Classification of Carbohydrates: Carbohydrates are organic compounds found in the major part of fruits, vegetables, legumes, and cereal grains. They carry out many functions in all living organisms. These are large biomolecules, consisting of carbon (C), hydrogen (H), and oxygen (O) in their basic structure.
Chemically, they are simple organic compounds that are aldehydes or ketones with many hydroxyl groups added on each carbon atom which are not part of either of these functional groups. The basic formula is Cm(H2O)n where “m” is different from “n”. The hydrogen and oxygen ratio is 2 : 1, hence they are known as hydrates of carbon.
So, carbohydrate is defined as polyhydroxy aldehydes or polyhydroxy ketones which give these on hydrolysis and contains at least one chiral carbon atom. Carbohydrates are produced in green plants by photosynthesis and serve as a major source of energy in animals. They also serve as structural components, such as cellulose in plants and chitin in some animals.
Classification of Carbohydrates:
Carbohydrates are of three types namely simple sugar, polysaccharides, and oligosaccharides (Fig.1). The simplest carbohydrates are the three-carbon sugars, i.e. monosaccharides, and further cannot be hydrolyzed to simple sugars.
Depending on the number of carbon atoms, monosaccharides are further classified into the following:
Bioses are two-carbon compounds but do not occur in free form in nature.
Trioses are three-carbon compounds. They are in the form of phosphoric esters.
Tetroses are four-carbon compounds.
Examples: Erythrose, Threose.
Pentoses are five-carbon compounds and occur in plants. They are the products of polysaccharides such as hemicelluloses, mucilage, etc.
Examples: Ribose, Arabinose, Xylose, etc.
Hexoses are six-carbon molecules and are abundantly available. On hydrolysis, they produce starch and inulin.
Examples: Glucose, Fructose, etc.
Monosaccharides are classified according to three different characteristics: the placement of the carbonyl group, the number of carbon atoms they contain, and their chiral handedness. If the carbonyl group is an aldehyde, the monosaccharide is an aldose, and if the carbonyl group is a ketone, the monosaccharide is a ketose. Aldose or Aldo sugar-containing compounds are glucose, galactose, ribose, etc., whereas ketose or keto sugar-containing compound is fructose. Depending on the chemical nature of the sugar, they are also classified as reducing and non-reducing sugars.
Reducing sugar: Contain a hemiacetal or hemiketal group. Sugars include glucose, galactose, fructose, maltose, lactose.
Non-reducing sugar: Hemiacetal groups are absent. Sucrose and all polysaccharides are present in this group. Heptoses contain seven carbon atoms and so on. Disaccharides are the compounds in which two monosaccharides are joined together and these are the simplest polysaccharides. Examples: Sucrose, lactose, etc.
Sucrose = Glucose and Fructose; Lactose = Galactose and Glucose; Maltose = Glucose and Glucose
Trisaccharides are oligosaccharides composed of three monosaccharides with two glycosidic bonds connecting in between them. Examples: Raffinose (Glucose + Fructose + Galactose).
Polysaccharides are polymeric carbohydrates that are composed of long or branched chains of monosaccharide units bound together by glycosidic bonds. They are of two types viz. Structural polysaccharides and Digestible polysaccharides. Former one are digestible by herbivorous species – cellulose, lignin, dextrans, mannans, inulin, pentosans, pectic acids, Algic acids, agar, and chitin. Later one is starch.
Oligosaccharides are saccharide polymers containing a small number of simple sugars. Some examples are fructooligosaccharides (FOS), which are found in many vegetables.
Function of Carbohydrates:
- They are used for energy storage and production. Starch and glycogen, respectively in plants and animals, are stored as carbohydrates from which glucose can be mobilized for energy production.
- They exert a protein-saving action.
- The presence of carbohydrates is necessary for normal lipid metabolism.
- Glucose is indispensable for the maintenance of the integrity of nervous tissue and red blood cells.
- Two sugars, ribose, and deoxyribose are part of the bearing structure, respectively of the RNA and DNA, and present in the nucleotide structure.
- They take part in detoxifying processes. For example, at the hepatic level glucuronic acid, synthesized from glucose, combines with endogenous substances, like hormones, bilirubin, etc.
- Carbohydrates are also found linked to many proteins and lipids. Within cells, they act as signals that determine the metabolic fate or intracellular localization of the molecules which are bound.
- Two homopolysaccharides, cellulose and chitin, serve as structural elements.
- The cellulose in plants is used to manufacture paper, wood for construction, and fabrics.
Chemical Tests of Carbohydrates :
Molisch Test: Molisch reagent is mixed with a dilute solution of carbohydrate. The test reagent dehydrates pentoses to form furfural (top reaction) and dehydrates hexoses to form 5-hydroxymethyl furfural (bottom reaction). The furfurals further react with the alpha-naphthol present in the test reagent to produce a purple product. Pentoses and hexoses from the five-member oxygen-containing ring on dehydration. This test is known as the Molisch test and is used to detect carbohydrates in several substances.
Fehling’s Test: Fehling’s solution (containing Cu2+) changes color from blue to red/brown in the presence of reducing sugars.
Benedict’s Test: This test is performed to identify the reducing sugars. A few ml of a sample solution is placed in a test tube. Two ml of Benedict’s reagent (a solution of sodium citrate and sodium carbonate mixed with a solution of copper sulfate) is added. The solution is then heated in a boiling water bath for three minutes. A reddish precipitate will form within three minutes.
Barfoed’s Test: This test is performed to distinguish between mono and disaccharides. Barfoed’s reagent is copper acetate in acetic acid and is not as reactive as Benedict’s reagent. A positive reaction may only be a light red precipitate. Monosaccharides produce the red precipitate in 2 to 3 minutes; disaccharides produce the precipitate in 10 minutes.
Iodine Test for Starch and Other Polysaccharides: Starch is a polysaccharide that can be easily identified by the iodine test. Cellulose does not form a color complex with Iodine solution. Starch reacts with iodine solution and forms a blue-violet color. Many glucose in starch trap iodine molecules and form the color.
Seliwanoff’s Test: Seliwanoff’s test is used to distinguish aldohexoses from ketohexoses. Ketohexose-like fructose will form a deep red color with Seliwanoff’s reagent (a solution of resorcinol in HCl), while an aldohexose will show a light pink color and takes a longer time to develop the color.
Bial’s Test: This test is used to distinguish pentoses and hexoses. Pentoses give a positive test with Bial’s reagent (contains orcinol, HCl, and ferric chloride). In the presence of concentrated HCl, pentoses form a five-membered ring, known as furfural. A positive test is the formation of a bluish color within 5 minutes without the formation of a precipitate. Hexoses generally react to form green, red, or brown products.
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