As you know, carbohydrates or glucids are polyhydroxylated aldehydes or ketones, their derivatives and polymers. Glucose is a typical example of carbohydrates.
Observe that glucose has an aldehyde group (drawed in red) and five hydroxyl groups.
Most of carbohydrates are present with a cyclic structure in nature, as a consequence of internal linkages between the carbonyl carbon (of the aldehyde or ketone group) with one of the hydroxyl groups in the same molecule. This graphic represents glucose in a cyclic form:
Considering the polymerization degree (PD) of carbohydrates, they can be classified in Monosaccharides, Oligosaccharides and Polysaccharides.
Monosaccharides:
Monosaccharides are formed by a single molecule. It means that when hydrolyzed they can not release simpler molecules. Examples of this group of carbohydrates are glucose, ribose and fructose, among others.
Monosaccharides can be subclassified according to different criteria, for example:
According to the main (carbonyl) function:
If the carbonyl group belongs to an aldehyde function, the monosaccharide is classified as an aldose. Glucose is a typical aldose. If the main function is a ketone, then the monosaccharide is classified as a ketose.
Fructose (see structure below) is a ketose, since it is a polyhydroxylated ketone.
According to the number of carbons:
Monosaccharides can be classified in trioses, tetroses, pentoses, hexoses, heptoses and octoses, according to the number of carbons in the molecule.
According to the steric series:
According to the type of esteroisomers, monosaccharides can be classified as L or D (most of the carbohydrates in the animal kingdom belongs to D series)
According to the kind or anomer:
According to the position of the anomeric hydroxyl, monosaccharides can be classified as Alpha or Beta.
Usually, these criteria are combined for describing a monosaccharide, e.g. a compound can be described as a Beta-D-aldohexose.
Oligosaccharides:
They are formed by 2-9 monomers linked through glycosidic linkages; in other words, when hydrolyzed these compounds release 2 to 9 monosaccharides (some texts say up to 20; in fact, oligosaccharides release “a few” monosaccharides).
According to the number of monosaccharides in the oligosaccharide, oligosaccharides can be dissacharides, trisaccharides, tetrasaccharides, etc. Disaccharides, formed by just 2 monosaccharides, are the most important subgroup of oligosaccharides. Disaccharides that appear in nature are lactose, or milk sugar (formed by galactose and glucose), and sucrose or table sugar (formed by fructose and glucose). Other important disaccharides are produced as result of starch digestion: maltose and isomaltose. These disaccharides are ,both of them, formed by two molecules of glucose, but linked in different ways. Cellobiose is a third dissacharide formed also by two molecules of glucose, but linked in such a way that animals can not break, unless animals have in the digestive system specific microorganisms that hydrolyze these linkages, as herbivors have (Cellobiose is formed as result of the digestion of cellulose).
Polysaccharides:
Polysaccharides are carbohydrates formed by more than 9 monosaccharides (some texts say more than 10 monosaccharides, other texts say more than 20…in fact, they usually are formed by a lot of monosaccharides!). When the polysaccharides are formed by the same type of monosaccharides, they are called homopolysaccharides.
Amylose
The molecules that form starch, glycogen and cellulose are formed by hundreds of molecules of just one type of monosaccharides (glucose, in this cases), linked through glycosidic linkages. These polysaccharides are typical examples of homopolysaccharides.
If the polysaccharide is formed by different types of monosaccharides, then it is called a heteropolysaccharide. Hyaluronic acid, formed by thousands of alternant units of glucuronic acid and N-acetylglucosamine, is an example of heteropolysaccharides.
Hyaluronic acid is an important component of the extracellular matrix in the skin and the conective tissue. This compound has received lately a lot of attention from the media as an antiaging agent.
The Biochemistry Questions Site 
Amilose should be spelled as amylose
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Monosaccharides
These are the only sugars that can be absorbed and utilized by the body. Disaccharides and polysaccharides must be ultimately broken down into monosaccharides in the digestive process known as hydrolysis. Only then can they be utilized by the body. Three monosaccharides are particularly important in the study of nutritional science: glucose, fructose and galactose
Disaccharides
Sucrose: cane and beet sugar, maple syrup, molasses, and sorghum. Hydrolyzed to glucose and fructose; a non-reducing sugar.
Maltose: malted products and germinating cereals; an intermediate product of starch digestion. Hydrolyzed to two molecules of glucose; a reducing sugar; does not occur free in tissues.
Lactose: milk and milk products; formed in the body from glucose nature. Hydrolyzed to glucose and galactose; may occur in urine during pregnancy; a reducing sugar
Polysaccharides (more than 10 sugar units)
Digestible
Glycogen: meat products and seafoods; polysaccharides of the animal body, often called animal starch; storage form of carbohydrates in body, mainly in liver and muscle.
Starch: cereal grains, unripe fruits, vegetables, legumes, and tubers. Most important food source of carbohydrates; storage form of carbohydrates in plants. Composed chiefly of amylose and amylopectin; hydrolyzed to glucose.
Dextrin: toasted bread, intermediate product of starch digestion. Formed in course of hydrolytic breakdown of starch.
Partially Digestible
Inulin: tubers and roots of dahlias, artichokes, dandelions, onions, and garlic. Hydrolizable to fructose; used in physiologic investigation for determination of glomerular filtration rate.
Mannosan: legumes and plant gums. Hydrolyzable to mannose but digestion incomplete; further splitting by bacteria may occur in large bowel.
Indigestible
Cellulose: skins of fruits, outercoverings of seeds, plus stalks and leaves of vegetables. Not subject to attack of digestive enzymes in man, thus an important source of “bulk” in diet; may be partially split to glucose by bacterial action in large bowel.
Hemicellulose and pectin: woody fibers and leaves. Less polymerized than cellulose; may be digested to some extent by microbial enzymes, yielding xylose.
Future Classification
In 2001, Hoffman, et al.* proposed a classification system that would be more useful for equine nutrition.
This classification system has 3 parts:
Hydrolyzable CarbohydratesThese carbohydrates can be digested in the small intestine and would include hexoses (molecules with a backbone of 6 carbon atoms), disaccharides, some of the oligosaccharides, as well as the nonresistant starches (starches that can be broken down by enzymes in the stomach). The energy yield from these compounds is very high, because they are broken down into monosaccharides.
Rapidly Fermented CarbohydratesThese carbohydrates are easy for microbes in the large intestine to digest. They include pectin, fructan, and the oligosaccharides that are not digested in the small intestine.
Slowly Fermented CarbohydratesThese carbohydrates include cellulose and hemicellulose that is broken down slowly by the microbes in the large intestine. These carbohydrates are an important source of acetate, a fatty acid.
There are a variety of interrelated classification schemes. The most useful classification scheme divides the carbohydrates into groups according to the number of individual simple sugar units. Monosaccharides contain a single unit; disaccharides contain two sugar units; and polysaccharides contain many sugar units as in polymers – most contain glucose as the monosaccharide unit.
Functional Groups
Aldoses contain the aldehyde group – Monosaccharides in this group are glucose, galactose, ribose, and glyceraldehyde.
Ketoses contain the ketone group
Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between two and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Definitions of how large a carbohydrate must be to fall into each category vary according to personal opinion. Examples of oligosaccharides include the trisaccharide raffinose and the tetrasaccharide stachyose.Polysaccharides represent an important class of biological polymers. Their function in living organisms is usually either structure or storage related. Starch is used as a storage polysaccharide in plants, being found in the form of both amylose and the branched amylopectin. In animals, the structurally similar but more densely branched glycogen is used instead.
Disaccharides
Carbohydrates are one of the important biomolecules and it is necessary to know their functions and classification. Carbohydrates are classified as monosaccharides, disaccharides, oligosaccharides and polysaccharides. Monosaccharides exhibit isomerism due to the presence of asymmetric carbon atoms. Monosaccharides undergo various reactions to form important derivatives like sugar acids, sugar alcohols, amino sugars and deoxy sugars. Disaccharides can be reducing or non-reducing. Maltose and Lactose are reducing sugars. Sucrose is a non-reducing sugar. Polysaccharides are classified into homopolysaccharides and heteropolysaccharides. Mucopolysaccharides are negatively charged heteropolysaccharides. They are also known as glycosaminoglycans. Glycoproteins mostly contain oligosaccharides which are tightly bound by proteins. In proteoglycans, proteins are covalently bound with mucopolysaccharides.
Polysaccharides
Consist of repeating units of monosaccharides or their derivatives held together by glycosidic bonds. Ex: Starch, Hyaluronic acid, homopolysaccharides contain only a single type of monosaccharide. Ex: Starch, glycogen, cellulose, chitin. Starch is found in cereals, legumes, potatoes. Here, glucose is the monosaccharide unit. They are made up of amylase and amylopectin with α (1-4) linage in linear structure of amylase and α (1-4) and α (1-6) linkage in branched structure amylopectin. In cellulose, polymer of glucose is linked by β (1-4) glycosidic linkage. Cellulose is present in plant cell wall and functions as a dietary fiber. Glycogen is the main storage polysaccharide in animals and is abundant in the liver. Glycogen is more extensively branched. Heteropolysaccharides are composed of different types of monosaccharides or their derivatives. Mucopolysaccharides are known as glycosaminoglycans. They are negatively charged large complexes. Ex: Hyaluronic acid, heparin, chondroitin sulfate, dermatan sulfate, keratin sulfate etc. Glycoproteins mostly contain oligosaccharides which are tightly bound to proteins. Ex: Immunoglobulins, mucin of stomach. They function as enzymes, hormones, receptors, structural proteins, transport proteins. In proteoglycans, proteins are covalently bound with mucopolysaccharides. Found in the bone, elastin and collagen.
Pentosans (chief constituents of gums and mucilages). Anhydrides of xylose – Xylans. Anhydrides of arabinose – Arabans.
Hexosans.
Anhydrides of glucose-Starch, Cellulose, Glycogen, Dextrin (and other “dextrans”). Anhydrides of mannose – Mannans. Anhydrides of galactose- Galactans (pectins). Anhydrides of fructose – Inulin (and other “levulans”).
1 Occurs free in nature.
2 Not yet found free in nature (or only in small amounts) but obtained by hydrolysis or fermentation of natural product.
3 Known only (with certainty) as a
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