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Carbohydrates | Wikipedia audio article

October 8, 2019


A carbohydrate () is a biomolecule consisting
of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom
ratio of 2:1 (as in water) and thus with the empirical formula Cm(H2O)n (where m may be
different from n). This formula holds true for monosaccharides. Some exceptions exist;
for example, deoxyribose, a sugar component of DNA, has the empirical formula C5H10O4.
The carbohydrates are technically hydrates of carbon; structurally it is more accurate
to view them as aldoses and ketoses. The term is most common in biochemistry, where
it is a synonym of saccharide, a group that includes sugars, starch, and cellulose. The
saccharides are divided into four chemical groups: monosaccharides, disaccharides, oligosaccharides,
and polysaccharides. Monosaccharides and disaccharides, the smallest (lower molecular weight) carbohydrates,
are commonly referred to as sugars. The word saccharide comes from the Greek 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, as in the
monosaccharides fructose (fruit sugar) and glucose (starch sugar) and the disaccharides
sucrose (cane or beet sugar) and lactose (milk sugar).
Carbohydrates perform numerous roles in living organisms. Polysaccharides serve for the storage
of energy (e.g. starch and glycogen) and as structural components (e.g. cellulose in plants
and chitin in arthropods). The 5-carbon monosaccharide ribose is an important component of coenzymes
(e.g. ATP, FAD and NAD) and the backbone of the genetic molecule known as RNA. The related
deoxyribose is a component of DNA. Saccharides and their derivatives include many other important
biomolecules that play key roles in the immune system, fertilization, preventing pathogenesis,
blood clotting, and development.They 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, such as bread, pizza or pasta. Sugars appear
in human diet mainly as table sugar (sucrose, extracted from sugarcane or sugar beets),
lactose (abundant in milk), glucose and fructose, both of which occur naturally in honey, 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. Although it is not digestible, insoluble dietary fiber helps
to maintain a healthy digestive system by easing defecation. Other polysaccharides contained
in dietary fiber include resistant starch and inulin, which feed some bacteria in the
microbiota of the large intestine, and are metabolized by these bacteria to yield short-chain
fatty acids.==Terminology==
In scientific literature, the term “carbohydrate” has many synonyms, like “sugar” (in the broad
sense), “saccharide”, “ose”, “glucide”, “hydrate of carbon” or “polyhydroxy compounds with
aldehyde or ketone”. Some of these terms, specially “carbohydrate” and “sugar”, are
also used with other meanings. In food science and in many informal contexts,
the term “carbohydrate” often means any food that is particularly rich in the complex carbohydrate
starch (such as cereals, bread and pasta) or simple carbohydrates, such as sugar (found
in candy, jams, and desserts). Often in lists of nutritional information,
such as the USDA National Nutrient Database, 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 or lactic acid, which are not normally considered
carbohydrates. It also includes dietary fiber which is a carbohydrate but which does not
contribute much in the way of food energy (kilocalories), even though it is often included
in the calculation of total food energy just as though it were a sugar.
In the strict sense, “sugar” is applied for sweet, soluble carbohydrates, many of which
are used in food.==Structure==
Formerly the name “carbohydrate” was used in chemistry for any compound with the formula
Cm (H2O)n. Following this definition, some chemists considered formaldehyde (CH2O) to
be the simplest carbohydrate, while others claimed that title for 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), sulphate
(e.g. glycosaminoglycans), carboxylic acid (e.g. sialic acid) and deoxy modifications
(e.g. fucose and sialic acid). Natural saccharides are generally built of
simple carbohydrates called monosaccharides 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 or ketone with many hydroxyl groups added, usually one on each carbon atom
that is not part of the aldehyde or ketone functional group. Examples of monosaccharides
are glucose, fructose, and glyceraldehydes. However, some biological substances commonly
called “monosaccharides” do not conform to this formula (e.g. uronic acids and deoxy-sugars
such as 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 (CH2O)6).The open-chain
form of a monosaccharide often coexists with a closed ring form where the aldehyde/ketone
carbonyl group carbon (C=O) and hydroxyl group (–OH) react forming a hemiacetal with a
new C–O–C bridge. Monosaccharides can be linked together into
what are called polysaccharides (or oligosaccharides) 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, a component of DNA, is a modified version of ribose; chitin
is composed of repeating units of N-acetyl glucosamine, a 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 polymerization, and may be divided initially into three principal
groups, namely sugars, oligosaccharides and polysaccharides
DP *=Degree of polymerization==Monosaccharides==Monosaccharides are the simplest carbohydrates
in that they cannot be hydrolyzed to smaller carbohydrates. They are aldehydes or ketones
with two or more hydroxyl groups. The general chemical formula 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===Monosaccharides are classified according to
three different characteristics: the placement of its carbonyl group, the number of carbon
atoms it contains, and its chiral handedness. If the carbonyl group is an aldehyde, the
monosaccharide is an aldose; if the carbonyl group is a ketone, the monosaccharide is a
ketose. Monosaccharides with three carbon atoms are called trioses, those with four
are called tetroses, five are called pentoses, six are hexoses, and so on. These two systems
of classification are often combined. For example, glucose is an aldohexose (a six-carbon
aldehyde), ribose is an aldopentose (a five-carbon aldehyde), and fructose is a ketohexose (a
six-carbon ketone). Each carbon atom bearing a 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 isomers 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 stereoisomers.
In the case of glyceraldehydes, an aldotriose, there is one pair of possible stereoisomers,
which are enantiomers and epimers. 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. This usage
of “d-” and “l-” is no longer followed in carbohydrate chemistry.===Ring-straight chain isomerism===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 or hemiketal, forming a heterocyclic ring with an oxygen bridge
between two carbon atoms. Rings with five and six atoms are called furanose and pyranose
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, 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 anomers. In the α anomer, the -OH substituent on the
anomeric carbon rests on the opposite side (trans) 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 source of fuel for metabolism, being used both as an energy
source (glucose being the most important in nature) and in biosynthesis. When monosaccharides
are not immediately needed by many cells they are often converted to more space-efficient
forms, often polysaccharides. In many animals, including humans, this storage form is glycogen,
especially in liver and muscle cells. In plants, starch is used for the same purpose. The most
abundant carbohydrate, cellulose, is a structural component of the cell wall of plants and many
forms of algae. Ribose is a component of RNA. Deoxyribose is a component of DNA. Lyxose
is a component of lyxoflavin found in the human heart. Ribulose and xylulose occur in
the pentose phosphate pathway. Galactose, a component of milk sugar lactose, is found
in galactolipids in plant cell membranes and in glycoproteins in many tissues. Mannose
occurs in human metabolism, especially in the glycosylation of certain proteins. Fructose,
or fruit sugar, is found in many plants and in 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==Two joined monosaccharides are called a disaccharide
and these are the simplest polysaccharides. Examples include sucrose and lactose. They
are composed of two monosaccharide units bound together by a covalent bond known as a glycosidic
linkage formed via a dehydration reaction, resulting in the loss of a hydrogen atom from
one monosaccharide and a hydroxyl group from the other. The 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 molecule and one 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 pyranose and fructose is a furanose
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 of both monosaccharides participates in the glycosidic bond.Lactose, a disaccharide
composed of one D-galactose molecule and one 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 cellulobiose (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. 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 monosaccharides and disaccharides
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 or other polysaccharides like 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. Fiber enhances
digestion, among other benefits.The Institute of Medicine recommends that American and Canadian
adults get between 45–65% of 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 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 (monosaccharides
and disaccharides) and complex if they are polysaccharides (or oligosaccharides).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) raise blood glucose slowly,
while some complex carbohydrates (starches), especially if processed, raise blood sugar
rapidly. 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.The 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.The 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 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 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 are associated with
increased mortality, but they may miss the health advantages – such as increased intake
of dietary fiber – afforded by high-quality carbohydrates found in legumes and 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 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 denotes the various
biochemical processes responsible for the formation, breakdown and interconversion of
carbohydrates in living organisms. The most important carbohydrate is glucose,
a simple sugar (monosaccharide) 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 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 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 ATP. Organisms capable of anaerobic and aerobic
respiration metabolize glucose and 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 (G6P) and Fructose
6-Phosphate (F6P) to Fructose 1,6-biphosphate (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: Carbohydrate acetalisation
Cyanohydrin reaction Lobry de Bruyn–van Ekenstein transformation
Amadori rearrangement Nef reaction
Wohl degradation Koenigs–Knorr reaction
Carbohydrate digestion==See also

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