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is a
disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose ...
found in animal milk. It consists of a molecule of
D-galactose Galactose (, ''wikt:galacto-, galacto-'' + ''wikt:-ose#Suffix 2, -ose'', "milk sugar") sometimes abbreviated Gal, is a monosaccharide Carbohydrate, sugar that is about as sweetness, sweet as glucose, and about 65% as sweet as sucrose. It is an al ...
and a molecule of
D-glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water ...

D-glucose
bonded by beta-1-4
glycosidic linkage A glycosidic bond or glycosidic linkage is a type of covalent bond A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may ...
. A carbohydrate () is a
biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a Nobel Prize in Chemistry, Nobel Prize i ...
consisting of
carbon Carbon (from la, carbo "coal") is a chemical element with the Symbol (chemistry), symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—making four electrons available to form covalent bond, covalent chemical bonds. ...
(C),
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight of , hydrogen is the lightest element in the periodic table. Hydrogen is the abundance of the chemical elements, mos ...

hydrogen
(H) and
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...

oxygen
(O) atoms, usually with a hydrogen–oxygen
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

atom
ratio of 2:1 (as in water) and thus with the
empirical formula In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
(where ''m'' may or may not be different from ''n''). However, not all carbohydrates conform to this precise stoichiometric definition (e.g.,
uronic acid 200px, Glucose (before oxidization) 300px, The Fischer projections of glucose and glucuronic acid">glucose.html" ;"title="Fischer projections of glucose">Fischer projections of glucose and glucuronic acid. Glucose's terminal carbon's primary a ...
s, deoxy-sugars such as
fucose Fucose is a hexose In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the ...

fucose
), nor are all chemicals that do conform to this definition automatically classified as carbohydrates (e.g.
formaldehyde Formaldehyde ( , also ) ( systematic name methanal) is a naturally occurring organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen che ...
and
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, ...

acetic acid
). The term is most common in
biochemistry Biochemistry or biological chemistry, is the study of chemical process In a scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. La ...

biochemistry
, where it is a synonym of saccharide, a group that includes
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide found in animal milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by beta-1-4 glycosidic linkage. A carbohydrate () ...

sugar
s,
starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates ...
, and
cellulose Cellulose is an organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ...
. The saccharides are divided into four chemical groups:
monosaccharide Monosaccharides (from Greek '' monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Tab ...
s,
disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose ...
s,
oligosaccharide An oligosaccharide (/ˌɑlɪgoʊˈsækəˌɹaɪd/; from the Greek wikt:ὀλίγος#Ancient Greek, ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a carbohydrate, saccharide polymer containing a small number (typicall ...
s, and
polysaccharide , a beta-glucan polysaccharide Image:amylose 3Dprojection.svg">350px, Amylose is a linear polymer of glucose mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components of starch, the o ...
s. Monosaccharides and disaccharides, the smallest (lower
molecular weight A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atom An atom is the smallest unit of ordinary matter In ...
) carbohydrates, are commonly referred to as sugars. The word ''saccharide'' comes from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of 2018; Athens is ...
word ''σάκχαρον'' (''sákkharon''), meaning "sugar". While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides very often end in the suffix
-ose The suffix In linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for studying and modeling them. The traditional areas of linguistic ana ...
, which was originally taken from
glucose Glucose is a simple sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regular sugar, refers to sucrose, a disaccharide composed of glucose and ...

glucose
, from
Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages () ...
''γλεῦκος'' (''gleûkos'', “wine, must”), and is used for almost all sugars, e.g.
fructose Fructose, or fruit sugar, is a simple ketonic simple sugar found in many plants, where it is often bonded to glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosac ...

fructose
(fruit sugar),
sucrose Sucrose is a type of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regular sugar, refers to sucrose, a disaccharide composed of glucose a ...

sucrose
(
cane Cane or caning may refer to: *Walking stick or walking cane, a device used primarily to aid walking *Assistive cane, a walking stick used as a mobility aid for better balance *White cane, a mobility or safety device used by many people who are bl ...

cane
or
beet The beetroot is the taproot A taproot is a large, central, and dominant root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into th ...
sugar), ribose, amylose,
lactose Lactose, a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three comm ...

lactose
(milk sugar), etc. Carbohydrates perform numerous roles in living organisms. Polysaccharides serve for the storage of
energy In physics, energy is the physical quantity, quantitative physical property, property that must be #Energy transfer, transferred to a physical body, body or physical system to perform Work (thermodynamics), work on the body, or to heat it. En ...

energy
(e.g.
starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates ...
and
glycogen A view of the atomic structure of a single branched strand of glucose units in a glycogen molecule. Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysacchar ...

glycogen
) and as structural components (e.g.
cellulose Cellulose is an organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ...
in plants and
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin ( carbon, C8H13 O5 N)n ( ) is a long-chain polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consis ...

chitin
in arthropods). The 5-carbon monosaccharide
ribose Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, -ribose, is a component of the ribonucleotides from which RNA is built, and so this c ...

ribose
is an important component of
coenzyme A cofactor is a non-protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including ...
s (e.g.
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...
,
FAD In biochemistry Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biolo ...

FAD
and
NAD NAD or Nad may refer to: Geography * Nad, County Cork, a village in Ireland * North American Datum The North American Datum (NAD) is the horizontal datum now used to define the geodetic network in North America. A datum is a formal descripti ...
) and the backbone of the genetic molecule known as
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of ...

RNA
. The related
deoxyribose Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide with idealized formula H−(C=O)−(CH2)−(CHOH)3−H. Its name indicates that it is a deoxy sugar, meaning that it is derived from the sugar ribose by loss of an oxygen atom. De ...
is a component of DNA. Saccharides and their derivatives include many other important
biomolecules , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a Nobel Prize in Physics A biomolecul ...
that play key roles in the
immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biolog ...
,
fertilization Fertilisation or fertilization (see spelling differences Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orth ...

fertilization
, preventing
pathogenesis Pathogenesis is the process by which a disease or Disease#Disorder, disorder develops. It can include factors which contribute not only to the onset of the disease or disorder, but also to its progression and maintenance. The word comes from the A ...

pathogenesis
,
blood clotting Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechani ...
, and
development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking#Development, Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material i ...
. Carbohydrates are central to
nutrition Nutrition is the biochemical and physiological process by which an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, pr ...
and are found in a wide variety of natural and processed foods. Starch is a polysaccharide. It is abundant in cereals (wheat, maize, rice), potatoes, and processed food based on cereal
flour Kinako Flour is a Powder (substance), powder made by Mill (grinding), grinding raw grains, List of root vegetables, roots, beans, Nut (fruit), nuts, or seeds. Flours are used to make many different foods. Cereal flour, particularly wheat fl ...

flour
, such as bread, pizza or pasta. Sugars appear in human diet mainly as table sugar (sucrose, extracted from
sugarcane Sugarcane or sugar cane refers to several species and hybrids of tall Perennial plant, perennial grass in the genus ''Saccharum'', tribe Andropogoneae, that are used for sugar Sugar industry, production. The plants are 2-6 m (6-20 ft) tall wi ...

sugarcane
or
sugar beet A sugar beet is a plant whose root contains a high concentration of sucrose Sucrose is common sugar. It is a disaccharide, a molecule composed of two monosaccharides: glucose and fructose. Sucrose is produced naturally in plants, from which ta ...
s), lactose (abundant in milk), glucose and fructose, both of which occur naturally in
honey Honey is a sweet, viscous food substance made by honey bees and some related insects. Bees produce honey from the sugary secretions of plants (floral nectar) or from secretions of other insects (such as honeydew), by regurgitation, enzym ...
, many fruits, and some vegetables. Table sugar, milk, or honey are often added to drinks and many prepared foods such as jam, biscuits and cakes. Cellulose, a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble
dietary fiber Dietary fiber (British spelling fibre) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzyme Digestive enzymes are a group of enzyme Enzymes () are proteins that act as biologica ...
. Although it is not digestible in humans (ruminates generally, and some insects -- notably termites, have more complex digestions and can digest cellulose), cellulose and insoluble dietary fiber generally help maintain a healthy digestive system by facilitating movement through the large colon (ie,
defecation frame, Anatomy of the anus and rectum Defecation is the final act of digestion, by which organisms eliminate solid, semisolid, or liquid waste Waste (or wastes) are unwanted or unusable materials. Waste is any substance which is discarde ...
). Other polysaccharides contained in dietary fiber include
resistant starch Resistant starch (RS) is starch, including its degradation products, that escapes from digestion in the small intestine of healthy individuals. Resistant starch occurs naturally in foods but is also added to foods by the addition of dried raw foo ...
and
inulin Inulins are a group of naturally occurring polysaccharides produced by many types of plants, industrially most often extracted from chicory. The inulins belong to a class of dietary fibers known as fructans. Inulin is used by some plants as a ...

inulin
, which feed some bacteria in the
microbiota Microbiota are "ecological communities of commensal, symbiotic and pathogenic microorganisms A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usu ...
of the
large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the remaining waste material is stored as feces Feces (American and ...

large intestine
, and are
metabolized Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such as signaling and self-sustaining processes, ...

metabolized
by these bacteria to yield
short-chain fatty acidsShort-chain fatty acids (SCFAs) are fatty acid fatty acids have perfectly straight chain structure. Unsaturated ones are typically bent, unless they have a trans configuration. In chemistry Chemistry is the scientific discipline involved w ...
.


Terminology

In
scientific literature : ''For a broader class of literature, see Academic publishing Academic publishing is the subfield of publishing Publishing is the activity of making information, literature, music, software and other content available to the public for sa ...
, the term "carbohydrate" has many synonyms, like "sugar" (in the broad sense), "saccharide", "ose", "glucide", "hydrate of carbon" or " polyhydroxy compounds with
aldehyde Chemically, an aldehyde is a compound containing a functional group with the structure −CHO, consisting of a carbonyl center (a carbon double-bonded to oxygen Oxygen is the chemical element with the chemical symbol, symbol O and ...
or
ketone In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they ...
". Some of these terms, specially "carbohydrate" and "sugar", are also used with other meanings. In
food science Food science is the basic science and applied science Applied science is the use of the scientific method The scientific method is an Empirical evidence, empirical method of acquiring knowledge that has characterized the development of ...
and in many informal contexts, the term "carbohydrate" often means any food that is particularly rich in the complex carbohydrate
starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates ...
(such as cereals, bread and pasta) or simple carbohydrates, such as sugar (found in candy,
jam Fruit preserves are preparations of fruit In botany, a fruit is the seed-bearing structure in flowering plants (also known as angiosperms) formed from the ovary after flowering. Fruits are the means by which angiosperms disseminate s ...

jam
s, and desserts). This informality is sometimes confusing since it confounds chemical structure and digestibility in humans. Often in lists of nutritional information, such as the
USDA National Nutrient DatabaseThe USDA National Nutrient Database for Standard Reference is a database produced by the United States Department of Agriculture The United States Department of Agriculture (USDA), also known as the Agriculture Department, is the United States fed ...
, the term "carbohydrate" (or "carbohydrate by difference") is used for everything other than water, protein, fat, ash, and ethanol. This includes chemical compounds such as
acetic Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen che ...

acetic
or
lactic acid Lactic acid is an organic acid. It has a molecular formula CH3CH(OH)COOH. It is white in the solid state and it is miscibility, miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial s ...

lactic acid
, which are not normally considered carbohydrates. It also includes
dietary fiber Dietary fiber (British spelling fibre) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzyme Digestive enzymes are a group of enzyme Enzymes () are proteins that act as biologica ...
which is a carbohydrate but which does not contribute
food energy Food energy is chemical energy that animals (including human Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, hairlessness, and intelligence allowing the use o ...
(
kilocalories The calorie is a unit of energy defined as the amount of heat needed to raise the temperature of a quantity of water by one degree. For historical reasons, two main definitions of calorie are in wide use. The small calorie or gram calorie (usua ...
) in humans, even though it is often included in the calculation of total food energy just as though it did (ie, as if it were a digestible and absorbable carbohydrate such as a sugar). In the strict sense, "
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide found in animal milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by beta-1-4 glycosidic linkage. A carbohydrate () ...

sugar
" is applied for sweet, soluble carbohydrates, many of which are used in human food.


Structure

Formerly the name "carbohydrate" was used in
chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, molecules and i ...
for any compound with the formula C''m'' (H2O)''n''. Following this definition, some chemists considered
formaldehyde Formaldehyde ( , also ) ( systematic name methanal) is a naturally occurring organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen che ...
(CH2O) to be the simplest carbohydrate, while others claimed that title for
glycolaldehyde Glycolaldehyde is the organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, ...

glycolaldehyde
. Today, the term is generally understood in the biochemistry sense, which excludes compounds with only one or two carbons and includes many biological carbohydrates which deviate from this formula. For example, while the above representative formulas would seem to capture the commonly known carbohydrates, ubiquitous and abundant carbohydrates often deviate from this. For example, carbohydrates often display chemical groups such as: ''N''-acetyl (e.g.
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin ( carbon, C8H13 O5 N)n ( ) is a long-chain polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consis ...

chitin
),
sulfate The sulfate or sulphate ion is a polyatomic anion An ion () is a particle, atom or molecule with a net electric charge, electrical charge. The charge of the electron is considered negative by convention. The negative charge of an ion is ...

sulfate
(e.g.
glycosaminoglycans . For polysaccharide nomenclature see here. R1, R2, R3 may have different values. Image:Hyaluronan.svg, 240px, Hyaluronic acid, Hyaluronan (-4GlcUA''β''1-3GlcNAc''β''1-)n Glycosaminoglycans (GAGs) or mucopolysaccharides are long linear polysac ...
),
carboxylic acid A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with R referring to the alkyl, alkenyl, aryl, or other group. Carboxy ...
and deoxy modifications (e.g.
fucose Fucose is a hexose In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the ...

fucose
and
sialic acidSialic acids are a class of alpha-keto acid sugars Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide found in animal milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by be ...

sialic acid
). Natural saccharides are generally built of simple carbohydrates called
monosaccharide Monosaccharides (from Greek '' monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Tab ...
s with general formula (CH2O)''n'' where ''n'' is three or more. A typical monosaccharide has the structure H–(CHOH)''x''(C=O)–(CHOH)''y''–H, that is, an
aldehyde Chemically, an aldehyde is a compound containing a functional group with the structure −CHO, consisting of a carbonyl center (a carbon double-bonded to oxygen Oxygen is the chemical element with the chemical symbol, symbol O and ...
or
ketone In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they ...
with many
hydroxyl A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group ...

hydroxyl
groups added, usually one on each
carbon Carbon (from la, carbo "coal") is a chemical element with the Symbol (chemistry), symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—making four electrons available to form covalent bond, covalent chemical bonds. ...
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

atom
that is not part of the aldehyde or ketone
functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions reg ...
. Examples of monosaccharides are
glucose Glucose is a simple sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regular sugar, refers to sucrose, a disaccharide composed of glucose and ...

glucose
,
fructose Fructose, or fruit sugar, is a simple ketonic simple sugar found in many plants, where it is often bonded to glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosac ...

fructose
, and
glyceraldehyde Glyceraldehyde (glyceral) is a triose is an aldotriose because the carbonyl group is at the end of the chain Image:Dihydroxyacetone.png">150px, Dihydroxyacetone is a ketone, ketotriose because the carbonyl group is the center of the chain. A tr ...

glyceraldehyde
s. However, some biological substances commonly called "monosaccharides" do not conform to this formula (e.g.
uronic acid 200px, Glucose (before oxidization) 300px, The Fischer projections of glucose and glucuronic acid">glucose.html" ;"title="Fischer projections of glucose">Fischer projections of glucose and glucuronic acid. Glucose's terminal carbon's primary a ...
s and deoxy-sugars such as
fucose Fucose is a hexose In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the ...

fucose
) and there are many chemicals that do conform to this formula but are not considered to be monosaccharides (e.g. formaldehyde CH2O and
inositol Inositol, or more precisely ''myo''-inositol, is a carbocycle, carbocyclic sugar that is abundant in the brain and other mammalian tissues; it mediates cell signal transduction in response to a variety of hormones, neurotransmitters, and growth fa ...
(CH2O)6). The
open-chainIn chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo ...
form of a monosaccharide often coexists with a closed ring form where the
aldehyde Chemically, an aldehyde is a compound containing a functional group with the structure −CHO, consisting of a carbonyl center (a carbon double-bonded to oxygen Oxygen is the chemical element with the chemical symbol, symbol O and ...
/
ketone In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they ...
carbonyl In organic chemistry, a carbonyl group is a functional group In organic chemistry, a functional group is a substituent or moiety in a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecul ...
group carbon (C=O) and hydroxyl group (–OH) react forming a
hemiacetal of a hemiacetal Image:Hemiketal-2D-skeletal.png, 150px, Hemiketal A hemiacetal or a hemiketal is a compound that results from the addition of an alcohol to an aldehyde or a ketone, respectively. The Greek word ''hèmi'', meaning half, refers to th ...

hemiacetal
with a new C–O–C bridge. Monosaccharides can be linked together into what are called
polysaccharide , a beta-glucan polysaccharide Image:amylose 3Dprojection.svg">350px, Amylose is a linear polymer of glucose mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components of starch, the o ...
s (or
oligosaccharide An oligosaccharide (/ˌɑlɪgoʊˈsækəˌɹaɪd/; from the Greek wikt:ὀλίγος#Ancient Greek, ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a carbohydrate, saccharide polymer containing a small number (typicall ...
s) in a large variety of ways. Many carbohydrates contain one or more modified monosaccharide units that have had one or more groups replaced or removed. For example,
deoxyribose Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide with idealized formula H−(C=O)−(CH2)−(CHOH)3−H. Its name indicates that it is a deoxy sugar, meaning that it is derived from the sugar ribose by loss of an oxygen atom. De ...
, a component of
DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings ...
, is a modified version of
ribose Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, -ribose, is a component of the ribonucleotides from which RNA is built, and so this c ...

ribose
;
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin ( carbon, C8H13 O5 N)n ( ) is a long-chain polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consis ...

chitin
is composed of repeating units of N-acetyl glucosamine, a
nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772. Although Carl Wilhelm Scheele and Henry Cavendish had independentl ...

nitrogen
-containing form of glucose.


Division

Carbohydrates are polyhydroxy aldehydes, ketones, alcohols, acids, their simple derivatives and their polymers having linkages of the acetal type. They may be classified according to their
degree of polymerizationThe degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule. For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymerization is given by DP ...

degree of polymerization
, and may be divided initially into three principal groups, namely sugars, oligosaccharides and polysaccharides.


Monosaccharides

is an aldohexose with the formula (C·H2O)6. The red atoms highlight the
aldehyde Chemically, an aldehyde is a compound containing a functional group with the structure −CHO, consisting of a carbonyl center (a carbon double-bonded to oxygen Oxygen is the chemical element with the chemical symbol, symbol O and ...
group and the blue atoms highlight the asymmetric center furthest from the aldehyde; because this -OH is on the right of the
Fischer projection The Fischer projection, devised by Emil Fischer in 1891, is a two-dimensional representation of a Three-dimensional space, three-dimensional organic molecule by Graphical projection, projection. Fischer projections were originally proposed for the ...

Fischer projection
, this is a D sugar. Monosaccharides are the simplest carbohydrates in that they cannot be hydrolysis, hydrolyzed to smaller carbohydrates. They are aldehydes or ketones with two or more hydroxyl groups. The general
chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes ...
of an unmodified monosaccharide is (C•H2O)n, literally a "carbon hydrate". Monosaccharides are important fuel molecules as well as building blocks for nucleic acids. The smallest monosaccharides, for which n=3, are dihydroxyacetone and D- and L-glyceraldehydes.


Classification of monosaccharides

The α and β
anomer An anomer is a type of geometric variation found at certain atoms in carbohydrate molecules. An epimer is a stereoisomer that differs in configuration at any single stereogenic center. An anomer is an epimer at the hemiacetal/hemiketal carbon ...
s of glucose. Note the position of the hydroxyl group (red or green) on the anomeric carbon relative to the CH2OH group bound to carbon 5: they either have identical absolute configurations (R,R or S,S) (α), or opposite absolute configurations (R,S or S,R) (β).
Monosaccharides are classified according to three different characteristics: the placement of its
carbonyl In organic chemistry, a carbonyl group is a functional group In organic chemistry, a functional group is a substituent or moiety in a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecul ...
group, the number of
carbon Carbon (from la, carbo "coal") is a chemical element with the Symbol (chemistry), symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—making four electrons available to form covalent bond, covalent chemical bonds. ...
atoms it contains, and its
chiral that is chiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand," a familiar chiral object. An object or a system is ''chiral'' if it is disting ...
handedness. If the carbonyl group is an
aldehyde Chemically, an aldehyde is a compound containing a functional group with the structure −CHO, consisting of a carbonyl center (a carbon double-bonded to oxygen Oxygen is the chemical element with the chemical symbol, symbol O and ...
, the monosaccharide is an
aldoseAn aldose is a monosaccharide Monosaccharides (from Greek '' monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many o ...
; if the carbonyl group is a
ketone In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they ...
, the monosaccharide is a
ketose A ketose is a monosaccharide containing one ketone In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, s ...

ketose
. Monosaccharides with three carbon atoms are called
triose is an aldotriose because the carbonyl group is at the end of the chain Image:Dihydroxyacetone.png">150px, Dihydroxyacetone is a ketone, ketotriose because the carbonyl group is the center of the chain. A triose is a monosaccharide, or simple sug ...
s, those with four are called
tetroseA tetrose is a monosaccharide Monosaccharides (from Greek '' monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many o ...

tetrose
s, five are called
pentoseIn chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo ...

pentose
s, six are
hexose In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
s, and so on. These two systems of classification are often combined. For example,
glucose Glucose is a simple sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regular sugar, refers to sucrose, a disaccharide composed of glucose and ...

glucose
is an
aldohexose In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
(a six-carbon aldehyde),
ribose Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, -ribose, is a component of the ribonucleotides from which RNA is built, and so this c ...

ribose
is an
aldopentoseIn chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo ...

aldopentose
(a five-carbon aldehyde), and
fructose Fructose, or fruit sugar, is a simple ketonic simple sugar found in many plants, where it is often bonded to glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosac ...

fructose
is a ketohexose (a six-carbon ketone). Each carbon atom bearing a
hydroxyl group A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group ...

hydroxyl group
(-OH), with the exception of the first and last carbons, are asymmetric, making them stereo centers with two possible configurations each (R or S). Because of this asymmetry, a number of
isomer In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they und ...

isomer
s may exist for any given monosaccharide formula. Using Le Bel-van't Hoff rule, the aldohexose D-glucose, for example, has the formula (C·H2O)6, of which four of its six carbons atoms are stereogenic, making D-glucose one of 24=16 possible
stereoisomer In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in spa ...
s. In the case of
glyceraldehyde Glyceraldehyde (glyceral) is a triose is an aldotriose because the carbonyl group is at the end of the chain Image:Dihydroxyacetone.png">150px, Dihydroxyacetone is a ketone, ketotriose because the carbonyl group is the center of the chain. A tr ...

glyceraldehyde
s, an aldotriose, there is one pair of possible stereoisomers, which are
enantiomers In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they und ...

enantiomers
and
epimerIn stereochemistry, an epimer is one of a pair of diastereomers. The two epimers have opposite configuration at only one stereogenic center out of at least two. All other stereogenic centers in the molecules are the same in each. Epimerization is ...
s. 1, 3-dihydroxyacetone, the ketose corresponding to the aldose glyceraldehydes, is a symmetric molecule with no stereo centers. The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group: in a standard Fischer projection if the hydroxyl group is on the right the molecule is a D sugar, otherwise it is an L sugar. The "D-" and "L-" prefixes should not be confused with "d-" or "l-", which indicate the direction that the sugar rotates plane
polarized light Polarization ( also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillation Oscillation is the repetitive variation, typically in time, of some measure about a central value (ofte ...
. This usage of "d-" and "l-" is no longer followed in carbohydrate chemistry.


Ring-straight chain isomerism

can exist in both a straight-chain and ring form. The aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a
hemiacetal of a hemiacetal Image:Hemiketal-2D-skeletal.png, 150px, Hemiketal A hemiacetal or a hemiketal is a compound that results from the addition of an alcohol to an aldehyde or a ketone, respectively. The Greek word ''hèmi'', meaning half, refers to th ...

hemiacetal
or
hemiketal of a hemiacetal Image:Hemiketal-2D-skeletal.png, 150px, Hemiketal A hemiacetal or a hemiketal is a compound that results from the addition of an alcohol File:Alcohol general.svg, upright=0.8, The bond angle between a hydroxyl group (-OH) and ...

hemiketal
, forming a
heterocyclic 125px, Pyridine, a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, elements as members of its ring(s). Heterocyclic chemistry is the branch of organi ...
ring with an oxygen bridge between two carbon atoms. Rings with five and six atoms are called
furanose A furanose is a collective term for carbohydrates is a disaccharide found in animal milk. It consists of a molecule of galactose, D-galactose and a molecule of glucose, D-glucose bonded by beta-1-4 glycosidic linkage. A carbohydrate () is a bio ...
and
pyranosePyranose is a collective term for saccharides that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom. There may be other carbons external to the ring. The name derives from its similarity ...
forms, respectively, and exist in equilibrium with the straight-chain form. During the conversion from straight-chain form to the cyclic form, the carbon atom containing the carbonyl oxygen, called the
anomeric carbon An anomer is a type of geometric variation found at certain atoms in carbohydrate is a disaccharide found in animal milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by beta-1-4 glycosidic linkage. A carbohy ...
, becomes a stereogenic center with two possible configurations: The oxygen atom may take a position either above or below the plane of the ring. The resulting possible pair of stereoisomers is called
anomer An anomer is a type of geometric variation found at certain atoms in carbohydrate molecules. An epimer is a stereoisomer that differs in configuration at any single stereogenic center. An anomer is an epimer at the hemiacetal/hemiketal carbon ...
s. In the ''α anomer'', the -OH substituent on the anomeric carbon rests on the opposite side (
trans Trans- is a Latin prefix meaning "across", "beyond", or "on the other side of". Used alone, trans may refer to: Media * ''Trans'' (film), US film * Trans (Festival), a former festival in Belfast, Northern Ireland, United Kingdom * Trans Corp, a ...
) of the ring from the CH2OH side branch. The alternative form, in which the CH2OH substituent and the anomeric hydroxyl are on the same side (cis) of the plane of the ring, is called the ''β anomer''.


Use in living organisms

Monosaccharides are the major fuel source for
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main purposes of metabolism are: the conversion of the energy in food to energy available to run cellu ...

metabolism
, being used both as an energy source (
glucose Glucose is a simple sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regular sugar, refers to sucrose, a disaccharide composed of glucose and ...

glucose
being the most important in nature as it is the product of photosynthesis in plants) and in
biosynthesisBiosynthesis is a multi-step, enzyme-Catalysis, catalyzed process where substrate (chemistry), substrates are converted into more complex Product (chemistry), products in living organisms. In biosynthesis, simple Chemical compound, compounds are modi ...

biosynthesis
. When monosaccharides are not immediately needed, they are often converted to more space-efficient (ie, less water soluble) forms, often
polysaccharide , a beta-glucan polysaccharide Image:amylose 3Dprojection.svg">350px, Amylose is a linear polymer of glucose mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components of starch, the o ...
s. In many animals, including humans, this storage form is
glycogen A view of the atomic structure of a single branched strand of glucose units in a glycogen molecule. Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysacchar ...

glycogen
, especially in liver and muscle cells. In plants,
starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates ...
is used for the same purpose. The most abundant carbohydrate,
cellulose Cellulose is an organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ...
, is a structural component of the
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to a ...
of plants and many forms of algae.
Ribose Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally-occurring form, -ribose, is a component of the ribonucleotides from which RNA is built, and so this c ...

Ribose
is a component of
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of ...

RNA
.
Deoxyribose Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide Monosaccharides (from Greek '' monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar Sugar is the generic name for Sweetness, sweet ...
is a component of
DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings ...
.
Lyxose Lyxose is an aldopentose — a monosaccharide containing five carbon atoms, and including an aldehyde functional group. It has chemical formula 5105. It is a C'-2 carbon epimer of the sugar xylose. Lyxose occurs only rarely in nature, for exam ...

Lyxose
is a component of lyxoflavin found in the human heart.
Ribulose Ribulose is a ketopentose — a monosaccharide containing five carbon atoms, and including a ketone functional group. It has chemical formula 5105. Two enantiomers are possible, -ribulose (-erythro-pentulose) and -ribulose (-erythro-pentulose) ...

Ribulose
and
xylulose Xylulose is a ketopentose, a monosaccharide Monosaccharides (from Greek '' monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carb ...

xylulose
occur in the
pentose phosphate pathwayIn chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atom, atoms. The chemical formula of all pentoses is , and their molecular weight is 150.13 g/mol.
.
Galactose Galactose (, '' galacto-'' + ''wikt:-ose#Suffix 2, -ose'', "milk sugar") sometimes abbreviated Gal, is a monosaccharide Carbohydrate, sugar that is about as sweetness, sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a ...
, a component of milk sugar
lactose Lactose, a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three comm ...

lactose
, is found in
galactolipid Galactolipids are a type of glycolipid Glycolipids are lipids with a carbohydrate is a disaccharide found in animal milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by beta-1-4 glycosidic linkage. A car ...
s in and in
glycoprotein Glycoproteins are protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme ...
s in many tissues.
Mannose Mannose is a sugar monomer of the aldohexose series of carbohydrates. It is a C-2 epimer of glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subc ...

Mannose
occurs in human metabolism, especially in the glycosylation of certain proteins. Fructose, or fruit sugar, is found in many plants and humans, it is metabolized in the liver, absorbed directly into the intestines during digestion, and found in semen. Trehalose, a major sugar of insects, is rapidly hydrolyzed into two glucose molecules to support continuous flight.


Disaccharides

Image:sucrose 3Dprojection.png, Sucrose, also known as table sugar, is a common disaccharide. It is composed of two monosaccharides:
D-glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water ...

D-glucose
(left) and fructose, D-fructose (right). Two joined monosaccharides are called a
disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose ...
and these are the simplest polysaccharides. Examples include
sucrose Sucrose is a type of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regular sugar, refers to sucrose, a disaccharide composed of glucose a ...

sucrose
and
lactose Lactose, a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three comm ...

lactose
. They are composed of two monosaccharide units bound together by a covalent bond known as a
glycosidic linkage A glycosidic bond or glycosidic linkage is a type of covalent bond A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may ...
formed via a dehydration reaction, resulting in the loss of a
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight of , hydrogen is the lightest element in the periodic table. Hydrogen is the abundance of the chemical elements, mos ...

hydrogen
atom from one monosaccharide and a
hydroxyl group A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group ...

hydroxyl group
from the other. The chemical formula, formula of unmodified disaccharides is C12H22O11. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable. Sucrose, pictured to the right, is the most abundant disaccharide, and the main form in which carbohydrates are transported in plants. It is composed of one
D-glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water ...

D-glucose
molecule and one fructose, D-fructose molecule. The systematic name for sucrose, ''O''-α-D-glucopyranosyl-(1→2)-D-fructofuranoside, indicates four things: * Its monosaccharides: glucose and fructose * Their ring types: glucose is a
pyranosePyranose is a collective term for saccharides that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom. There may be other carbons external to the ring. The name derives from its similarity ...
and fructose is a
furanose A furanose is a collective term for carbohydrates is a disaccharide found in animal milk. It consists of a molecule of galactose, D-galactose and a molecule of glucose, D-glucose bonded by beta-1-4 glycosidic linkage. A carbohydrate () is a bio ...
* How they are linked together: the oxygen on carbon number 1 (C1) of α-D-glucose is linked to the C2 of D-fructose. * The ''-oside'' suffix indicates that the
anomeric carbon An anomer is a type of geometric variation found at certain atoms in carbohydrate is a disaccharide found in animal milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by beta-1-4 glycosidic linkage. A carbohy ...
of both monosaccharides participates in the glycosidic bond. Lactose, a disaccharide composed of one
D-galactose Galactose (, ''wikt:galacto-, galacto-'' + ''wikt:-ose#Suffix 2, -ose'', "milk sugar") sometimes abbreviated Gal, is a monosaccharide Carbohydrate, sugar that is about as sweetness, sweet as glucose, and about 65% as sweet as sucrose. It is an al ...
molecule and one
D-glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water ...

D-glucose
molecule, occurs naturally in mammalian milk. The systematic name for lactose is ''O''-β-D-galactopyranosyl-(1→4)-D-glucopyranose. Other notable disaccharides include maltose (two D-glucoses linked α-1,4) and cellobiose (two D-glucoses linked β-1,4). Disaccharides can be classified into two types: reducing and non-reducing disaccharides. If the functional group is present in bonding with another sugar unit, it is called a reducing disaccharide or biose.


Nutrition

Carbohydrate consumed in food yields 3.87 kilocalories of energy per gram for simple sugars, and 3.57 to 4.12 kilocalories per gram for complex carbohydrate in most other foods. Relatively high levels of carbohydrate are associated with processed foods or refined foods made from plants, including sweets, cookies and candy, table sugar, honey, soft drinks, breads and crackers, jams and fruit products, pastas and breakfast cereals. Lower amounts of carbohydrate are usually associated with unrefined foods, including beans, tubers, rice, and unrefined fruit. Animal-based foods generally have the lowest carbohydrate levels, although milk does contain a high proportion of
lactose Lactose, a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three comm ...

lactose
. Organisms typically cannot metabolize all types of carbohydrate to yield energy. Glucose is a nearly universal and accessible source of energy. Many organisms also have the ability to metabolize other
monosaccharide Monosaccharides (from Greek '' monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Tab ...
s and
disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose ...
s but glucose is often metabolized first. In ''Escherichia coli'', for example, the lac operon will express enzymes for the digestion of lactose when it is present, but if both lactose and glucose are present the ''lac'' operon is repressed, resulting in the glucose being used first (see: Diauxie). Polysaccharides are also common sources of energy. Many organisms can easily break down starches into glucose; most organisms, however, cannot metabolize
cellulose Cellulose is an organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ...
or other polysaccharides like
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin ( carbon, C8H13 O5 N)n ( ) is a long-chain polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consis ...

chitin
and arabinoxylans. These carbohydrate types can be metabolized by some bacteria and protists. Ruminants and termites, for example, use microorganisms to process cellulose. Even though these complex carbohydrates are not very digestible, they represent an important dietary element for humans, called
dietary fiber Dietary fiber (British spelling fibre) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzyme Digestive enzymes are a group of enzyme Enzymes () are proteins that act as biologica ...
. Fiber enhances digestion, among other benefits. The Institute of Medicine recommends that American and Canadian adults get between 45 and 65% of food energy, dietary energy from whole-grain carbohydrates. The Food and Agriculture Organization and World Health Organization jointly recommend that national dietary guidelines set a goal of 55–75% of total energy from carbohydrates, but only 10% directly from sugars (their term for simple carbohydrates). A 2017 The Cochrane Database of Systematic Reviews, Cochrane Systematic Review concluded that there was insufficient evidence to support the claim that whole grain diets can affect cardiovascular disease.


Classification

Nutritionists often refer to carbohydrates as either simple or complex. However, the exact distinction between these groups can be ambiguous. The term ''complex carbohydrate'' was first used in the U.S. Senate Select Committee on Nutrition and Human Needs publication ''Dietary Goals for the United States'' (1977) where it was intended to distinguish sugars from other carbohydrates (which were perceived to be nutritionally superior). However, the report put "fruit, vegetables and whole-grains" in the complex carbohydrate column, despite the fact that these may contain sugars as well as polysaccharides. This confusion persists as today some nutritionists use the term complex carbohydrate to refer to any sort of digestible saccharide present in a whole food, where fiber, vitamins and minerals are also found (as opposed to processed carbohydrates, which provide energy but few other nutrients). The standard usage, however, is to classify carbohydrates chemically: simple if they are sugars (
monosaccharide Monosaccharides (from Greek '' monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Tab ...
s and
disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose ...
s) and complex if they are
polysaccharide , a beta-glucan polysaccharide Image:amylose 3Dprojection.svg">350px, Amylose is a linear polymer of glucose mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components of starch, the o ...
s (or
oligosaccharide An oligosaccharide (/ˌɑlɪgoʊˈsækəˌɹaɪd/; from the Greek wikt:ὀλίγος#Ancient Greek, ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a carbohydrate, saccharide polymer containing a small number (typicall ...
s). In any case, the simple vs. complex chemical distinction has little value for determining the nutritional quality of carbohydrates. Some simple carbohydrates (e.g.
fructose Fructose, or fruit sugar, is a simple ketonic simple sugar found in many plants, where it is often bonded to glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosac ...

fructose
) raise blood glucose rapidly, while some complex carbohydrates (starches), raise blood sugar slowly. The speed of digestion is determined by a variety of factors including which other nutrients are consumed with the carbohydrate, how the food is prepared, individual differences in metabolism, and the chemistry of the carbohydrate. Carbohydrates are sometimes divided into "available carbohydrates", which are absorbed in the small intestine and "unavailable carbohydrates", which pass to the
large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the remaining waste material is stored as feces Feces (American and ...

large intestine
, where they are subject to fermentation by the Human gastrointestinal microbiota, gastrointestinal microbiota. The United States Department of Agriculture, USDA's ''Dietary Guidelines for Americans 2010'' call for moderate- to high-carbohydrate consumption from a balanced diet that includes six one-ounce servings of grain foods each day, at least half from whole grain sources and the rest from Enriched grain, enriched.United States Department of Health and Human Services, DHHS and United States Department of Agriculture, USDA
''Dietary Guidelines for Americans 2010''
.
The glycemic index, glycemic index (GI) and glycemic load concepts have been developed to characterize food behavior during human digestion. They rank carbohydrate-rich foods based on the rapidity and magnitude of their effect on blood glucose levels. Glycemic index is a measure of how quickly food
glucose Glucose is a simple sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regular sugar, refers to sucrose, a disaccharide composed of glucose and ...

glucose
is absorbed, while glycemic load is a measure of the total absorbable glucose in foods. The insulin index is a similar, more recent classification method that ranks foods based on their effects on insulin, blood insulin levels, which are caused by glucose (or starch) and some amino acids in food.


Health effects of dietary carbohydrate restriction

Low-carbohydrate diets may miss the health advantages – such as increased intake of
dietary fiber Dietary fiber (British spelling fibre) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzyme Digestive enzymes are a group of enzyme Enzymes () are proteins that act as biologica ...
– afforded by high-quality carbohydrates found in legumes and pulse (legume), pulses, whole grains, fruits, and vegetables. Disadvantages of the diet might include halitosis, headache and constipation, and in general the potential adverse effects of carbohydrate-restricted diets are under-researched, particularly for possible risks of osteoporosis and cancer incidence. Carbohydrate-restricted diets can be as effective as low-fat diets in helping achieve weight loss over the short term when overall calorie intake is reduced. An Endocrine Society scientific statement said that "when calorie intake is held constant [...] body-fat accumulation does not appear to be affected by even very pronounced changes in the amount of fat vs carbohydrate in the diet." In the long term, effective weight loss or maintenance depends on calorie restriction, not the ratio of macronutrients in a diet. The reasoning of diet advocates that carbohydrates cause undue fat accumulation by increasing blood insulin levels, and that low-carbohydrate diets have a "metabolic advantage", is not supported by evidence-based medicine, clinical evidence. Further, it is not clear how low-carbohydrate dieting affects cardiovascular health, although two reviews showed that carbohydrate restriction may improve lipid markers of cardiovascular disease risk. Carbohydrate-restricted diets are no more effective than a conventional healthy diet in preventing the onset of type 2 diabetes, but for people with type 2 diabetes, they are a viable option for losing weight or helping with glycemic control. There is limited evidence to support routine use of low-carbohydrate dieting in managing type 1 diabetes. The American Diabetes Association recommends that people with diabetes should adopt a generally healthy diet, rather than a diet focused on carbohydrate or other macronutrients. An extreme form of low-carbohydrate diet – the ketogenic diet – is established as a medical diet for treating epilepsy. Through celebrity endorsement during the early 21st century, it became a fad diet as a means of weight loss, but with risks of undesirable side effects, such as low energy levels and increased hunger, insomnia, nausea, and gastrointestinal discomfort. The British Dietetic Association named it one of the "top 5 worst celeb diets to avoid in 2018".


Metabolism

Carbohydrate metabolism is the series of biochemistry, biochemical processes responsible for the anabolism, formation, catabolism, breakdown and interconversion of carbohydrates in life, living organisms. The most important carbohydrate is
glucose Glucose is a simple sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated sugar, or regular sugar, refers to sucrose, a disaccharide composed of glucose and ...

glucose
, a simple sugar (
monosaccharide Monosaccharides (from Greek '' monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Tab ...
) that is metabolized by nearly all known organisms. Glucose and other carbohydrates are part of a wide variety of metabolic pathways across species: plants synthesize carbohydrates from carbon dioxide and water by photosynthesis storing the absorbed energy internally, often in the form of
starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is the most abundant monosaccharide, a subcategory of carbohydrates ...
or lipids. Plant components are consumed by animals and fungi, and used as fuel for cellular respiration. Oxidation of one gram of carbohydrate yields approximately 16 kJ (4 kcal) of Chemical energy, energy, while the oxidation of one gram of lipids yields about 38 kJ (9 kcal). The human body stores between 300 and 500 g of carbohydrates depending on body weight, with the skeletal muscle contributing to a large portion of the storage. Energy obtained from metabolism (e.g., oxidation of glucose) is usually stored temporarily within cells in the form of adenosine triphosphate, ATP. Organisms capable of anaerobic and aerobic respiration metabolize glucose and
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...

oxygen
(aerobic) to release energy, with carbon dioxide and water as byproducts.


Catabolism

Catabolism is the metabolic reaction which cells undergo to break down larger molecules, extracting energy. There are two major metabolic pathways of monosaccharide catabolism: glycolysis and the citric acid cycle. In glycolysis, oligo- and polysaccharides are cleaved first to smaller monosaccharides by enzymes called glycoside hydrolases. The monosaccharide units can then enter into monosaccharide catabolism. A 2 ATP investment is required in the early steps of glycolysis to phosphorylate Glucose to Glucose 6-phosphate, Glucose 6-Phosphate (Glucose 6-phosphate, G6P) and Fructose 6-phosphate, Fructose 6-Phosphate (Fructose 6-phosphate, F6P) to Fructose 1,6-bisphosphate, Fructose 1,6-biphosphate (Fructose 1,6-bisphosphate, FBP), thereby pushing the reaction forward irreversibly. In some cases, as with humans, not all carbohydrate types are usable as the digestive and metabolic enzymes necessary are not present.


Carbohydrate chemistry

Carbohydrate chemistry is a large and economically important branch of organic chemistry. Some of the main organic reactions that involve carbohydrates are: * Amadori rearrangement * Carbohydrate acetalisation * Carbohydrate digestion * Cyanohydrin reaction * Koenigs–Knorr reaction * Lobry de Bruyn–Van Ekenstein transformation * Nef reaction * Wohl degradation


See also

* Bioplastic * Carbohydrate NMR * Gluconeogenesis - A process where glucose can be synthesized by non-carbohydrate sources. * Glycobiology * Glycogen * Glycoinformatics * Glycolipid * Glycome * Glycomics * Glycosyl * Macromolecule * Saccharic acid


References


Further reading

*


External links


Carbohydrates, including interactive models and animations
(Require
MDL Chime

IUPAC-IUBMB Joint Commission on Biochemical Nomenclature (JCBN): Carbohydrate Nomenclature



Carbohydrates and Glycosylation – The Virtual Library of Biochemistry, Molecular Biology and Cell Biology

Functional Glycomics Gateway
a collaboration between the Consortium for Functional Glycomics and Nature Publishing Group {{Authority control Carbohydrates, Nutrition