Carbohydrates (main functions)

Carbohydrates play a variety of functions in nature. Because of this, they are indispensable elements for living beings. The main functions of carbohydrates are:

- Metabolic fuel – various monosaccharides may be used as a source of chemical energy through its catabolism. Logically, the main carbohydrate used as metabolic fuel is glucose. However, there are several other monosaccharides that can also be used as metabolic fuel (more information on this subject here), such as fructose, mannose or galactose;

 

- Nucleotide components– this function is performed by two different pentose, ribose and deoxyribose. Actually, only one of these carbohydrates is a “pure” monosaccharide (ribose), the other is a derivative of monosaccharide (deoxyribose). Soon, I will write a post about this... Both ribose and deoxyribose are pentoses, that means, they are monosaccharides with 5 carbons. Ribose enters in the composition of ribonucleotides (and consequently RNA) while deoxyribose takes part of the composition of deoxyribonucleotides (and hence the DNA);

- Metabolic fuel reserve - some polysaccharides play the function of metabolic fuel reserve. In this context, there are two molecules that deserve a highlight: starch and glycogen. Both are composed of a single type of monosaccharide, glucose. Starch is the reserve polysaccharide of glucose in plant cells, while glycogen is the reserve polysaccharide in animal cells;

 

- Structural function - some polysaccharides play structural functions, including cellulose and peptidoglycan. The first is the main component of cell walls of plant cells, while the latter is the main component of the cell wall of prokaryotes;

 

- Protection - some polysaccharides play a protective function, such as chitin, which is the main component of the exoskeleton of insects;

- Lubrication and hydration - due to their rich composition in hydrophilic functional groups, carbohydrates have the ability to interact with a large number of water molecules. Because of this feature, various polysaccharides form viscous and highly hydrated mixtures. These polysaccharides are referred to as glycosaminoglycans and are essential for the skin, joints, etc.

 

- Recognition and cell adhesion - there are several molecules involved in cell adhesion and recognition. These molecules are found on the cell surface and have carbohydrate components, being called glycoproteins or glycolipids.

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Carbohydrates (general characteristics)

Carbohydrates, also referred to as sugars, are a class of biomolecules characterized by the presence of many polar groups in its composition. The building block of the carbohydrates are the monosaccharides, since any carbohydrate has one, or more than one, monosaccharide. Consequently, they can be grouped into different classes, namely, monosaccharides, oligosaccharides and polysaccharides.

When only one or a few monosaccharides are present, usually the carbohydrate has a sweet taste and is therefore referred to as sugars. In fact, when looking for a label of a food product, it is common an information like "Carbohydrates of which sugars". This information may cause some confusion, because in fact there is some ambiguity in the designation of sugar. If some people call sugars to carbohydrates, there are those who use this designation only to carbohydrates that are sweet.
Carbohydrates are the most abundant class of biomolecules in nature, being also the most abundant class of biomolecules in our food, and should correspond to 45-75% of total energy intake.
Carbohydrates exist in a free form, i.e. without being linked to other types of molecules. In this case, they are referred to as poly-hydroxyaldehydes or poly-hydroxyketones, since they present several hydroxyl groups and one carbonyl group which can be aldehyde or ketone, respectively (if you have any questions about these functional groups, you can find more information about them HERE and HERE). 

If carbohydrates are combined with other molecules, the resulting molecule is referred to as a glycoconjugate, being the most well-known glycoconjugates the glycoproteins and glycolipids.

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