Medical brain notes 25

  CARBOHYDRATES Introduction Carbohydrates are widely distributed in plants in which they are formed from carbon dioxide of the atmosphere and water by photosynthesis. Plants use carbohydrates as the precursor for the synthesis of proteins, lipids and other organic compounds. Animals obtain their carbohydrates from plants.   Functions of carbohydrates Carbohydrates have a variety of functions in the animal and human body. ·             They supply energy for body functions and for doing work. ·             They are structural components of many organisms. ·             They exert a sparing action on proteins. ·             They provide the carbon skeleton for the synthesis of some nonessential amino acids and fats. ·        ...

  Classification of carbohydrates Carbohydrates are classified as follows They are i. Monosaccharides ii. Disaccharides iii. Oligo saccharides and iv. Polysaccharides (Fig. 4.1).

  Monosaccharides Monosaccharides are defined as polyhydroxy aldehydes or ketones, which cannot be further hydrolysed to simple sugars. Monosaccharides are divided into two groups according to their functional groups (Fig. 4.2). They are also classified based on the number of carbon atoms present in the monosaccharides  1. Aldotriose eg: Glycerose 1. Ketotriose :eg.Dihydroxy acetone 2. Aldotetrose eg : Erythrose 2. Ketotetrose : eg : Erythrulose 3. Aldopentose eg : Ribose 3. Ketopentose : eg. Ribulose 4. Aldohexose eg: Glucose 4. Ketohexose : eg. Fructose 1. Aldoses are sugars containing aldehyde group eg : glucose, galactose, mannose. 2. Ketoses are sugars containing ketone group eg : fructose and sorbose. Carbohydrates posses asymmetric carbon atoms. A carbon atom to which four different atoms or groups of atoms are attached is said to be an asymmetric carbon. Vant Hoff’s Rule of ‘n’ The number of possible isomers of any given compound depends upon the number of asymmetric c...

  Physical properties of monosaccharides 1. Colour and shape Monosaccharides are colourless and crystalline compounds.   2. Solubility They are readily soluble in water.   3. Taste They have sweet tase.   4. Stereo isomerism D-glucose and L-glucose are mirror images of each other. The presence of asymmetric carbon atoms in a compound give rise to the formation of isomers of that compound. Such compound which are identical in composition and differs only in spatial configuration are called “stereo isomers’’. For example glucose can exist in two forms as shown below. D-series and L-series The orientation of the H and OH groups around the carbon atom just adjacent to the terminal primary alcohol carbon, eg. C 5  in glucose determines the series. The D and L forms of glyceraldehyde are given below. when the -OH group of this C 2  is at the right, it belongs to D-series, when the -OH group is on the left it belongs to L-series.   5. Optical Isomerism A beam...

  Chemical properties of glucose Glucose contain active groups. The active groups are responsible for their chemical properties. There are three types of active groups in glucose. They are   1. Glucoside formation   Glucose reacts with methanol in the presence of HCl and gives  α  and  β  glucoside. Glucoside formation is due to the reaction of alcohol with glucoside -OH group of glucose. β , D glucose is forms   β , D-methyl glucoside. In the same way, fructose forms   fructoside.   2. Oxidation Glucose when treated with bromine water, forms gluconic acid. The aldehyde group is oxidised to carboxylic group. Br 2  + H 2 O < - - - > HOBr + HBr First, bromine forms hypobromous acid (HOBr), with water and oxidises the glucose to gluconic acid. When glucose is oxidised with nitric acid, saccharic acid is formed. When glucose is oxidised with hydrogen peroxide (H 2 O 2 ), glucuronic acid is formed. In this reaction only the primary...

  Haworth projection formula of glucose and fructose According  to  Haworth,  glucose  exists  in  six  membered  ring  structure  called pyranoform (Fig.4.5) and fructose in five membered furano form. (Fig. 4.6) 2. Fructose

  Disaccharides Disaccharides are sugars containing two molecules of monosaccharides. Disaccharides are formed by the condensation of two molecules of monosaccharides with the elimination of one molecule of water. In disaccharides, monosaccharides are linked by the glycosidic bonds. The properties of the disaccharides depend to a great extent on the type of linkage. If the two potential aldehyde or ketone of both monosaccharides are involved in the linkage, the sugar will not exhibit reducing properties and will not be able to form osazones. eg. Sucrose. But if one of them is not bound in this way, it will permit reduction and osazone formation by the sugars eg. lactose and maltose which are known as reducing disaccharides.   1. Maltose Maltose is composed of two glucose molecules combined by  a - 1,4 glycosidic linkage. It is commonly called malt sugar. Malt from sprouting barley is the major source of maltose. It is a rather sweet sugar and is highly soluble in water. T...