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Lactose 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 (/ˌkɑːrboʊˈhaɪdreɪt/) is a sugar (saccharide) or a sugar derivative.[1] For the simplest carbohydrates, the carbon-to-hydrogen-to-oxygen atomic ratio is 1:2:1, i.e. they are often represented by the empirical formula C(H2O)n. Together with amino acids, fats, and nucleic acids, the carbohydrates are one of the major families of biomolecules.[2]

Carbohydrates perform numerous roles in living organisms.[3] Polysaccharides serve as an energy store (e.g., starch and glycogen) and as structural components (e.g., cellulose in plants and chitin in arthropods and fungi). 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 play key roles in the immune system, fertilization, preventing pathogenesis, 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 is often added to drinks and many prepared foods such as jam, biscuits and cakes.

Terminology

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The term "carbohydrate" has many synonyms and the definition can depend on context. Terms associated with carbohydrate include "sugar", "saccharide", "glucan",[5] and "glucide".[6] In food science and the term "carbohydrate" often means any food that is rich in the starch (such as cereals, bread and pasta) or simple carbohydrates, or fairly simple sugars such as sucrose (found in candy, jams, and desserts). Carbohydrates can also refer to dietary fiber, like cellulose.[7][8]

Saccharides

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The starting point for discussion of carbohydrates are the saccharides. Monosaccharides are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. Monosaccharides usually have the formula Cm (H2O)n. Disaccharides (e.g. sucrose) are common as are polysaccharides/oligosaccharides (e.g., starch, cellulose). Saccharides are polyhydroxy aldehydes, ketones as well as derived polymers having linkages of the acetal type. They may be classified according to their degree of polymerization. Many polyols are also classified as carbohydrates. In many carbohydrates the OH groups are appended to or replaced by N-acetyl (e.g., chitin), sulfate (e.g., glycosaminoglycans), carboxylic acid and deoxy modifications (e.g., fucose and sialic acid).[6]

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 Glycogen, Cellulose, Hemicellulose, Pectins, Hydrocolloids

Complex carbohydrates

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Heparin, a carbohydrate, is a blood anticoagulant.[9]
Main articles: Glycoconjugates and Glycosylation

Sugars may be linked to other types of biological molecules to form glycoconjugates. The enzymatic process of glycosylation creates sugars/saccharides linked to themselves and to other molecules by the glycosidic bond, thereby producing glycans. Glycoproteins, proteoglycans and glycolipids are the most abundant glycoconjugates found in mammalian cells. They are found predominantly on the outer cell membrane and in secreted fluids. Glycoconjugates have been shown to be important in cell-cell interactions due to the presence on the cell surface of various glycan binding receptors in addition to the glycoconjugates themselves.[10][11] In addition to their function in protein folding and cellular attachment, the N-linked glycans of a protein can modulate the protein's function, in some cases acting as an on-off switch.[12]

History

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Emil Fischer, who elucidated the structure of glucose, with colleagues and student in their laboratory of Ludwig Maximilian University of Munich in 1877.

The history of carbohydrates, to some extent, is the history of sugar cane, which was first grown in New Guinea. The mass cultivation occurred in India where techniques were developed for the isolatoin of crystalline sugar.[13] Cane sugar and its cultivation reached Europe around the 13th Century and then expanded to the New World, where industrialization occurred.

The chemistry and biochemistry of carbohydrates can be traced to 1811. On that year Constantin Kirchhoff discovered that grape sugar (glucose) forms when starch is boiled with acid. The starch industry started the following year. Henri Braconnot discovered in 1819 that sugar is formed through the action of sulfuric acid on cellulose. William Prout, after chemical analyses of sugar and starch by Joseph Louis Gay-Lussac and Thénard, gave this group of substances the group name "saccharine." 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.[14] Emil Fischer received the 1902 Nobel Prize in Chemistry for his work on sugars and purines. For the discovery of glucose metabolism, Otto Meyerhof received the 1922 Nobel Prize in Physiology or Medicine. Hans von Euler-Chelpin, together with Arthur Harden, received the 1929 Nobel Prize in Chemistry "for their research on sugar fermentation and the role of enzymes in this process." In 1947, both Bernardo Houssay for his discovery of the role of the pituitary gland in carbohydrate metabolism and Carl and Gerty Cori for their discovery of the conversion of glycogen received the Nobel Prize in Physiology or Medicine. For the discovery of sugar nucleotides in carbohydrate biosynthesis, Luis Leloir received the 1970 Nobel Prize in Chemistry.

The term glycobiology[15] was coined in 1988 by Raymond Dwek to recognize the coming together of the traditional disciplines of carbohydrate chemistry and biochemistry.[16] This coming together was as a result of a much greater understanding of the cellular and molecular biology of glycans. "Glycoscience" is a field that explores the structures and functions of glycans.[17]

Nutrition

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Grain products: rich sources of carbohydrates

Carbohydrate consumed in food yields 3.87 kilocalories of energy per gram for simple sugars,[18] and 3.57 to 4.12 kilocalories per gram for complex carbohydrate in most other foods.[19] 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. Refined carbohydrates from processed foods such as white bread or rice, soft drinks, and desserts are readily digestible, and many are known to have a high glycemic index, which reflects a rapid assimilation of glucose. By contrast, the digestion of whole, unprocessed, fiber-rich foods such as beans, peas, and whole grains produces a slower and steadier release of glucose and energy into the body.[20] 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 such as chitin and arabinoxylans. These carbohydrate types can be metabolized by some bacteria and protists. Ruminants and termites, for example, use microorganisms to process cellulose, fermenting it to caloric short-chain fatty acids. Even though humans lack the enzymes to digest fiber, dietary fiber represents an important dietary element for humans. Fibers promote healthy digestion, help regulate postprandial glucose and insulin levels, reduce cholesterol levels, and promote satiety.[21]

The Institute of Medicine recommends that American and Canadian adults get between 45 and 65% of dietary energy from whole-grain carbohydrates.[22] 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).[23] A 2017 Cochrane Systematic Review concluded that there was insufficient evidence to support the claim that whole grain diets can affect cardiovascular disease.[24]

Carbohydrates are one of the main components of insoluble dietary fiber. Although it is not digestible by humans, cellulose and insoluble dietary fiber generally help maintain a healthy digestive system by facilitating bowel movements.[7] 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.[7][25][26]

Classification

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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).[27] 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. 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).[7][28] Carbohydrates are sometimes divided into "available carbohydrates", which are absorbed in the small intestine and "unavailable carbohydrates", which pass to the large intestine, where they are subject to fermentation by the gastrointestinal microbiota.[7]

Glycemic index

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The glycemic index (GI) and glycemic load concepts characterize the potential for carbohydrates in food to raise blood glucose compared to a reference food (generally pure glucose).[29] Expressed numerically as GI, carbohydrate-containing foods can be grouped as high-GI (score more than 70), moderate-GI (56–69), or low-GI (less than 55) relative to pure glucose (GI=100).[29] Consumption of carbohydrate-rich, high-GI foods causes an abrupt increase in blood glucose concentration that declines rapidly following the meal, whereas low-GI foods with lower carbohydrate content produces a lower blood glucose concentration that returns gradually after the meal.[29]

Glycemic load is a measure relating the quality of carbohydrates in a food (low- vs. high-carbohydrate content – the GI) by the amount of carbohydrates in a single serving of that food.[29]

Health effects of dietary carbohydrate restriction

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Main article: Low-carbohydrate diet

Low-carbohydrate diets may miss the health advantages – such as increased intake of dietary fiber and phytochemicals – afforded by high-quality plant foods such as legumes and pulses, whole grains, fruits, and vegetables.[30][31] A "meta-analysis, of moderate quality," included as adverse effects of the diet halitosis, headache and constipation.[32][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.[33] 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."[33] In the long term, low-carbohydrate diets do not appear to confer a "metabolic advantage," and effective weight loss or maintenance depends on the level of calorie restriction,[33] not the ratio of macronutrients in a diet.[34] The reasoning of diet advocates that carbohydrates cause undue fat accumulation by increasing blood insulin levels, but a more balanced diet that restricts refined carbohydrates can also reduce serum glucose and insulin levels and may also suppress lipogenesis and promote fat oxidation.[35] However, as far as energy expenditure itself is concerned, the claim that low-carbohydrate diets have a "metabolic advantage" is not supported by clinical evidence.[33][36] 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.[37][38]

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.[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 ketogenic diet – is established as a medical diet for treating epilepsy.[43] 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.[scientific citation needed][43] The British Dietetic Association named it one of the "top 5 worst celeb diets to avoid in 2018".[43]

Sources

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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

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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 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.[49] Energy obtained from metabolism (e.g., oxidation of glucose) is usually stored temporarily within cells in the form of ATP.[50] Organisms capable of anaerobic and aerobic respiration metabolize glucose and oxygen (aerobic) to release energy, with carbon dioxide and water as byproducts.

Catabolism

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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.

Analytical tools

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Many techniques are used in the analysis of glycans.[51] NMR spectroscopy is common, the major challenge being spectral overlap.[52] [53]

High-resolution mass spectrometry (MS) and high-performance liquid chromatography (HPLC)

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MS and HPLC are commonly applied to glycan cleaved either enzymatically or chemically from the target.[54] In case of glycolipids, they can be analyzed directly without separation of the lipid component.

N-glycans from glycoproteins are analyzed routinely by high-performance-liquid-chromatography (reversed phase, normal phase and ion exchange HPLC) after tagging the reducing end of the sugars with a fluorescent compound (reductive labeling).[55] A large variety of different labels were introduced in the recent years, where 2-aminobenzamide (AB), anthranilic acid (AA), 2-aminopyridin (PA), 2-aminoacridone (AMAC) and 3-(acetylamino)-6-aminoacridine (AA-Ac) are just a few of them.[56] Different labels have to be used for different ESI modes and MS systems used.[57]

O-glycans are usually analysed without any tags.

Fractionated glycans from high-performance liquid chromatography (HPLC) instruments can be further analyzed by MALDI-TOF-MS(MS) to get further information about structure and purity. Sometimes glycan pools are analyzed directly by mass spectrometry without prefractionation, although a discrimination between isobaric glycan structures is more challenging or even not always possible. Anyway, direct MALDI-TOF-MS analysis can lead to a fast and straightforward illustration of the glycan pool.[58]

High performance liquid chromatography online coupled to mass spectrometry is useful. By choosing porous graphitic carbon as a stationary phase for liquid chromatography, even non derivatized glycans can be analyzed. Detection is here done by mass spectrometry, but in instead of MALDI-MS, electrospray ionisation (ESI) is more frequently used.[59][60][61]

Multiple reaction monitoring (MRM)

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Although MRM has been used extensively in metabolomics and proteomics, its high sensitivity and linear response over a wide dynamic range make it especially suited for glycan biomarker research and discovery. MRM is performed on a triple quadrupole (QqQ) instrument, which is set to detect a predetermined precursor ion in the first quadrupole, a fragmented in the collision quadrupole, and a predetermined fragment ion in the third quadrupole. It is a non-scanning technique, wherein each transition is detected individually and the detection of multiple transitions occurs concurrently in duty cycles. This technique is being used to characterize the immune glycome.[12][62]

Chemical synthesis and manipulation of carbohydrates

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Carbohydrate synthesis is a sub-field of organic chemistry concerned specifically with the generation of natural and unnatural carbohydrate structures. Carbohydrate chemistry is a large and economically important branch of organic chemistry. This can include the synthesis of monosaccharide residues or structures containing more than one monosaccharide, known as oligosaccharides. Selective formation of glycosidic linkages and selective reactions of hydroxyl groups are very important, and the usage of protecting groups is extensive.

Some of the main organic reactions that involve carbohydrates are:

Related topics

See also

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References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Carbohydrate

View on Grokipedia
from Grokipedia
Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that yield them on , characterized by the general (CH₂O)ₙ, where n typically ranges from 3 to 7 for simple sugars. They represent the most abundant class of organic compounds on , originating primarily from in , and serve as essential macronutrients alongside proteins and fats in human diets. Classification of carbohydrates is based on their degree of polymerization and structural features. Monosaccharides, the simplest form, include aldoses like glucose (with an aldehyde group) and ketoses like fructose (with a ketone group), often existing in cyclic forms such as pyranose or furanose rings due to intramolecular hemiacetal formation. Disaccharides, such as sucrose (glucose-fructose) and lactose (galactose-glucose), consist of two monosaccharide units linked by glycosidic bonds formed through dehydration synthesis. Polysaccharides, including storage forms like starch in plants and glycogen in animals, as well as structural ones like cellulose in plant cell walls and chitin in fungal and arthropod exoskeletons, are long chains or branched polymers of monosaccharides that can exceed 100,000 daltons in molecular weight. In biological systems, carbohydrates fulfill diverse and critical roles beyond energy provision. They act as the primary metabolic fuel, with glucose undergoing glycolysis to yield ATP—approximately 2 ATP net per molecule in anaerobic conditions and up to 32 ATP via full oxidative metabolism. Daily human requirements include about 120 grams for brain function alone, underscoring their indispensability. Structurally, they contribute to cell walls (e.g., cellulose comprising over 50% of Earth's organic carbon), membranes via glycoproteins, and cell-to-cell recognition through oligosaccharides on cell surfaces. Additionally, pentose sugars like ribose form the backbone of RNA and DNA, while carbohydrates supply carbon atoms for synthesizing other biomolecules such as proteins and lipids.

Fundamentals

Terminology

Carbohydrates are biomolecules composed primarily of carbon, , and oxygen atoms, typically in a of 2:1, which gives rise to their general Cn(H2O)nC_n(H_2O)_n, where nn represents the number of carbon atoms. This composition underscores their role as organic compounds essential in biological systems, though the formula serves more as an approximation than a strict rule. The term "carbohydrate" derives from the phrase "hydrate of carbon," coined to reflect the apparent hydration of carbon in the , as early analyses of simple sugars like glucose (C6H12O6C_6H_{12}O_6) suggested a water-like structure bound to carbon. However, this nomenclature is a for many complex carbohydrates, such as , where the ratio deviates significantly due to branching, , or additional modifications, rendering the "hydrate" analogy imprecise for non-monosaccharide forms. Standard IUPAC nomenclature for carbohydrates classifies monosaccharides based on the functional group of their carbonyl: aldoses contain an aldehyde group at carbon-1, while ketoses feature a ketone group, typically at carbon-2. Names incorporate prefixes denoting the carbon chain length, such as triose for three-carbon sugars, tetrose for four-carbon, pentose for five-carbon, and extending to decose for ten-carbon variants; these stems combine with "-ose" for aldoses or "-ulose" with a locant for ketoses. Configurational prefixes like D- or L- further specify stereochemistry relative to glyceraldehyde. Carbohydrates are broadly distinguished as simple or complex based on molecular size and structure: simple carbohydrates include monosaccharides (single sugar units) and disaccharides (two linked units), which are rapidly digested, whereas complex carbohydrates encompass oligosaccharides (three to ten units) and (longer chains), which require more extensive breakdown. This classification highlights differences in digestibility and energy release, with simple forms providing quick glucose availability and complex forms sustaining prolonged .

History

The early history of carbohydrate research began in the late with the isolation of various sugars from natural sources by Swedish chemist . Between 1780 and 1790, Scheele systematically extracted and identified key sugars, such as from milk, and discovered from citrus fruits, contributing to the understanding of plant-derived saccharides through his analytical techniques. In the early 19th century, French chemist Joseph Louis Proust advanced the field by recognizing the consistent elemental composition of carbohydrates, noting their general approximating C_n(H_2O)_n, which reflected a fixed ratio of carbon, , and oxygen. This observation underpinned the and led Proust to coin the term "hydrate de carbone" around 1800 to describe these compounds, highlighting their hydrated carbon-like structure. A major breakthrough occurred in the 1890s through the work of German chemist , who elucidated the structures of monosaccharides and established the D/L configuration system for . Fischer's synthesis of glucose in 1890 and his use of to form osazones allowed him to determine the open-chain structures and relative configurations of aldohexoses, including glucose and its isomers, laying the foundation for carbohydrate . In the 1920s, English chemist Walter Norman Haworth built on linear models by demonstrating that monosaccharides predominantly exist in cyclic ring forms, such as and structures. Haworth's studies and analyses confirmed the six-membered ring for glucose and extended these insights to like and , revealing their linear, repeating glycosidic linkages. Advancements in enzymology and accelerated after , building on earlier discoveries like the proposed by Carl and in 1929, which described the interconversion of , glucose, and lactate between muscle and liver. Postwar research emphasized enzymatic mechanisms, such as action in breakdown, enabling deeper insights into metabolic pathways without relying on prior structural details.

Chemical Structure

General Structure

Carbohydrates are polyhydroxy s or s, or compounds that produce such units upon , with the general Cn(H2O)nC_n(H_2O)_n, where nn typically ranges from 3 to 7 for simple units. In their open-chain forms, aldoses feature an aldehyde group (-CHO) at carbon 1, while ketoses contain a ketone group (=O) at carbon 2, both accompanied by multiple hydroxyl (-OH) groups on the remaining carbons. This linear representation highlights the carbonyl functionality central to carbohydrate reactivity. The convention depicts these open-chain structures as vertical chains with the most oxidized carbon (carbonyl) at the top, horizontal lines representing bonds projecting out of the plane, and the carbon chain in the plane of the paper. This notation standardizes the portrayal of carbohydrate without specifying three-dimensional orientations beyond the implied tetrahedral . In solution, most carbohydrates exist predominantly in cyclic forms due to intramolecular reactions forming s, where a hydroxyl group reacts with the carbonyl to create a ring. Five-membered rings arise from reaction between the carbonyl and the hydroxyl on carbon 4 or 5 (depending on the ), while six-membered rings form with the hydroxyl on carbon 5 or 6, stabilizing the structure through the hemiacetal linkage. The anomeric carbon, the former carbonyl carbon in the ring, gives rise to two anomers: alpha, where the hydroxyl group is trans to the -CH2OH substituent in the standard for D-sugars, and beta, where it is cis. These configurations result from the formation and interconvert via in open-chain intermediates. Glycosidic bonds link carbohydrate units by connecting the anomeric carbon of one (as a hemiacetal-derived ) to a hydroxyl oxygen of another via , forming alpha-glycosidic bonds if derived from the alpha anomer or beta-glycosidic bonds from the beta anomer. This linkage prevents further at the involved anomeric carbon, defining the bond's . Oligosaccharides consist of 3 to 10 units joined by glycosidic bonds, while comprise longer chains of hundreds to thousands of units, often exhibiting branched structures through multiple glycosidic linkages per . The general for such polymers approximates (C6H10O5)m(C_6H_{10}O_5)_m for hexose-based structures, reflecting the loss of in bond formation.

Stereochemistry and Isomerism

Carbohydrates, particularly , possess multiple chiral centers arising from their polyhydroxy or structures, leading to a rich array of stereoisomers. This is crucial for their three-dimensional arrangement and influences their physical and chemical properties. The convention is used to depict these open-chain forms, with horizontal lines representing bonds coming out of the plane and vertical lines going into the plane. The D and L configurations of carbohydrates are assigned based on the orientation of the hydroxyl group at the highest numbered chiral carbon in the , relative to as the reference standard. D-, the simplest , has its hydroxyl group on the right side of the projection and rotates plane-polarized light in the positive direction (dextrorotatory). Thus, sugars with the same configuration at this carbon as D- are classified as D-sugars, while those matching L- are L-sugars; most naturally occurring carbohydrates belong to the D series. Among the stereoisomers, epimers are diastereomers that differ in configuration at only one chiral center. Anomers are a specific type of that differ solely at the anomeric carbon (the carbonyl carbon in the open-chain form, C1 in aldoses), which becomes a new chiral center upon cyclization to form hemiacetals. In solution, anomers undergo , an interconversion between α (where the anomeric hydroxyl is trans to the CH₂OH group in the standard ) and β forms via the open-chain intermediate, reaching an equilibrium mixture. For D-glucose, this equilibrium comprises approximately 36% α-D-glucopyranose and 64% β-D-glucopyranose, with the open-chain form constituting less than 0.02%. Mutarotation exemplifies ring-straight chain tautomerism, where the cyclic opens to the aldehydic or ketonic form and recyclizes, governed by equilibrium constants that strongly favor the cyclic structures (K > 100 for most aldoses). The β-anomer often predominates in due to favorable and reduced steric hindrance, though the —a stereoelectronic stabilization favoring axial orientation of the anomeric substituent—can influence the ratio in non-aqueous environments. The of carbohydrates results in optical activity, the ability to rotate the plane of polarized light, with the direction and magnitude depending on the specific configuration and anomeric form. Pure α-D-glucose exhibits a [α]_D of +112°, while β-D-glucose has [α]_D of +18.7°; during , the value shifts to the equilibrium [α]_D of +52.7° for the mixture. This property arises from the asymmetric electronic environment around chiral centers and is a key tool for identifying and distinguishing isomers. In their cyclic forms, especially pyranose rings (six-membered), carbohydrates adopt preferred conformations to minimize steric strain. The chair conformation is overwhelmingly favored over the boat form, as it allows bulky hydroxyl and hydroxymethyl groups to occupy equatorial positions, reducing 1,3-diaxial interactions; for β-D-glucopyranose, all substituents are equatorial in the ^4C_1 . The conformation, while possible, is destabilized by interactions and is higher in energy by approximately 25-30 kJ/mol compared to the chair, making it rarely populated under physiological conditions. Interconversion between chair forms (via or twist-boat transition states) inverts axial and equatorial positions but preserves anomeric configuration.

Classification

Monosaccharides

Monosaccharides are the simplest form of carbohydrates, consisting of single sugar units that cannot be hydrolyzed into smaller carbohydrates. They are polyhydroxy aldehydes or ketones, typically containing three to seven carbon atoms, and serve as the building blocks for more complex carbohydrates. Monosaccharides are classified based on the number of carbon atoms in their backbone and the nature of their carbonyl functional group. By carbon number, they include trioses (3 carbons, such as glyceraldehyde), tetroses (4 carbons), pentoses (5 carbons, such as ribose), hexoses (6 carbons, such as glucose), and heptoses (7 carbons). Regarding the functional group, those with an aldehyde at the end of the chain are aldoses, while those with a ketone group are ketoses; for example, glucose is an aldose and fructose is a ketose. Common examples of monosaccharides include glucose, which functions as the primary blood sugar in humans and the main energy source for cells; , known as fruit sugar and prominent in and fruits; , a component of sugars; and , essential for the structure of . Glucose, for instance, exhibits anomeric isomerism in its cyclic forms, where the hydroxyl group at C1 can be axial or equatorial. Physically, monosaccharides are generally colorless, crystalline solids that are highly soluble in due to their multiple hydroxyl groups, which enable hydrogen bonding. Many possess a sweet taste, with being notably sweeter than glucose, approximately 1.7 times on a molar basis, making it a preferred in lower quantities. Chemically, most monosaccharides act as reducing sugars because their free or group can tautomerize to an aldehyde form, allowing them to reduce oxidizing agents like and participate in reactions such as the . In the , reducing monosaccharides react with under heat to form advanced end products, contributing to the browning, flavor, and aroma of cooked foods. Non-reducing sugars lack this free , but free monosaccharides are inherently reducing unless modified. In nature, monosaccharides like glucose occur widely, with glucose serving as the primary simple sugar produced by in , where and are converted into glucose using sunlight energy.

Disaccharides

Disaccharides are carbohydrates composed of two units linked together by a , formed through a dehydration synthesis reaction that releases a molecule. This linkage typically involves the anomeric carbon of one and a hydroxyl group on the other, resulting in a dimer with distinct chemical properties compared to its constituent . The in disaccharides can be classified by its configuration (α or β) and position of linkage, such as 1→4 or 1→6, which determines the molecule's specificity, digestibility, and biological function. For instance, α-1→4 linkages are common in energy-related disaccharides, while β-1→4 bonds often appear in structural or dietary contexts; these bonds are stereospecific, with the α form involving axial orientation and β involving equatorial, influencing recognition and . Common disaccharides include , , and , each with unique compositions and linkages. consists of α-D-glucose and β-D-fructose linked by an α-1→2 between their anomeric carbons, making it a non- as both anomeric positions are involved in the bond. comprises β-D-galactose and D-glucose connected via a β-1→4 , where the glucose unit retains a free anomeric carbon, classifying it as a . is formed by two D-glucose molecules joined by an α-1→4 , also a due to the free group on one glucose. Hydrolysis of disaccharides breaks the , yielding the constituent monosaccharides, and can occur via or enzymatic action. Specific enzymes facilitate this in biological systems: (also known as ) hydrolyzes into glucose and , primarily in the ; cleaves into and glucose; and breaks into two glucose molecules. These enzymes are proteins in mammals, enabling efficient of dietary disaccharides. The reducing properties of disaccharides depend on whether a free anomeric carbon () is available to form an open-chain or , which can reduce agents like . Maltose and are reducing disaccharides because one unit has an intact , whereas is non-reducing due to its linkage at both anomeric positions. In nature, disaccharides serve specific roles related to storage and nutrition. is the primary carbohydrate in mammalian , comprising about 4-5% of cow's milk and providing for infants. Sucrose occurs widely in plants as a transport sugar, found in high concentrations in (up to 20% by weight) and sugar beets, as well as in fruits and like apples and carrots. appears transiently as an intermediate during the enzymatic breakdown of in germinating seeds and grains, such as .

Oligosaccharides

Oligosaccharides are short-chain carbohydrates consisting of three to ten units connected primarily through glycosidic bonds, distinguishing them from disaccharides and longer . These molecules often feature branched architectures, which arise from diverse linkage types such as α-1,6 or β-2,1, allowing for structural complexity and functional specificity. Unlike linear forms, branched oligosaccharides enhance solubility and enable precise interactions in biological systems. The variety of glycosidic linkages in oligosaccharides contributes to their resistance to hydrolysis by human digestive enzymes, as many involve β-configurations or uncommon positions that evade and other hydrolases in the upper . This indigestibility allows oligosaccharides to reach the colon intact, where they undergo microbial . Representative examples include , a trisaccharide (galactose-α-1,6-glucose-α-1,2-β-fructose) that functions as a storage carbohydrate in seeds, providing energy during , and stachyose, a tetrasaccharide that similarly supports reserve in . Fructooligosaccharides (FOS), chains of units with a terminal glucose, act as prebiotics by selectively stimulating the proliferation of beneficial such as species. In biological contexts, s play critical roles in cell recognition and signaling, exemplified by the ABO blood group antigens on erythrocyte surfaces. These antigens are branched chains attached to proteins or , where specific terminal sugars—such as α-N-acetylgalactosamine for A or α-galactose for B—determine immune compatibility and facilitate self/non-self discrimination. are extracted from natural sources like soybeans and beans, where family members predominate; however, their presence in these foods leads to in humans due to colonic bacterial breakdown producing gases like and .

Polysaccharides

Polysaccharides are polymeric carbohydrates composed of more than ten units linked together by glycosidic bonds, forming long chains that can be linear or branched. These macromolecules serve primarily as or structural components in living organisms, with their properties determined by the type of , the configuration of glycosidic linkages, and the degree of branching. Unlike shorter oligosaccharides, often exhibit insolubility in and form extensive networks or granules essential for biological functions. Among storage polysaccharides, starch predominates in plants, consisting of two components: amylose, a linear polymer of α-D-glucose units connected by α-1,4-glycosidic bonds, and amylopectin, a highly branched structure with α-1,4-linked chains and α-1,6 branches at branch points. This combination allows plants to store glucose efficiently in granules within seeds, roots, and tubers for later mobilization during growth or stress. In animals, glycogen fulfills a similar role as the primary energy reserve, stored mainly in liver and muscle tissues; it is a highly branched polymer of α-D-glucose with α-1,4 linkages in linear segments and α-1,6 branches, enabling rapid enzymatic breakdown to release glucose when energy demands increase. Structural provide rigidity and protection, with being the most abundant organic polymer on Earth, forming linear chains of β-D-glucose units joined by β-1,4-glycosidic bonds that enable hydrogen bonding into strong microfibrils in plant cell walls. , another key structural polysaccharide, consists of β-1,4-linked N-acetyl-D-glucosamine units and forms tough, flexible frameworks in the exoskeletons of arthropods and cell walls of fungi, contributing to mechanical support and defense. These β-linkages confer resistance to compared to the α-linkages in storage forms. The biodegradability of varies by structure and organism; for instance, is highly degradable by microbial enzymes like in soil and fungi, facilitating nutrient in ecosystems, but humans lack endogenous and thus cannot digest , relying on for limited that yields rather than glucose. This indigestibility positions as , promoting gut health without caloric contribution. Industrially, is extracted from corn via wet milling processes that separate germ, , and protein to yield purified for food, adhesives, and biofuels, while is isolated from wood or agricultural residues through pulping to produce and textiles.

Biological Functions

Structural Roles

Carbohydrates serve essential structural roles in biological systems, forming rigid frameworks that maintain cellular integrity and facilitate interactions. In plant cell walls, , a linear composed of β-1,4-linked glucose units, provides tensile strength and supports , enabling plant growth and rigidity. In bacterial cell walls, , a of and N-acetylmuramic acid cross-linked by bridges, forms a mesh-like structure that withstands and preserves cell shape. Similarly, , a β-1,4-linked of , constitutes the primary structural component of fungal cell walls, where it associates with glucans to confer mechanical strength and resistance to environmental stress, and of exoskeletons, providing a protective, lightweight armor. Additionally, carbohydrates are integral to the structure of nucleic acids, which carry genetic information. Ribose, a five-carbon monosaccharide, forms the sugar-phosphate backbone of ribonucleic acid (RNA), while its deoxy form (2-deoxyribose) does the same in deoxyribonucleic acid (DNA). These backbones provide stability and enable the attachment of nucleotide bases, facilitating the double-helical structure of DNA and the functional folding of RNA. Beyond cell walls, carbohydrates contribute to architecture and recognition through glycoproteins and glycolipids. Glycoproteins, where chains are covalently attached to proteins, and glycolipids, with carbohydrates linked to anchors, are embedded in the plasma membrane's outer leaflet, exposing their glycan moieties for specific interactions. These structures mediate cell-cell recognition, adhesion, and signaling; for instance, gangliosides—sialic acid-containing glycolipids—modulate responses and binding on neuronal and immune cells. In the (ECM), glycosaminoglycans (GAGs) such as form hydrated networks that provide structural support and influence tissue mechanics. , an unsulfated, high-molecular-weight GAG, binds water extensively to create a viscoelastic scaffold that facilitates and maintains tissue hydration in connective tissues like and . Other GAGs, often bound to core proteins in proteoglycans, further organize the ECM by interacting with and to regulate tissue architecture. Oligosaccharides also play a critical role in cell surface identity, particularly as blood group determinants on red blood cells (RBCs). The ABO blood group antigens are chains attached to proteins and on the RBC surface, where specific terminal sugars—such as α-N-acetylgalactosamine for or α-galactose for —determine compatibility in transfusions and influence immune recognition. These glycan structures extend from the membrane, acting as molecular tags for self/non-self discrimination.

Energy Storage and Metabolism

Carbohydrates, primarily in the form of glucose, serve as the source for most living organisms through metabolic pathways that convert them into usable in the form of (ATP), providing approximately 4 kcal per gram of carbohydrate metabolized. These pathways include both catabolic processes for release and anabolic processes for storage, ensuring a balance between immediate needs and long-term reserves. The initial breakdown of glucose occurs via , a ten-step enzymatic process in the that converts one of glucose into two molecules of pyruvate, yielding a net gain of 2 ATP and 2 NADH molecules per glucose. The overall reaction for glycolysis is: C6H12O6+2NAD++2ADP+2Pi→2CH3COCOOH+2NADH+2ATP+2H2O+2H+\text{C}_6\text{H}_{12}\text{O}_6 + 2 \text{NAD}^+ + 2 \text{ADP} + 2 \text{P}_i \rightarrow 2 \text{CH}_3\text{COCOOH} + 2 \text{NADH} + 2 \text{ATP} + 2 \text{H}_2\text{O} + 2 \text{H}^+ This anaerobic phase provides quick energy but limited yield, with pyruvate then entering further depending on oxygen availability. In aerobic conditions, pyruvate is transported into the mitochondria, decarboxylated to , and enters the (also known as the Krebs or tricarboxylic acid cycle), where it is fully oxidized to CO₂, generating additional NADH, FADH₂, and 2 ATP per glucose via . The electron carriers NADH and FADH₂ donate electrons to the in , driving ATP synthesis through a proton gradient; the total yield from complete aerobic oxidation of one glucose molecule is approximately 30–32 ATP, accounting for inefficiencies in the process. For energy storage, excess glucose is converted to glycogen through , a process primarily in liver and muscle cells where glucose-6-phosphate is activated and polymerized into branched glycogen chains for rapid mobilization when needed. Conversely, synthesizes glucose from non-carbohydrate precursors like lactate, , or , mainly in the liver during to maintain blood glucose levels, bypassing irreversible steps of through specialized enzymes. Hormonal regulation tightly controls these pathways to maintain blood glucose homeostasis; insulin, secreted by pancreatic beta cells in response to high blood glucose, promotes , , and while inhibiting and . , released by alpha cells during low blood glucose, opposes insulin by stimulating , , and inhibiting to raise blood glucose levels. In anaerobic conditions, such as during intense exercise or in oxygen-limited environments like yeast , pyruvate is reduced to regenerate NAD⁺ for continued , producing lactate in animal cells () or ethanol and CO₂ in microorganisms (), with no additional ATP beyond the net 2 from .

Nutritional Aspects

Carbohydrates are classified in the diet as simple or complex based on their and impact on glucose levels. Simple carbohydrates include monosaccharides like glucose and disaccharides such as and , which are quickly digested and absorbed, leading to rapid increases in sugar. Complex carbohydrates encompass oligosaccharides and like starches and fibers, which require more extensive breakdown and result in slower, more sustained energy release. The () provides a scale to assess this effect, with pure glucose assigned a value of 100; foods with low (≤55), such as most fibers, cause gradual rises in glucose, while high- foods (≥70) like some refined sugars provoke sharp spikes. Digestion of carbohydrates begins in the with salivary , which hydrolyzes starches into and dextrins at an optimal of around 6.7. This process continues in the , where pancreatic further breaks down starches, and enzymes complete the : sucrase cleaves into glucose and , while splits into glucose and . These monosaccharides are then absorbed by enterocytes into the bloodstream for distribution. Deficiencies in , common in adults, can lead to , causing gastrointestinal discomfort from undigested reaching the colon. Health authorities recommend that carbohydrates comprise 45-65% of total daily caloric intake for adults, equating to approximately 200-300 grams on a 2,000-calorie diet, with an emphasis on whole sources to support sustained energy and metabolic health. As an essential macronutrient in a balanced diet, carbohydrates provide the primary source of energy; however, excess intake can contribute to obesity, while deficiency may result in weakness and fatigue due to low blood glucose levels. , a non-digestible complex carbohydrate, should be consumed at 25-30 grams per day, primarily from fruits, , and whole grains, to promote digestive regularity and overall well-being. The aligns with this by advocating at least 25 grams of daily alongside reduced intake of free sugars to mitigate noncommunicable diseases. Low-carbohydrate diets, often restricting intake to under 130 grams daily, can induce —a state where the body burns fat for fuel—leading to short-term , particularly in individuals with or risk, by reducing insulin levels and appetite. However, such diets may increase risk, especially in those on insulin or , due to diminished glucose availability. Conversely, high intake of refined sugars, classified as free sugars, is linked to elevated diabetes risk through and ; 2020s guidelines from the and WHO urge limiting added sugars to less than 10% of calories (ideally 5%) to curb this. Dietary fiber offers protective health effects, including improved gut microbiota diversity that enhances barrier function and reduces inflammation, as well as binding bile acids to lower LDL cholesterol levels and cardiovascular disease risk. Soluble fibers like beta-glucans from oats exemplify this by slowing cholesterol absorption in the small intestine. These benefits, along with aiding digestion and preventing constipation, underscore fiber's role in preventing chronic conditions beyond basic nutrition.

Sources and Applications

Natural Sources

Carbohydrates represent the most abundant class of organic compounds on , accounting for over half of the biosphere's . In plant biomass, they comprise approximately 75% of dry weight, mainly as like and in cell walls, alongside storage forms such as . From an evolutionary perspective, carbohydrates trace their origins to , the ancient process in which autotrophic organisms convert and into glucose using , establishing glucose as the foundational building block for more complex carbohydrates across kingdoms. Plants serve as the primary natural reservoirs of carbohydrates, with diverse forms distributed throughout their tissues. Starch accumulates as an energy storage molecule in seeds of grains like , , and corn, as well as in underground storage organs such as potatoes and other tubers. , a linear of glucose, provides structural rigidity in the cell walls of , leaves, and stems, forming the bulk of structural . Fruits, meanwhile, are rich in simple sugars including , glucose, and , which aid in and energy provision during ripening. Animals synthesize carbohydrates in limited quantities, primarily for internal storage and specific secretions. Glycogen, a branched analogous to plant , is stored in the liver and skeletal muscles to maintain blood glucose levels and support rapid energy demands. In mammals, —a of glucose and —occurs exclusively in , providing an essential energy source for nursing offspring. Microorganisms contribute significantly to carbohydrate diversity, with bacteria producing extracellular polysaccharides like alginate, synthesized by genera such as and for protective capsules, biofilms, and environmental adaptation.

Industrial Synthesis and Uses

Carbohydrates are synthesized industrially through chemical and enzymatic methods to produce specific monosaccharides and oligosaccharides for various applications. The Kiliani-Fischer synthesis, developed in the late , remains a foundational chemical approach for elongating the carbon chain of aldoses to produce higher monosaccharides, such as converting an aldopentose to aldohexoses like glucose and , by adding a intermediate followed by and reduction. This method is particularly useful in laboratory-scale production of rare sugars, enabling the creation of epimeric mixtures that can be separated for targeted synthesis. Enzymatic synthesis has advanced significantly for oligosaccharides, leveraging glycosyltransferases to catalyze the regioselective and stereospecific transfer of moieties from nucleotide-activated donors to acceptor substrates, often under mild aqueous conditions without the need for protecting groups. Glycosyltransferases, classified into families based on structural motifs, facilitate the construction of complex glycan structures, with efforts improving substrate specificity and yield for scalable production. These biocatalytic processes are increasingly automated, allowing for high-throughput assembly of oligosaccharides that mimic natural glycans. In industrial applications, starch hydrolysis is a cornerstone process for producing (HFCS), where is first liquefied with alpha-amylase to dextrins, then saccharified with glucoamylase to glucose, and finally isomerized to using , yielding syrups with 42-55% content for widespread use as sweeteners in beverages and processed foods. , derived from plant sources like wood pulp, is processed into fibers for textiles such as and viscose, where it is dissolved in and to form regenerated fibers valued for their breathability and absorbency in apparel and home furnishings. In paper production, provides the structural backbone, with pulping and refining steps yielding high-strength sheets for , , and hygiene products, accounting for over 90% of global paper composition. Chitosan, obtained by deacetylation of from crustacean shells, is utilized in biomedical fields for its and properties, serving as a in for wound dressings that promote and regeneration, and as a vector in systems for controlled release of therapeutics like antibiotics and genes. Fermentation processes convert into bioethanol through enzymatic to glucose followed by yeast-mediated anaerobic , producing up to 400 liters of per metric ton of corn in dry-grind facilities, which serves as a renewable additive to reduce dependence. Emerging biotechnological advancements in the 2020s focus on producing prebiotic oligosaccharides, such as galacto- and xylo-oligosaccharides, via engineered enzymes like inulosucrases and glycoside hydrolases in microbial hosts, enabling high-yield from inexpensive substrates like , with yields exceeding 200 g/L for applications in functional foods that support health. Additionally, recent advances include the direct synthesis of platform chemicals like 2,5-furandicarboxylic acid from carbohydrates for production, enhancing sustainable material alternatives as of 2025. These methods incorporate and immobilized biocatalysts to enhance and purity, addressing challenges for commercial prebiotic supplements.

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