Carbohydrates, sugars, saccharides – organic chemicals consisting of carbon atoms and hydrogen and oxygen, usually in the ratio H: O = 2: 1. They are compounds that simultaneously contain numerous hydroxyl and carbonyl groups, and sometimes semi-acetal bridges. The general summary formula for carbohydrates is CxH2yOy or Cx (H2O) y (however, carbohydrates not meeting this formula, e.g. deoxyribose, are known).
By the number of sugar units in the molecule, carbohydrates are divided into:
• simple sugars, or monosaccharides (monosaccharides),
• oligosaccharides: disaccharides, trisaccharides, tetra-, penta-, hexa-, hepta-, octa-, nona- and decasaccharides,
• polysaccharides, or polysaccharides.
Nomenclature in dietetics and food industry
The term “sugars” is commonly used to describe monosaccharides and disaccharides in foods, with the exception of polyhydric alcohols. So it appears on American and European labels and in databases (e.g. USDA National Nutrient Database) of food products. The word “carbohydrates” (in terminology “available carbohydrate”) means all carbohydrates that undergo metabolism in the human body, including polyhydric alcohols. Thus, non-absorbable fiber is not one of them. The concept of “total carbohydrates” includes all types of carbohydrates regardless of their physiological and nutritional properties and the variable content of non-carbohydrate substances in food. The content of such a sum of carbohydrates is determined as the difference in the sum of all other ingredients (water, protein, fat, ash and alcohol).
Simple sugars due to the number of carbon atoms in a single molecule are divided into:
• trioses with 3 carbon atoms, e.g. glycerol aldehyde,
• tetroses with 4 carbon atoms, e.g. threose,
• pentoses with 5 carbon atoms, e.g. ribose, ribulose,
• hexose with 6 carbon atoms, e.g. glucose, galactose and fructose,
• heptoses with 7 carbon atoms, eg sedoheptulose.
Most biologically important monosaccharides have 5 or 6 carbon atoms, although 3 and 4 carbon monosaccharides are also important in cell physiology (photosynthesis, Krebs cycle), and monosaccharides and their derivatives with more than 6 carbon atoms are also found.
Monosaccharides can also be divided into:
• aldoses with an aldehyde group (-CHO), e.g. deoxyribose, ribose, glucose, galactose,
• ketoses with a ketone group (= C = O), e.g. ribulose, fructose.
All monosaccharides have reducing properties, i.e. they give positive results for both Tollens and Trommer tests. The aldehyde group in aldoses reduces the Tollens and Trommer reagents while undergoing oxidation to the carboxyl group. In an alkaline environment during the abovementioned ketosis attempts undergo enolization reactions, forming epimers – two aldoses and one ketose; resulting aldoses give a positive test result.
Almost all monosaccharides are optically active. Usually, only one of the two stereoisomers is biologically active.
Complex sugars are formed by glycosidic linkage of two or more simple sugar molecules. Hydrolysis of complex sugars leads to breaking of glycosidic bonds. However, it runs more difficult, the longer the sugar chain is and the more branched it is.
Sugar molecules can connect with glycosidic bonds in two ways:
type α – the particles face the same side up, e.g. in maltose and starch particles; this type of binding conditions the characteristic bending of the polysaccharide chain
type β – the molecules are turned upside down once.
Polysaccharides in which β bonds dominate (e.g. cellulose and cellobiose) form linear chains that adhere closely to each other and are connected by numerous hydrogen bonds, which makes them insoluble in water and mechanically resistant. In contrast, starch, in which α-bonds dominate, is much more soluble and its complete hydrolysis is possible due to acids and enzymes.
Disaccharides (disaccharides) are made up of 2 simple sugars . These include, for example, sucrose, lactose, maltose, cellobiose, and rutinosis. Most disaccharides (except sucrose) show reducing properties.
Polysaccharides include: starch, glycogen, cellulose, pectin, chitin, as well as many sugar derivatives.
Polysaccharide chains are divided into:
• amylose – an unbranched chain, easily soluble in water
• amylopectins – a highly branched chain (there are also 1,6 glycosidic bonds), insoluble in water.
Polysaccharides do not show reducing properties. This is associated with a very small amount of free functional groups in long sugar chains.
Carbohydrates perform the following functions in organisms:
• energy – e.g. total oxidation of 1 mole glucose to CO2 in a cell allows 36 moles of ATP to be obtained, and the energy gain of this reaction is 2872 kJ.
• spare – energy is mainly starch and inulin in plants, and glycogen in animals (including humans)
• transport – in plants the transport form of sugar is sucrose, and in animals and humans, glucose
• building materials (cellulose, hemicellulose, chitin (carbohydrate derivative))
• they are part of DNA and RNA, and are a modification of some proteins.
• inhibit blood clotting – heparin
• they are energy (fructose) and nutritional (maltose, lactose, raffinose) materials.
Sugars and sugar alcohols are the basic form (60% -95%) in which coal is transported in plants. In most plants, this function is performed by sucrose, sometimes being practically the only transport carbohydrate. Raffinose, stachyiosis, gentianose, umbelliferosis and fructan perform this function to a lesser extent, while the status of fructose is unclear. Of sugar alcohols, sorbitol and mannitol are important in the transport of carbon inside plants.
Digestion and absorption
Monosaccharides are absorbed without prior chemical treatment. Disaccharides are digested under the influence of brush border enzymes. Starch is digested with α-amylase. Some carbohydrates can be digested as a result of fermentation carried out by microorganisms living in the digestive tract.
Human carbohydrate digestibility
From a nutritional point of view, carbohydrates can be divided into human-assimilable (e.g. starch, fructose) and non-absorbable, i.e. fiber called dietary fiber. The fiber consists of cellulose, pectins and other non-digestible compounds having a beneficial effect on the digestive system.
Carbohydrate derivatives are called sugars whose hydroxyl groups of monomers have been replaced by other functional groups, e.g. chitin (acetylamino groups), pectins (contains galacturonic acid units, partially methylated) and heparin (amino and sulfate groups).