAL LETTER X LATIN SMALL LETTER X
Encodings
decimal
hex
decimal
hex
Unicode 88 U+0058 120 U+0078
UTF-8 88 58 120 78
Numeric character reference X X x x
EBCDIC family 231 E7 167 A7
ASCII 1 88 58 120 78
1 Also for encodings based on ASCII, including the DOS, Windows, ISO-8859 and Macintosh families of encodings.
In the C programming language, 'x' preceded by zero (0x or 0X) is used to denote hexadecimal literal values.
Other representations[edit]
NATO phonetic Morse code
X-ray –··–
Signal flag Flag semaphore American manual alphabet (ASL fingerspelling) Braille
dots-1346
See also[edit]
XX
XXX
XXXX
X mark
Commonly used as a prefix term in nouns related to the X Window System and Unix
References[edit]
Jump up ^ "X", Oxford English Dictionary, 2nd edition (1989); Merriam-Webster's Third New International Dictionary of the English Language, Unabridged (1993); "ex", op. cit.
Jump up ^ Venezky, Richard (1 January 1970). The Structure of English Orthography. Walter de Gruyter. p. 40. ISBN 978-3-11-080447-8.
Jump up ^ Mička, Pavel. "Letter frequency (English)". Algoritmy.net. Retrieved 9 February 2014.
Jump up ^ "Dizionario di ortografia e pronunzia" [Dictionary of Spelling and Pronunciation]. Dizionario di ortografia e pronunzia (in Italian). Retrieved 9 February 2014.
Jump up ^ Cajori, Florian (1928). A History of Mathematical Notations. Chicago: Open Court Publishing. p. 381. See History of algebra.
Jump up ^ "X marks the spot". Retrieved 21 June 2016.
Jump up ^ Gordon, Arthur E. (1983). Illustrated Introduction to Latin Epigraphy. University of California Press. p. 44. Retrieved 3 October 2015.
Jump up ^ King, David A. (2001). The Ciphers of the Monks. p. 282. In the course of time, I, V and X became identical with three letters of the alphabet; originally, however, they bore no relation to these letters.
Jump up ^ http://arkitip.com/product/x-mark-of-collaboration-issue-no-0053x/
Jump up ^ Constable, Peter (2004-04-19). "L2/04-132 Proposal to add additional phonetic characters to the UCS" (PDF).
Jump up ^ Everson, Michael; Dicklberger, Alois; Pentzlin, Karl; Wandl-Vogt, Eveline (2011-06-02). "L2/11-202: Revised proposal to encode "Teuthonista" phonetic characters in the UCS" (PDF).
Jump up ^ Anderson, Deborah; Everson, Michael (2004-06-07). "L2/04-191: Proposal to encode six Indo-Europeanist phonetic characters in the UCS" (PDF).
External links[edit]
Media related to X at Wikimedia Commons
The dictionary definition of X at Wiktionary
The dictionary definition of x at Wiktionary
"X". The American Cyclopædia. 1879.
[hide]
v
t
e
Latin script
History
Spread
Romanization
Roman numerals
Alphabets (list)
Classical Latin alphabet
ISO basic Latin alphabet
phonetic alphabets
International Phonetic Alphabet
X-SAMPA
Spelling alphabet
Letters (list)
Letters of the ISO basic Latin alphabet
Aa Bb Cc Dd Ee Ff Gg Hh Ii Jj Kk Ll Mm Nn Oo Pp Qq Rr Ss Tt Uu Vv Ww Xx Yy Zz
Letter X with diacritics
X́x́ X̂x̂ Ẍẍ X̌x̌ Ẋẋ X̧x̧ X̱x̱ X̣x̣ ᶍ
Multigraphs
Digraphs
ch
cz
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ISO/IEC 27002
From Wikipedia, the free encyclopedia
Jump to: navigation, search
ISO/IEC 27002 is an information security standard published by the International Organization for Standardization (ISO) and by the International Electrotechnical Commission (IEC), titled Information technology – Security techniques – Code of practice for information security management.
This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2013) (Learn how and when to remove this template message)
The ISO/IEC 27000-series standards are descended from a corporate security standard donated by Shell to a UK government initiative in the early 1990s[1] . The Shell standard was developed into British Standard BS 7799 in the mid-1990s, and was adopted as ISO/IEC 17799 in 2000. The ISO/IEC standard was revised in 2005, and renumbered ISO/IEC 27002 in 2007 to align with the other ISO/IEC 27000-series standards. It was revised again in 2013.
ISO/IEC 27002 provides best practice recommendations on information security management for use by those responsible for initiating, implementing or maintaining information security management systems (ISMS). Information security is defined within the standard in the context of the C-I-A triad:
the preservation of confidentiality (ensuring that information is accessible only to those authorized to have access), integrity (safeguarding the accuracy and completeness of information and processing methods) and availability (ensuring that authorized users have access to information and associated assets when required).[2]
Contents
[hide]
1 Outline
1.1 Outline for ISO/IEC 27002:2013
2 Implementation example of ISO/IEC 27002
2.1 Physical and Environmental security
2.2 Human Resource security
2.3 Access control
3 National Equivalent Standards
4 Certification
5 Ongoing development
6 See also
7 References
8 External links
Outline[edit]
Outline for ISO/IEC 27002:2013[edit]
The standard starts with 5 introductory chapters:
0. Introduction
1. Scope
2. Normative references
3. Terms and definitions
4. Structure of this standard
These are followed by 14 main chapters:
5. Information Security Policies
6. Organization of Information Security
7. Human Resource Security
8. Asset Management
9. Access Control
10. Cryptography
11. Physical and environmental security
12. Operation Security- procedures and responsibilities, Protection from malware, Backup, Logging and monitoring, Control of operational software, Technical vulnerability management and Information systems audit coordination
13. Communication security - Network security management and Information transfer
14. System acquisition, development and maintenance - Security requirements of information systems, Security in development and support processes and Test data
15. Supplier relationships - Information security in supplier relationships and Supplier service delivery management
16. Information security incident management - Management of information security incidents and improvements
17. Information security aspects of business continuity management - Information security continuity and Redundancies
18. Compliance - Compliance with legal and contractual requirements and Information security reviews
Within each chapter, information security controls and their objectives are specified and outlined. The information security controls are generally regarded as best practice means of achieving those objectives. For each of the controls, implementation guidance is provided.
Specific controls are not mandated since:
Each organization is expected to undertake a structured information security risk assessment process to determine its specific requirements before selecting controls that are appropriate to its particular circumstances. The introduction section outlines a risk assessment process although there are more specific standards covering this area such as ISO/IEC 27005. The use of information security risk analysis to drive the selection and implementation of information security controls is an important feature of the ISO/IEC 27000-series standards: it means that the generic good practice advice in this standard gets tailored to the specific context of each user organization, rather than being applied by rote. Not all of the 39 control objectives are necessarily relevant to every organization for instance, hence entire categories of control may not be deemed necessary. The standards are also open ended in the sense that the information security controls are 'suggested', leaving the door open for users to adopt alternative controls if they wish, just so long as the key control objectives relating to the mitigation of information security risks, are satisfied. This helps keep the standard relevant despite the evolving nature of information security threats, vulnerabilities and impacts, and trends in the use of certain information security controls.
It is practically impossible to list all conceivable controls in a general purpose standard. Industry-specific implementation guidelines for ISO/IEC 27001:2013 and ISO/IEC 27002 offer advice tailored to organizations in the telecomms industry (see ISO/IEC 27011) and healthcare (see ISO 27799).
Most organizations implement a wide range of information security-related controls, many of which are recommended in general terms by ISO/IEC 27002. Structuring the information security controls infrastructure in accordance with ISO/IEC 27002 may be advantageous since it:
Is associated with a well-respected international standard
Helps avoid coverage gaps and overlaps
Is likely to be recognized by those who are familiar with the ISO/IEC standard
Implementation example of ISO/IEC 27002[edit]
Here are a few examples of typical information security policies and other controls relating to three parts of ISO/IEC 27002. (Note: this is merely an illustration. The list of example controls is incomplete and not universally applicable.)
Physical and Environmental security[edit]
Physical access to premises and support infrastructure (communications, power, air conditioning etc.) must be monitored and restricted to prevent, detect and minimize the effects of unauthorized and inappropriate access, tampering, vandalism, criminal damage, theft etc.
The list of people authorized to access secure areas must be reviewed and approved periodically (at least once a year) by Administration or Physical Security Department, and cross-checked by their departmental managers.
Photography or video recording is forbidden inside Restricted Areas without prior permission from the designated authority.
Suitable video surveillance cameras must be located at all entrances and exits to the premises and other strategic points such as Restricted Areas, recorded and stored for at least one month, and monitored around the clock by trained personnel.
Access cards permitting time-limited access to general and/or specific areas may be provided to trainees, vendors, consultants, third parties and other personnel who have been identified, authenticated, and authorized to access those areas.
Other than in public areas such as the reception foyer, and private areas such as rest rooms, visitors should be escorted at all times by an employee while on the premises.
The date and time of entry and departure of visitors along with the purpose of visits must be recorded in a register maintained and controlled by Site Security or Reception.
Everyone on site (employees and visitors) must wear and display their valid, issued pass at all times, and must present their pass for inspection on request by a manager, security guard or concerned employee.
Access control systems must themselves be adequately secured against unauthorized/inappropriate access and other compromises.
Fire/evacuation drills must be conducted periodically (at least once a year).
Smoking is forbidden inside the premises other than in designated Smoking Zones.
Human Resource security[edit]
All employees must be screened prior to employment, including identity verification using a passport or similar photo ID and at least two satisfactory professional references. Additional checks are required for employees taking up trusted positions.
All employees must formally accept a binding confidentiality or non-disclosure agreement concerning personal and proprietary information provided to or generated by them in the course of employment.
Human Resources department must inform Administration, Finance and Operations when an employee is taken on, transferred, resigns, is suspended or released on long-term leave, or their employment is terminated.
Upon receiving notification from HR that an employee's status has changed, Administration must update their physical access rights and IT Security Administration must update their logical access rights accordingly.
An employee's manager must ensure that all access cards, keys, IT equipment, storage media and other valuable corporate assets are returned by the employee on or before their last day of employment.
Access control[edit]
User access to corporate IT systems, networks, applications and information must be controlled in accordance with access requirements specified by the relevant Information Asset Owners, normally according to the user's role.
Generic or test IDs must not be created or enabled on production systems unless specifically authorized by the relevant Information Asset Owners.
After a predefined number of unsuccessful logon attempts, security log entries and (where appropriate) security alerts must be generated and user accounts must be locked out as required by the relevant Information Asset Owners.
Passwords or pass phrases must be lengthy and complex, consisting of a mix of letters, numerals and special characters that would be difficult to guess.
Passwords or pass phrases must not be written down or stored in readable format.
Authentication information such as passwords, security logs, security configurations and so forth must be adequately secured against unauthorized or inappropriate access, modification, corruption or loss.
Privileged access rights typically required to administer, configure, manage, secure and monitor IT systems must be reviewed periodically (at least twice a year) by Information Security and cross-checked by the appropriate departmental managers.
Users must either log off or password-lock their sessions before leaving them unattended.
Password-protected screensavers with an inactivity timeout of no more than 10 minutes must be enabled on all workstations/PCs.
Write access to removable media (USB drives, CD/DVD writers etc.) must be disabled on all desktops unless specifically authorized for legitimate business reasons.
National Equivalent Standards[edit]
ISO/IEC 27002 has directly equivalent national standards in several countries. Translation and local publication often results in several months' delay after the main ISO/IEC standard is revised and released, but the national standard bodies go to great lengths to ensure that the translated content accurately and completely reflects ISO/IEC 27002.
Countries
Equivalent Standard
Australia
New Zealand
AS/NZS ISO/IEC 27002:2006
Brazil ISO/IEC NBR 17799/2007 – 27002
Indonesia SNI ISO/IEC 27002:2014
Chile NCH2777 ISO/IEC 17799/2000
China GB/T 22081-2008
Czech Republic ČSN ISO/IEC 27002:2006
Croatia HRN ISO/IEC 27002:2013
Denmark DS/ISO27002:2014 (DK)
Estonia EVS-ISO/IEC 17799:2003, 2005 version in translation
Germany DIN ISO/IEC 27002:2008
Japan JIS Q 27002
Lithuania LST ISO/IEC 27002:2009 (adopted ISO/IEC 27002:2005, ISO/IEC 17799:2005)
Netherlands NEN-ISO/IEC 27002:2013
Peru NTP-ISO/IEC 17799:2007
Poland PN-ISO/IEC 17799:2007, based on ISO/IEC 17799:2005
Russia ГОСТ/Р ИСО МЭК 17799-2005
Slovakia STN ISO/IEC 27002:2006
South Africa SANS 27002:2014/ISO/IEC 27002:2013[3]
Spain UNE 71501
Sweden SS-ISO/IEC 27002:2014
Turkey TS ISO/IEC 27002
Thailand UNIT/ISO
Ukraine СОУ Н НБУ 65.1 СУІБ 2.0:2010
United Kingdom BS ISO/IEC 27002:2005
Uruguay UNIT/ISO 17799:2005
Certification[edit]
ISO/IEC 27002 is an advisory standard that is meant to be interpreted and applied to all types and sizes of organization according to the particular information security risks they face. In practice, this flexibility gives users a lot of latitude to adopt the information security controls that make sense to them, but makes it unsuitable for the relatively straightforward compliance testing implicit in most formal certification schemes.
ISO/IEC 27001:2013 (Information technology – Security techniques – Information security management systems – Requirements) is a widely recognized certifiable standard. ISO/IEC 27001 specifies a number of firm requirements for establishing, implementing, maintaining and improving an ISMS, and in Annex A there is a suite of information security controls that organizations are encouraged to adopt where appropriate within their ISMS. The controls in Annex A are derived from and aligned with ISO/IEC 27002.
Ongoing development[edit]
Both ISO/IEC 27001:2013 and ISO/IEC 27002 are revised by ISO/IEC JTC1/SC27 every few years in order to keep them current and relevant. Revision involves, for instance, incorporating references to other issued security standards (such as ISO/IEC 27000, ISO/IEC 27004 and ISO/IEC 27005) and various good security practices that have emerged in the field since they were last published. Due to the significant 'installed base' of organizations already using ISO/IEC 27002, particularly in relation to the information security controls supporting an ISMS that complies with ISO/IEC 27001, any changes have to be justified and, wherever possible, evolutionary rather than revolutionary in nature.
See also[edit]
BS 7799, the original British Standard from which ISO/IEC 17799 and then ISO/IEC 27002 was derived
ISO/IEC 27000-series
IT baseline protection
IT risk management
List of ISO standards
Sarbanes–Oxley Act
Standard of Good Practice published by the Information Security Forum
ISO/IEC JTC 1/SC 27 – IT Security techniques
NIST Cybersecurity Framework
Cyber Risk Quantification
References[edit]
Jump up ^ "ISO27k timeline". ISO27001security.com. IsecT Ltd. Retrieved 9 March 2016.
Jump up ^ "ISC CISSP Official Study Guide". SYBEX. ISBN 978-1119042716. Retrieved 1 November 2016.
Jump up ^ "SANS 27002:2014 (Ed. 2.00)". SABS Web Store. Retrieved 25 May 2015.
External links[edit]
The ISO 17799 Newsletter
ISO/IEC 27002:2013
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