Carbohydrates- Monosacchrides, Polysacchrides, Oligosacchrides
Carbohydrates
Carbohydrates hamare sharir ke liye bahut zaroori hain. Carbohydrates hamare sharir ko urja dete hain aur hamare dimag aur manspeshiyon ko sahi se kaam karne me madad karte hain. Carbohydrates do prakar ke hote hain: simple aur complex.
Simple carbohydrates jaldi se pach jaate hain aur hamare khoon me glucose ki matra badha dete hain. Simple carbohydrates me shakkar, shahad, phal aur doodh jaise khadya padarth shamil hain. Simple carbohydrates ko hamari body ko turant urja dene ke liye istemal kiya ja sakta hai.
Complex carbohydrates dheere se pach jaate hain aur hamare khoon me glucose ki matra ko sthir rakhte hain. Complex carbohydrates me gehun, jau, chawal, makka aur sabji jaise khadya padarth shamil hain. Complex carbohydrates ko hamari body ko lambe samay tak urja dene ke liye istemal kiya ja sakta hai.
Carbohydrates ka sahi matra me sevan karna hamare swasthya ke liye labhkari hai. Carbohydrates hamare sharir ko poshan dete hain aur hamare bhojan ko swadisht banate hain. Carbohydrates ka atyadhik ya kam sevan karne se hamare sharir par nuksan ho sakta hai.
Carbohydrates are the most abundant organic molecules in nature. They are primarily composed of the elements carbon, hydrogen and oxygen. The name carbohydrate literally means 'hydrates of carbon'. The simplest carbohydrates are the monosaccharides that have the general formula (CH2O)n where n is 3 or more.
A monosaccharide or simple sugar, consists of a carbon chain with a number of hydroxyl
(OH) groups and either one aldehyde group (often written as –CHO) or one ketone group (C=O). A sugar that bears an aldehyde group is called an aldose whereas a sugar with a ketone group is a ketose. The smallest carbohydrates, for which n = 3, are called trioses. Thus glyceraldehydes (Fig. 1) is a triose that has an aldehyde group and so is an aldose. Thus it can also be called an aldotriose. Similarly, dihydroxyacetone (Fig. 1) is a ketotriose.
Fig. Structure of glyceraldehyde and dihydroxyacetone
Carbohydrates may be defined as polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis. The term 'sugar' is applied to carbohydrates soluble in water and sweet to taste. Sugars that contain a free aldehyde or ketone group in the open-chain configuration can
reduce cupric ions (Cu2+) to cuprous ions (Cu+) and hence are called reducing sugars. This is the basis of the Fehling’s and Benedict’s tests for reducing sugars. The reducing end of such a sugar chain is thus the end that bears the aldehyde or ketone group. Functions of Carbohydrates Carbohydrates participate in a wide range of functions:
1. They are the most abundant dietary source of energy (4 Cal/g) 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 microorganism.
5. Carbohydrates also serve as the storage form of energy (glycogen) to meet the immediate energy demands of the body.
6. The carbohydrates are the basic raw material of many important industries including sugar and sugar products, starch products, paper and wood pulp, textiles, plastics, food processing and fermentation.
Classification of Carbohydrates Carbohydrates are often referred to as saccharides (Greek: sakcharon-sugar). They are broadly classified into three major groups:
Monosaccharides
Oligosaccharides
Polysaccharides
#Classification
*Monosaccharides (Simple sugars)
1.Low molecular weight carbohydrates and cannot be hydrolyzed further.
2.Crystalline, soluble in water, andsweet in taste.
3.Classified into triose, tetrose, pentose, hexose and heptose depending upon the number of carbon atoms. They may be either aldoses or ketoses depending upon whether they contain a free aldehyde or ketone group, respectively
4.All monosaccharides are reducing in nature
*Oligosaccharides
1. Contain 2-10 Containmonosaccharides joined by glycosidic bonds. Low molecular weight carbohydrates which can be hydrolysed by enzymes or acids to yield monosaccharide.
2.Powdery or crystalline , soluble in water and sweet in taste.
3.Classified into disaccharide, trisaccharide, tetrasaccharide and pentasaccharide depending upon the number of monosaccharidesthey contain.
4.Some of them are reducing and some of them are non reducing in nature.
*Polysaccharides (Glycans)
1.contain manymonosaccharides joined by glycosidic bonds. They canbe hydrolysed by enzymes or acids.
2.Insoluble in water, tasteless, linear or branched
3.Classified into homoglycans and heteroglycans depending upon the kind of monosaccharides present. Depending upon the function, they are classified as storage andstructural polysaccharides.
4.Non reducing in nature and give deep blue (amylose) or red colour (amylopectin) with iodine.
(a). Monosaccharides
Structural aspects
Stereoisomerism is an important character of monosaccharides. Stereoisomers are the compounds that have the same structural formulae but differ in their spatial configuration. Glyceraldehyde and dihydroxyacetone have the same chemical composition, C3H6O3, but differ in structure (i.e. they are structural isomers). A carbon is said to be asymmetric when it is attached to four different atoms or groups. The number of asymmetric carbon atoms (n) determines the possible isomers of a given compound which is equal to 2n. Glucose contains 4 asymmetric carbons, and thus has 16 isomers.
D- and L- isomers
The D and L isomers are mirror images of each other. The spatial orientation of -H and -OH groups on the carbon atom (C5 for glucose) that is adjacent to the terminal primary alcohol carbon determines whether the sugar is D- or L-isomer. If the -OH group is on the right side, the sugar is of D-series, and if on the left side, it belongs to Lseries
The naturally occurring monosaccharides in the mammalian tissues are mostly of Dconfiguration. The term diastereomers is used to represent the stereoisomers that are not mirror images of one another.
Optical isomer
Optical activity is a characteristic feature of compounds with asymmetric carbon atom. When a beam of polarized light is passed through a solution of an optical isomer, it will be rotated either to the right or left. The term dextrorotatory (+) and levorotatory (-) are used to compounds that respectively rotate the plane of polarized light to the right or to the left. An optical isomer may be designated as D(+), D(-), L(+) and L(-).
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, since the dextroand levorotatorv activities cancel each other
Configuration of D-aldoses and D-ketoses
The configuration of few D-aldoses starting from D-glyceraldehyde is shown in Figure 2. This is a representation of Killiani-Fischer synthesis by increasing the chain length of an aldose, by one carbon at a time. Thus, starting with an aldotriose (3C), aldotetroses (4C), aldopentoses (5C) and aldohexoses (6C) are formed.
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| Fig. D-aldoses |
Similarly starting from dihydroxyacetone (triose), there are five keto-sugars which are physiologically important.
*Epimers
If two monosaccharides differ from each other in their configuration around a single specific carbon (other than anomeric) atom they are referred to as epimers to each other for instance, glucose and galactose are epimers with regard to carbon 4 (C4-epimers). The inter-conversion of epimers is known as epimerization and is done by enzymes called epimerases.
*Ring structures
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 α and β. The configuration of glucose is conveniently represented either by Fischer formulae or by Haworth projection formulae. Haworth projection formulae are depicted by a six-membered ring pyranose (based on pyran) or a five-membered ring furanose (based on furan). The cyclic forms of glucose are known as α -D-glucopyranose and α -D-glucofuranose.
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