Carbohydrate

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Not to be confused with hydrocarbon.

glycosidic linkage
.

A carbohydrate (

deoxy-sugars such as fucose), nor are all chemicals that do conform to this definition automatically classified as carbohydrates (e.g. formaldehyde and acetic acid
).

The term is most common in

cane or beet sugar), ribose, lactose
(milk sugar), etc.

Carbohydrates perform numerous roles in living organisms.

blood clotting, and development.[4]

Carbohydrates are central to nutrition and are found in a wide variety of natural and processed foods. Starch is a polysaccharide and 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.

short-chain fatty acids.[6][7]

Terminology

In scientific literature, the term "carbohydrate" has many synonyms, like "sugar" (in the broad sense), "saccharide", "ose",[1] "glucide",[8] "hydrate of carbon" or "polyhydroxy compounds with aldehyde or ketone". Some of these terms, especially "carbohydrate" and "sugar", are also used with other meanings.

In

jams
, 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 Database, the term "carbohydrate" (or "carbohydrate by difference") is used for everything other than water, protein, fat, ash, and ethanol.[9] 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 food energy
in humans, even though it is often included in the calculation of total food energy just as though it did (i.e., as if it were a digestible and absorbable carbohydrate such as a sugar). In the strict sense, "sugar" is applied for sweet, soluble carbohydrates, many of which are used in human food.

History

The history of the discovery regarding carbohydrates dates back around 10,000 years ago in Papua New Guinea during the cultivation of Sugarcane during the Neolithic agricultural revolution [citation needed]. The term "carbohydrate" was first proposed by German chemist Carl Schmidt (chemist) in 1844. In 1856, glycogen, a form of carbohydrate storage in animal livers, was discovered by French physiologist Claude Bernard.[citation needed]

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,[10] while others claimed that title for glycolaldehyde.[11] 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), sulfate (e.g. glycosaminoglycans), carboxylic acid and deoxy modifications (e.g. fucose and sialic acid).

Natural saccharides are generally built of simple carbohydrates called

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).[12]

The

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

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.[13]

The major dietary carbohydrates
Class
(degree of polymerization)		 Subgroup Components
Sugars (1–2) Monosaccharides Glucose, galactose, fructose, xylose
Disaccharides Sucrose, lactose, maltose, isomaltulose, trehalose
Polyols Sorbitol, mannitol
Oligosaccharides (3–9) Malto-oligosaccharides Maltodextrins
Other oligosaccharides Raffinose, stachyose, fructo-oligosaccharides
Polysaccharides (>9) Starch Amylose, amylopectin, modified starches
Non-starch polysaccharides
Hydrocolloids

Monosaccharides

Main article: Monosaccharide
D-glucose is an aldohexose with the formula (C·H2O)6. The red atoms highlight the aldehyde group and the blue atoms highlight the asymmetric center furthest from the aldehyde; because this -OH is on the right of the Fischer projection, this is a D sugar.

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

The

β anomers 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).[14]

Monosaccharides are classified according to three different characteristics: the placement of its

ketohexose
(a six-carbon ketone).

Each carbon atom bearing a

rotates plane polarized light. This usage of "d-" and "l-" is no longer followed in carbohydrate chemistry.[16]

Ring-straight chain isomerism

Glucose 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

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.[17]

During the conversion from straight-chain form to the cyclic form, the carbon atom containing the carbonyl oxygen, called the

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 fuel source for

tissues. 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

Sucrose, also known as table sugar, is a common disaccharide. It is composed of two monosaccharides: D-glucose (left) and D-fructose (right).
Main article: Disaccharide

Two joined monosaccharides are called 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 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

Grain products: rich sources of carbohydrates

Carbohydrate consumed in food yields 3.87 kilocalories of energy per gram for simple sugars,[19] and 3.57 to 4.12 kilocalories per gram for complex carbohydrate in most other foods.[20] 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 digestible carbohydrate are usually associated with unrefined foods as these foods have more fiber, including beans, tubers, rice, and unrefined fruit[dubious – discuss].[21] 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

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.[22]

The

Cochrane Systematic Review concluded that there was insufficient evidence to support the claim that whole grain diets can affect cardiovascular disease.[25]

Classification

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).[26] 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).[27]

In any case, the simple vs. complex chemical distinction has little value for determining the nutritional quality of carbohydrates.

gastrointestinal microbiota.[29]

The

enriched.[30]

The

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

Main article: Low-carbohydrate diet

Low-carbohydrate diets may miss the health advantages – such as increased intake of

better source needed
]

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.

cardiovascular health, although two reviews showed that carbohydrate restriction may improve lipid markers of cardiovascular disease risk.[37][38]

Carbohydrate-restricted diets are no more effective than a conventional

glycemic control.[39][40][41] There is limited evidence to support routine use of low-carbohydrate dieting in managing type 1 diabetes.[42] 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.[41]

An extreme form of low-carbohydrate diet – the

scientific citation needed][43] The British Dietetic Association named it one of the "top 5 worst celeb diets to avoid in 2018".[43]

Sources

Glucose tablets

Most dietary carbohydrates contain glucose, either as their only building block (as in the polysaccharides starch and glycogen), or together with another monosaccharide (as in the hetero-polysaccharides sucrose and lactose).[44] Unbound glucose is one of the main ingredients of honey. Glucose is extremely abundant and has been isolated from a variety of natural sources across the world, including male cones of the coniferous tree Wollemia nobilis in Rome,[45] the roots of Ilex asprella plants in China,[46] and straws from rice in California.[47]

Sugar content of selected common plant foods (in grams per 100 g)[48]
Food
item 		Carbohydrate,
total,A including
dietary fiber 		Total
sugars 		Free
fructose 		Free
glucose 		Sucrose Ratio of
fructose/
glucose 		Sucrose as
proportion of
total sugars (%)
Fruits
Apple 13.8 10.4 5.9 2.4 2.1 2.0 19.9
Apricot 11.1 9.2 0.9 2.4 5.9 0.7 63.5
Banana 22.8 12.2 4.9 5.0 2.4 1.0 20.0
Fig, dried 63.9 47.9 22.9 24.8 0.9 0.93 0.15
Grapes 18.1 15.5 8.1 7.2 0.2 1.1 1
Navel orange
 12.5 8.5 2.25 2.0 4.3 1.1 50.4
Peach 9.5 8.4 1.5 2.0 4.8 0.9 56.7
Pear 15.5 9.8 6.2 2.8 0.8 2.1 8.0
Pineapple 13.1 9.9 2.1 1.7 6.0 1.1 60.8
Plum 11.4 9.9 3.1 5.1 1.6 0.66 16.2
Vegetables
Beet
, red		 9.6 6.8 0.1 0.1 6.5 1.0 96.2
Carrot 9.6 4.7 0.6 0.6 3.6 1.0 77
Red pepper, sweet 6.0 4.2 2.3 1.9 0.0 1.2 0.0
Onion, sweet 7.6 5.0 2.0 2.3 0.7 0.9 14.3
Sweet potato 20.1 4.2 0.7 1.0 2.5 0.9 60.3
Yam 27.9 0.5 Traces Traces Traces — Traces
Sugar cane
 13–18 0.2–1.0 0.2–1.0 11–16 1.0 high
Sugar beet 17–18 0.1–0.5 0.1–0.5 16–17 1.0 high
Grains
Corn, sweet 19.0 6.2 1.9 3.4 0.9 0.61 15.0

^A The carbohydrate value is calculated in the USDA database and does not always correspond to the sum of the sugars, the starch, and the "dietary fiber".

Metabolism

Main article: Carbohydrate metabolism

Carbohydrate metabolism is the series of biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms.

The most important carbohydrate is

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.[49] 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:

See also

References

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Further reading

External links

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