Carbohydrates - Its classification & structure
Carbohydrates - carbohydrates are the most abundant organic \- molecules in nature. They are primarily composed of the elements carbon, hydrogen and oxygen.
Carbohydrates may be defined as polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis. the emperical formula of hydrates of carbon are (C.H2O)n where n < 3.
Function of carbohydrates -
1. They are the most abundant dietary source of energy (a Cal/S) for all organisms.
2. Carbohydrates are precursors for many organic compounds (fats, amino acids).
3. Carbohydrates (as glycoproteins and glycolipids) participate in the structure of cell membrane and cellular functions such as cell growth, adhesion and fertilization.
4. They are structural components of many organisms. These include the fiber (cellulose) of plants, exoskeleton of some insects and the cell wall of microorganisms.
5. Carbohydrates also serve as the storage form of energy (glycogen) to meet the immediate energy demands of the body.
CLASSIFICATION OF CARBOHYDRATES
They are broadly classified into three major groups monosaccharides, oligosaccharides and polysaccharide based on number of sugar unit present .
1. Monosaccharides
Monosaccharides (Greek : mono-one) are the simplest group of carbohydrates and are often referred to as simple sugars. They have the general formula Cn(H20)n, and they cannot be further hydrolysed.s, based on the functional group monosaccharides are divided into different categorie
Aldoses : When the functional group inmonosaccharides is an aldehyde e.g. glyceraldehyde, glucose
Ketoses :When the functional group is a keto e.g. dihydroxyacetone, fructose
2. Oligosaccharides
Oligosaccharides (Creek: oligo-few) contain 2-1O monosaccharide molecules which are liberated on hydrolysis , these are further subdivided to disaccharides, trisaccharides etc
3. Polysaccharides
Polysaccharides(Creek: poly-many) are polymers of mondficcharide units with high molecular weight (up to a million). They are usually tasteless (non-sugars) and form colloids with water. The polysaccharides are of two types - homopolysaccharides and heteropolysaccharides.
A. MONOSACCHARIDE - Structural aspect
Stereoisomerism is an important character of monosaccharides. Stereoisomers are the compounds that have the same structural formulae but differ in their spatial configuration.
A carbon is said to be asymmetric when it is attached to four different atoms or groups. Ihe number of asymmetric carbon atoms (n) determines the possible isomers of a given compound which is equal to 2n. Clucose contains 4 asymmetric carbons, and thus has 16 isomers
A.a. Glyceraldehyde-the reference carbohydrate
Clyceraldehyde (triose) is the simplest monosaccharide with one asymmetric carbon atom
D and L isomer
The D and L isomers are mirror images of each other. The structures of D- and L-glucose based on the reference monosaccharide, D- and L-glyceraldehyde (glycerose) are depicted in Fig
Racemic mixture : lf D- and L-isomers are present in equal concentration, it is known as racemic mixture or DL mixture. Racemic mixture does not exhibit any optical activity
Epimers
If two monosaccharides differ from eacother in their configuration around a singk specific carbon (other than anomeric) atom. L*ei are referred to as epimers to each other.For instance, glucose and galactose are efilwl with regard to carbon 4 (C-epimer)
A.b. Glucose structure
For a better understanding structure of glucose , let us consider the formation of hemiacetals and hemiketals respectively produced when an aldehyde or a ketone reacts with alcohol. The hydroxyl group of monosaccharides can react with its own aldehyde or keto functional group to form hemiacetal and hemiketal. Thus, the aldehyde group of glucose at C1 reacts with alcohol group at C5 to form two types of cyclic hemiacetals namely a and B, as depicted in Fig.. The configuration of glucose is conveniently represented either by Fischer formulae or by Haworth projection formulae
B. OLIGOSACCHARIDES
Among the oligosaccharides, disaccharides are the most common . As is evident from the name, a disaccharide consists of two monosaccharide units (similar or dissimilar) held together by a glycosidic bond.
Maltose is composed of two a-D-glucose units held together by cl (1 - 4) glycosidic bond. The free aldehyde group present on C1 of second glucose answers the reducing reaction.
Cellobiose is another disaccharide, identical in structure with maltose, except that the former has p (1 - 4) glycosidic linkage. Cellobiose is formed during the hydrolysis of cellulose.
Sucrose (cane sugar) is the sugar of commerce, mostly produced by sugar cane and sugar beets. Sucrose is made up of alphaD-glucose and betaD-fructose.
Lactose is more commonlv known as milk sugar since it is the disaccharide found in milk. Lactose is composed ol beta-D-galactose and beta-Dglucose held together by 0 (1 --4) glycosidic bond
C.POLYSACCHARIDES
Polysaccharides are linear as well as branched polymers. The occurrence of branches in polysaccharides is due to the fact that glycosidic linkages can be formed at any one of the hydroxyl groups of a monosaccharide.
Polysaccharides are of two types-
C.1. Homopolysaccharides which on hydrolysis yield only a single type of monosaccharide. They are named based on the nature of the monosaccharide unit. Thus, glucans are polymers of glucose whereas fructosans are polymers of fructose
C.2. Heteropofysaccharides on hydrolysis yield a mixture of a few monosaccharides or their derivatives.
C.1. Homoploysaccharides
C.1.a Starch-
Starch is the carbohydrate reserve of plants which is the most important dietary source for higher animals, including man.Starch consists of two polysaccharide components-water soluble amylose (1 5-20%) and a water insoluble amylopectin (80-85%).
C.1.b Glycogen-
Glycogen is the carbohydrate reserve in animals, hence often referred ro as animal starch. It is present in high concentration in liver, followed by muscle, brain etc. Clycogen is also found in plants that do not possess chlorophyll (e.g. yeast, fungi)
The structure of glycogen is similar to that of amylopectin with more number of branches. Glucose is the repeating unit in glycogen joined together by alpha (1 - 4) glycosidic bonds, and alpha (1 - 6) glycosidic bonds at branching point.
C.1.c Cellulose-
Cellulose occurs exclusively in plants and it is the most abundant organic substance in plant kingdom.Cellulose is composed of p-D-glucose units linked by 9 0 -+ 4) glycosidic bonds Cellulose cannot be digested by mammals including man-due to lack of the enzyme that cleave
C.2 Heteropolvsaccharides-
When the polysaccharides are composed of different types of sugars or their derivatives, they are referred to as heteropolvsaccharides or heteroglycans