The classification of carbohydrates It can be done according to their function, according to the number of carbon atoms, according to the position of the carbonyl group, according to the units that make them up, according to derivatives and according to foods.
Carbohydrates, carbohydrates or saccharides, are chemical compounds made up of carbon, hydrogen and oxygen atoms, the combustion of which results in the release of carbon dioxide and one or more water molecules. They are molecules widely distributed in nature and of fundamental importance for living beings, both from a structural and metabolic point of view..
Usually, the best way to represent the formula of any carbohydrate is Cx (H2O) and that, in a nutshell, means "hydrated carbon".
In plants, a large part of carbohydrates are produced during photosynthesis from carbon dioxide and water, after which they can be stored in high molecular weight complexes (starches, for example) or used to give structure and support to plant cells (cellulose, for example).
Animals also produce carbohydrates (glycogen, glucose, fructose, etc.), but they do so from substances such as fats and proteins. Despite this, the main source of metabolizable carbohydrates for animal organisms is that which comes from plants..
The most important natural sources of carbohydrates for man are, generally, cereals such as wheat, corn, sorghum, oats and others; tubers such as potatoes, cassava and plantain, for example; in addition to many seeds of legume plants such as lentils, beans, broad beans, etc..
Carnivorous animals, that is, those that feed on other animals, indirectly depend on carbohydrates to survive, since their prey, or the prey of their prey, are herbivorous animals capable of taking advantage of the structural and storage carbohydrates contained in herbs. they ingest and convert them into protein, muscle, and other body tissues.
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Carbohydrates can be classified, according to their general function, into two large classes: structural carbohydrates and universally digestible carbohydrates or polysaccharides..
Structural carbohydrates are those that are part of the wall of all plant cells, as well as the secondary deposits that characterize the tissues of different plant species and that fulfill a specific function of support and "scaffolding".
Among these, the main plant polysaccharide is cellulose, but lignin, dextrans, pentosans, agar (in algae) and chitin (in fungi and in many arthropods) also stand out..
Digestible carbohydrates, on the other hand, are those that heterotrophic organisms (other than autotrophs that "synthesize their own food") can acquire from plants and use to nourish their cells through different metabolic pathways..
The main digestible carbohydrate is starch, which is found in tubers, cereal seeds, and many other storage structures in plants. It is made up of two similar types of polysaccharides, amylose and amylopectin..
However, simpler sugars such as fructose, for example, present in large quantities in the fruits of many plant species, are also very important..
Honey, a substance produced by bees that has an important commercial value, is also a rich source of digestible carbohydrates, but of animal origin.
Glycogen, considered in many cases as "animal starch", is a reserve polysaccharide synthesized by animals and can be included in the group of digestible carbohydrates.
Depending on the number of carbon atoms, carbohydrates can be:
- Trios, with three carbons (example: glyceraldehyde)
- Tetrosas, with four carbons (example: erythrose)
- Pentosas, with five carbons (example: ribose)
- Hexoses, with six carbons (example: glucose)
- Heptoses, with seven carbons (example: sedoheptulose 1,7-bisphosphate)
Pentases and hexoses, in general, can be found in the form of stable rings thanks to the formation of an internal hemiacetal group, that is, through the union between an aldehyde group or a ketone group with an alcohol.
These rings can have 5 or 6 "links", so they can be of the furan type or of the pyran type, correspondingly, with which furanose and pyranose are formed..
The position of the carbonyl group (C = O) in monosaccharides is also a character used for their classification, because depending on this, the molecule can be a ketosis or an aldose. Thus there are, for example, aldohexoses and ketohexoses, as well as aldopentoses and ketopentoses..
If the carbon atom that forms the carbonyl group is in position 1 (or at one end), then it is an aldehyde. On the other hand, if it is in position 2 (or in any other internal carbon atom), it is a ketone group, so it becomes a ketosis.
Taking as an example the trioses, tetroses, pentoses and hexoses of the previous section, we have that the aldoses of these simple sugars are glyceraldehyde, erythrose, ribose and glucose, meanwhile the ketoses are dihydroxyacetone, erythrulose, ribulose and fructose, respectively.
According to the number of units that carbohydrates have, that is, according to the number of sugars that results from their hydrolysis, they can be classified as:
They are the simplest saccharides or sugars, as they are made up of a single "sugar unit". In this group there are sugars as metabolically relevant as glucose, whose metabolism involves the production of energy in the form of ATP in the cells of practically all living organisms. Galactose, mannose, fructose, arabinose, xylose, ribose, sorbose and others also stand out..
Disaccharides, as the prefix of their name implies, are saccharides made up of two sugar units. The main examples of these molecules are lactose, sucrose, maltose and isomaltose, cellobiose, gentiobiose, melibiose, trehalose and turanose..
They correspond to those carbohydrates that, when hydrolyzed, release more than two “sugar units”. Although perhaps not very well known, in this group raffinose, stachyose and verbascosa can be singled out. Some authors consider that disaccharides are also oligosaccharides.
Polysaccharides are composed of more than 10 sugar units and can be made up of repeating units of the same monosaccharide (homopolysaccharides) or by relatively complex mixtures of different monosaccharides (heteropolysaccharides). Examples of polysaccharides are starch, cellulose, hemicellulose, pectins and glycogen.
Usually, the union between the "sugar units" of disaccharides, oligosaccharides and polysaccharides occurs through a bond known as glycosidic bond, which takes place thanks to the loss of a water molecule.
As is true of many molecules of great importance in nature, carbohydrates can function as "building blocks" for other compounds that can perform similar or radically different functions. According to this, such derivatives can be classified, according to their characteristics, as follows:
They are generally phosphorylated monosaccharides, in which the phosphoryl group is attached to the saccharide through an ester bond. These are extremely important molecules for a large part of cellular metabolic reactions, since they behave as “activated compounds” whose hydrolysis is thermodynamically favorable..
Among the most prominent examples are glyceraldehyde 3-phosphate, glucose 6-phosphate, glucose 1-phosphate and fructose 6-phosphate.
They are the product of the oxidation of certain monosaccharides with particular oxidizing agents. Aldonic acids result from the oxidation of glucose with alkaline copper and these, in solution, are in equilibrium with lactones. When oxidation is directed by enzymatic catalysis, lactones and uronic acids can be produced.
They are formed by the oxidation of the carbonyl group of some monosaccharides; examples of these are erythritol, mannitol and sorbitol or glucitol.
They are derivatives of monosaccharides to which an amino group (NH2) has been attached, generally at the carbon of position 2 (especially in glucose). The most prominent examples are glucosamine, N-acetyl glucosamine, muramic acid, and N-acetyl muramic acid; there is also galactosamine.
They are derivatives of monosaccharides that are produced when they lose an oxygen atom in one of their hydroxyl groups, which is why they are known as "deoxy" or "deoxysugars".
Among the most important are those that make up the DNA backbone, that is, 2-deoxyribose, but there are also 6-deoxymanopyranose (rhamnose) and 6-deoxygalactofuranose (fucose).
These compounds result from the elimination of a water molecule by the union between the anomeric hydroxyl group of a monosaccharide and a hydroxyl group of another different hydroxylated compound..
Classic examples are ouabain and amygdalin, two widely used compounds that are extracted from an African bush and from the seeds of bitter almonds, correspondingly.
Finally, carbohydrates can also be classified according to their use in the course of preparing a culinary dish. In this sense, there are sweetening carbohydrates, such as sucrose (a disaccharide), fructose (a monosaccharide) and to a lesser extent maltose (another disaccharide).
Likewise, there are thickening carbohydrates and gelling carbohydrates, such as starches and pectins, for example..
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