Carbohydrates definition and important functions Digestion of Carbohydrates

CARBOHYDRATES

DEFINITION

Carbohydrates are defined as the aldehydic or ketonic derivatives of higher polyhydroxy alcohol or anhydrides of such derivatives.

IMPORTANT FUNCTIONS

Humans and all animals except carnivorous derive the major portion of their food calories from the various types of carbohydrates in their diets. Most of the energy for the metabolic activities of the cell in all organisms is derived from the oxidation of carbohydrates (in the form of glucose). In the storage organs of plants (in the form of starch) and in the liver and muscles of animals (in the form of glycogen), they are important food reserve. They also serve as an important component of skeletal structures in plants (in the form of cellulose) and that of outer structure of microorganisms.

Classification of Carbohydrates

Monosaccharides

These consist of single polyhydroxy aldehyde or ketone unit which cannot be broken down to simpler substances on hydrolysis. Monosaccharides containing an aldehyde group as the functional group are called aldoses and those containing a ketone group as the functional gro are called ketoses. They are further classified according to the number of carbon atoms present. Monosaccharides containing three to seven carbon atoms are called trioses, tetroses, pentoses, hexoses and heptoses respectively. Glucose (or dextrose) is a hexose and it is the principal sugar in blood serving the tissues as a major metabolic fuel. Fructose (or levulose) is an important component of fruit juices and honey. A cyclic hemiacetal structure accounts for their chemical properties.

PROPERTIES OF CARBOHYDRATES

Following are the important properties of some of the carbohydrates on which clinical chemistry methods are based:

1 Reducing property

Monosaccharides by virtue of free aldehydic or ketonic group in their structure reduce metallic cations, such as, Cu++ ions in alkaline solution at high temperature. All the monosaccharides and the two disaccharides, maltose and lactose reduce cupric ions present in Benedict's reagent and also in Fehling's reagent. The reaction takes place as follows:

2. Formation of osazone crystals 

The reducing sugars form characteristic osazone crystals. These are obtained by adding a mixture of phenylhydrazine hydrochloride and sodium acetate to the sugar solution and then by heating the mixture in a boiling water bath. These compounds have characteristic crystal structures, melting points, and precipitation times, and are valuable in the identification of sugars. Glucose, fructose and mannose give the same types of osazones (needle shaped), maltose forms sunflower type crystals and lactose forms cotton ball types of osazones. Hence, lactose can be differentiated from other reducing substances, if present in urine. 

3. Action of acids

Selivanoff's test: Selivanoff's reagent contains 50 mg of resorcinol in 12% (v/v) hydrochloric acid. Selivanoff's reagent reacts with fructose to form red colored compound. When 5 ml of Selivanoff's reagent is heated and boiled with 5 drops 0.25 ml of fructose solution or urine containing fructose (0.5%) for 20 to 30 seconds, a positive reaction is indicated by the production of red color, with or without separation of brown red color.

4.Reaction with orthotoluidine

Glucose and galactose react with orthotoluidine in hot acidic medium to form green colored complex.

5. Reaction with iodine (iodine test)

a) Starch reacts with dilute iodine solution to form blue colored starch iodine complex.

b) Glycogen gives a red color with iodine solution. 

c) Dextrin however gives red, blue or purple color with dilute iodine solution, depending upon the type of dextrin present.

Reaction with sulfuric acid (Molisch's test) Molish's test: It is a sensitive chemical test for the presence of carbohydrates, based on the dehydration of the carbohydrate by sulfuric acid to produce an aldehyde, which condenses with two molecules of phenol (mainly a-naphthol), resulting in a red- or purple-colored compound. All carbohydrates such as monosaccharides, disaccharides, and polysaccharides give a positive reaction.

Isomerism Isomerism is exhibited by various carbohydrates such as glyceraldehyde, glucose, fructose etc.

DIGESTION OF CARBOHYDRATES

For carrying out day-to-day work and for being in a state of health, diet consisting of carbohydrates, fats, proteins, vitamins, minerals and water is required for a human being. The human body requires nutrients sufficient to provide free energy to manufacture the daily requirements of high energy bonds (mainly ATP). Most food products are ingested in high molecular weight complex forms. These can not be absorbed from the digestive tract until they have been broken down into small molecules. This disintegration of the naturally occurring foodstuffs into assimilable forms constitutes the process of digestion. Carbohydrates are the first and most efficient source of energy needed for vital cellular processes. About 50 to 60 percent of the daily caloric intake is supplied by carbohydrates. This is equivalent to 250 to 500 gm in the form of commonly digestible carbohydrates such as polysaccharides, as po disaccharides and assimilable monosaccharides. The main sources of polysaccharides in the form of starch are cereals, grains, vegetables and tubers. Meat and seafood contain small quantities of glycogen i.e. another form of polysaccharide. Disaccharides such as lactose and sucrose are present in milk and sugar cane respectively. Monosaccharides such as glucose and fructose are present in various types of fruits.

In the oral cavity salivary amylase is capable of bringing about the hydrolysis of starch and glucose to maltose. This part of digestion in the oral cavity is of little significance because of the short time the salivary amylase can act on the food. Further digestion of carbohydrate takes place in the duodenum by pancreatic amylase. Starch and glucose are hydrolyzed to maltose, maltotriose and a mixture of branched and non branched oligosaccharides and some glucose at alkaline pH 7.5-8.0.

The intestinal juice secreted by the glands of Brunner and of Lieberkuhn contain various enzymes such as maltase which removes single glucose residues from a (1-4) linked oligosaccharides and disaccharides. Isomaltase hydrolyses, 1-6 bonds in ā-limit dextrins.Bita-galactosidase removes galactose from lactose and sucrase hydrolyses sucrose into glucose and fructose. The end products of carbohydrate digestion are glucose, fructose, mannose, galactose and pentose sugars.