CARBOHYDRATE CLASSIFICATION (WITH PICTURES) - NUTRITION - 2025

The classification of carbohydrates 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 a metabolic point of view.

Cyclic structure of glucose, a hexose (Source: Edgar181, via Wikimedia Commons)

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 bananas, for example; as well as many seeds of leguminous 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.

Classification according to their function

Carbohydrates can be classified, according to their general function, into two large classes: structural carbohydrates and universally digestible carbohydrates or polysaccharides.

Structural carbohydrates

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".

General structure of cellulose (Source: Vicente Neto via Wikimedia Commons)

Among these, the main plant polysaccharide is cellulose, but lignin, dextrans, pentosans, agar (in algae) and chitin (in fungi and many arthropods) also stand out.

Digestible carbohydrates

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. This 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 significant commercial value, is also a rich source of digestible carbohydrates, but of animal origin.

Glycogen is an important reserve polysaccharide in animals (Source: Alejandro Porto via Wikimedia Commons)

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.

Classification according to the number of carbon atoms

Depending on the number of carbon atoms, carbohydrates can be:

- Trioses, with three carbons (example: glyceraldehyde)

- Tetrosas, with four carbons (example: erythrose)

- Pentoses with five carbons (eg ribose)

- Hexoses, with six carbons (example: glucose)

- Heptoses, with seven carbons (example: sedoheptulose 1,7-bisphosphate)

Diagram of possible hemiacetal structures for glucose and mannose (Source: Karlhahn via Wikimedia Commons)

The pentoses and hexoses, in general, can be 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 the furanose and pyranose are formed.

Classification according to the position of the carbonyl group

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.

Aldosas and Cetosas (Source: Pjvelasco, via Wikimedia Commons)

If the carbon atom that forms the carbonyl group is in position 1 (or at one end), then it is an aldehyde. Instead, if it is in position 2 (or 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.

Classification according to the number of units that make them up

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:

Monosaccharides

They are the simplest saccharides or sugars, as they are made up of a single "sugar unit". In this group are metabolically relevant sugars 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

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.

Chemical structure of maltose, a disaccharide (Source: NEUROtiker via Wikimedia Commons)

Oligosaccharides

They correspond to those carbohydrates that, when hydrolyzed, release more than two “sugar units”. Although they are perhaps not well known, in this group raffinose, stachyose and verbascosa can be singled out. Some authors consider that disaccharides are also oligosaccharides.

Polysaccharides

Polysaccharides are composed of more than 10 sugar units and can be made up of repeating units of the same monosaccharide (homopolysaccharides) or of 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 a glycosidic bond, which takes place thanks to the loss of a water molecule.

Classification of its derivatives

Just 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:

Phosphate esters

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.

Prominent examples include glyceraldehyde 3-phosphate, glucose 6-phosphate, glucose 1-phosphate, and fructose 6-phosphate.

Acids and lactones

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.

Alditols, polyols or sugar alcohols

They are formed by the oxidation of the carbonyl group of some monosaccharides; examples of these are erythritol, mannitol, and sorbitol or glucitol.

Amino sugars

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.

Glucosamine chemical structure (Source: Edgar181 via Wikimedia Commons)

Deoxysugars

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 backbone of DNA, that is, 2-deoxyribose, but there are also 6-deoxymanopyranose (rhamnose) and 6-deoxygalactofuranose (fucose).

Glycosides

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 a different hydroxylated compound.

Classic examples are ouabain and amygdalin, two widely used compounds that are extracted from an African bush and the seeds of bitter almonds, correspondingly.

Classification according to its use in food preparation

Sugar cubes (Source: Dietmar Rabich / Wikimedia Commons / “Würfelzucker - 2018 - 3564” / CC BY-SA 4.0 via Wikimedia Commons)

Finally, carbohydrates can also be classified according to the use that can be given to them 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, as is the case with starches and pectins, for example.

References

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