Carbohydrate
A carbohydrate is a sugar or a sugar derivative. 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. Together with amino acids, fats, and nucleic acids, the carbohydrates are one of the major families of biomolecules.
Carbohydrates perform numerous roles in living organisms. Polysaccharides serve as an energy store and as structural components. The 5-carbon monosaccharide ribose is an important component of coenzymes 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.
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, potatoes, and processed food based on cereal flour, such as bread, pizza or pasta. Sugars appear in the human diet mainly as table sugar, lactose, 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
The term "carbohydrate" has many synonyms and the definition can depend on context. Terms associated with carbohydrate include "sugar", "saccharide", "glucan", and "glucide". In food science the term "carbohydrate" often means any food that is rich in starch or simple carbohydrates, or fairly simple sugars such as sucrose. Carbohydrates can also refer to dietary fiber, like cellulose.Saccharides
The starting point for the discussion of carbohydrates is the saccharides. Monosaccharides are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. Monosaccharides usually have the formula Cm n. Disaccharides are common as are polysaccharides/oligosaccharides. 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, sulfate, carboxylic acid and deoxy modifications.| Class | Subgroup | Components |
| Sugars | Monosaccharides | Glucose, galactose, fructose, xylose |
| Sugars | Disaccharides | Sucrose, lactose, maltose, isomaltulose, trehalose |
| Sugars | Polyols | Sorbitol, mannitol |
| Oligosaccharides | Malto-oligosaccharides | Maltodextrins |
| Oligosaccharides | Other oligosaccharides | Raffinose, stachyose, fructo-oligosaccharides |
| Polysaccharides | Starch | Amylose, amylopectin, modified starches |
| Polysaccharides | Non-starch polysaccharides | Glycogen, Cellulose, Hemicellulose, Pectins, Hydrocolloids |
Complex carbohydrates
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. 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.History
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 isolation of crystalline sugar. 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 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 in 1844. In 1856, glycogen, a form of carbohydrate storage in animal livers, was discovered by French physiologist Claude Bernard. 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 was coined in 1988 by Raymond Dwek to recognize the coming together of the traditional disciplines of carbohydrate chemistry and biochemistry. 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.
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. 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. 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. 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.
The Institute of Medicine recommends that American and Canadian adults get between 45 and 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. A 2017 Cochrane Systematic Review concluded that there was insufficient evidence to support the claim that whole grain diets can affect cardiovascular disease.
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. 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.