Carbohydrates: definition, classification and functions

What are carbohydrates?

Fig. 1 – Bread

Carbohydrates, together with lipids, proteins and nucleic acids, are one of the four major classes of biologically essential organic molecules found in all living organisms.
Carbohydrates, all coming from the process of photosynthesis, represent the major part of organic substance on Earth, are the most abundant organic components in the major part of fruits, vegetables, legumes and cereal grains, carry out many functions in all living organisms and are the major energy source in a Mediterranean-type diet. Finally, they provide flavor and texture in many processed foods.

Chemical classification of carbohydrates

Carbohydrates, also called Carbs, are defined as aldehydic or ketonic compounds with a some number of oxydrilic groups (so polyhydroxy aldehydes or ketones as well).
Many of them, but not all, have general formula (CH2O)n (only molecules with n>4 are considered carbohydrates); some, in addition to carbon (C), oxygen (O) and hydrogen (H), include nitrogen or sulfur.

On the basis of the number of forming units, three major classes of carbohydrates can be defined: monosaccharides, oligosaccharides and polysaccharides.

  • Monosaccharides or simply sugars are formed by only one polyhydroxy aldehydeidic or ketonic unit.
    The most abundant monosaccharide is D-glucose, also called dextrose.
  • Oligosaccharides are formed by short chains of monosaccharidic units (from 2 to 20) linked one to the next by chemical bounds, called glycosidic bounds.
    The most abundant oligosaccharides are disaccharides, formed by two monosaccharides, and especially in the human diet the most important are sucrose (common table sugar), lactose and maltose. Within cells many oligosaccharides formed by three or more units do not find themselves as free molecules but linked to other ones, lipids or proteins, to form glycoconjugates.
  • Polysaccharides are polymers consisting of 20 to 107 monosaccharidic units; they differ each other for the monosaccharides recurring in the structure, for the length and the degree of branching of chains or for the type of links between units.
    Whereas in the plant kingdom several types of polysaccharides are present, in vertebrates there are only a small number.
    Polysaccharides are defined

omopolysaccharides if they contain only one type of monosaccharide as starch, glycogen and chitin;

eteropolysaccharides, instead, contain two or more different kinds (e.g. hyaluronic acid).

Note: the term “saccharide” derives from the greek word “sakcharon”, which means sugar.

Physiological classification of carbohydrates

On the basis of their degree of polymerization, they can be classified as:

  • simple: mono- and disaccharides (also known as “sugars”) and tri- and tetrasaccharides (oligosaccharides);
  • complex: the polysaccharides.

A further classification lays the foundations on the possibility of being used directly for energy purpose, so:

  • available, as glucose, fructosegalactose between monosaccharides, sucroselactosemaltose and maltodextrin between oligosaccharides, and starch and glycogen between polysaccharides;
  • not available, as xylose (monosaccharide), lactulose (see lactose) and raffinose (respectively di- and trisaccharide), fiber (cellulose, hemicellulose, lignin, pectins etc.) and resistant or not digestible starch (polysaccharides). The members of this class, also if ingested, are not digestible nor absorbable and will be fermented by intestinal flora with release of short chain fatty acids and so yielding some energy.

Functions of carbohydrates 

  • They are used as material for energy storage and production.
    Starch and glycogen, respectively in plants and animals, are stored carbohydrates from which glucose can be mobilized for energy production. Glucose can supply energy both fueling ATP synthesis (ATP, the cell’s energy currency, has inside a phosphorylated sugar) and in the form of reducing power as NADPH.
    It should be noted that glucose, used as energy source, “burns” without yielding metabolic wastes, being turned in CO2 and water, and of course releasing energy.
    Monosaccharides supply 3.74 kcal/g, disaccharides 3.95 kcal/g, while starch 4.18 kcal/g; on average it is approached to 4 kcal/g.
  • They exert a protein-saving action: if present in adequate amount in daily nourishment, the body does not utilize proteins for energy purpose, an anti-economic and “polluting” fuel because it will need to eliminate nitrogen (ammonia) and sulfur present in some aminoacids.
  • Their presence is necessary for the normal lipid metabolism. More than 100 years ago Pasteur said: “Fats burn in the fire of carbohydrates“. This idea continues to receive confirmations from the recent scientific studies. Moreover, excess carbohydrates may be converted in fatty acids and triglycerides (processes that occur mostly in the liver).
  • Glucose is indispensable for the maintenance of the integrity of nervous tissue (some central nervous system areas are able to use only glucose for energy production) and red blood cells.
  • Two sugars, ribose and deoxyribose, are part of the bearing structure, respectively of the RNA and DNA and obviously find themselves in the nucleotide structure as well.
  • They take part in detoxifying processes. For example, at hepatic level glucuronic acid, synthesized from glucose, combines with endogenous substances, as hormones, bilirubin etc., and exogenous substances, as chemical or bacterial toxins or drugs, making them atoxic, increasing their solubility and allowing their elimination.
  • They are also found linked to many proteins and lipids. Within cells they act as signals that determine the metabolic fate or the intracellular localization of the molecules which are bound. On the cellular surface their presence is necessary for identification processes between cells that are involved e.g. in the recognition between spermatozoon and oocyte during fertilization, in the return of lymphocytes in the lymph nodes of provenance or still in the leukocyte adhesion to the lips of the lesion of a blood vessel.
  • Two homopolysaccharides, cellulose (the most abundant polysaccharide in nature) and chitin (probably, next to cellulose, the second most abundant polysaccharide in nature), serve as structural elements, respectively, in plant cell walls and exoskeletons of nearly a million species of arthropods (e.g. insects, lobsters, and crabs).
  • Heteropolysaccharides provide extracellular support for organisms of all kingdoms: in bacteria, the rigid layer of the cell wall is composed in part of a heteropolysaccharide contained two alternating monosaccharide units while in animals the extracellular space is occupied by several types of heteropolysaccharides, which form a matrix with numerous functions, as hold individual cells together and provide protection, support, and shape to cells, tissues, and organs.

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