Carbohydrate Lec 1-2 PDF

Alkitab University College of Science Biochemistry Lec 1-2 CARBOHYDRATE Dr. Yasir K. Khalil  CARBOHYDRATE carbohydrates are saccharides or sugars, either single monomeric units (Monosacchardes) such as glucose or fructose, or polymeric...

Alkitab University College of Science Biochemistry Lec 1-2 CARBOHYDRATE Dr. Yasir K. Khalil  CARBOHYDRATE carbohydrates are saccharides or sugars, either single monomeric units (Monosacchardes) such as glucose or fructose, or polymeric chains (disaccharides or polysaccharides) of sugar units such as glycogen, starch or amylopectin. Carbohydrates are nutritive polyhydroxy compounds widely distributed in plant and animal tissues. In plants carbohydrates are synthesized from CO2 and H2O by the process of photosynthesis. Carbohydrates are formed from C, H, and O, the latter two are present in the same proportion as in water is 2: 1. Classification of carbohydrates Carbohydrates are classified into: 1- Monosaccharides which are a carbohydrate units that can not be hydrolysed into more simpler forms. Monosaccharides are classified according to the number of carbon atoms into trioses, tetroses, pentoses, hexoses and etc. These monosaccharides can be either aldoses or ketoses according to aldehyde or ketone group present. 1  Trioses CHO CH2OH C C H OH O CH2OH CH2OH Glyceraldehde (aldose) Dihydroxyacetone (ketose) Tetroses CHO CH2OH HOHO C C HCOH HCOH CH2OH CH2OH Pent Erythrose (aldose) Erythrulose (ketose) oses CHO CHO CH2OH H C OH HCH C O H C OH H C OH H C OH H C OH H C OH aH OH CH2OH CH2OH CH2OH Ribose Deoxyribose Ribulase 2  Hex oses CHO CHOCHO CH2OH H C OH C O H C OHHO C H HO C H HO C HHO C H HO C H H CH2OH 2 CH2OH CH2OH O Galactose C Glucose Mannose Fructose H H C OHH C OH H C OH HOH HOHHOH H OH CHOH (1) Disaccharides Disaccharides are sugars formed from condensation of two Monosaccharide units i.e. on hydrolysis give rise two monosaccharide units, these are: i- Maltose ii- Sucrose iii- Lactose i. Maltose: An example of a disaccharide (non reducing sugar) and its structure is shown here. ii. Sucrose: Sucrose is cane sugar, beet sugar or table sugar. Sucrose is a-glucose-(1 ® 2)- b-fructose. As shown below 3 iii. Lactose sugar (milk sugar): Lactose is disaccharide sugar formed by condensation of glucose and galactose molecules. Lactose makes up around 0-8% of milk (by weight), although the amount varies among species and individuals. Lactose is a disaccharide derived from the condensation of galactose and glucose, which form a β-1→4 glycosidic linkage. (2) Oligosaccharides are condensation products of two to ten monosaccharides; )3(Polysaccharides are condensation products of more than ten monosaccharide units; examples are the starches, glycogen, dextrins and cellulose which may be linear or branched polymers. Besides they are important nutritive materials, they are important in manufacture of textile, artificial silk and paper. 4 Homopolysaccharides 1- Starch. Homopolysaccharide formed form a-glucose units (great number). Starch is the main carbohydrate present in the plant e.g barley, rice, maize and wheat and also in roots and tubers (potatoes & sweet potatoes). Starch nerve present in animal tissues. Starch granules is formed of: 2- Dextrins - Dextrins are produced during enzymatic hydrolysis of starch by salivary amylase or pancreatic amylase. Dextrins include, amylodextrin, erythrodextrin and achrodextrin. - 5  3- Dextrans - These are polysaccharides produces by the action of certain bacteria when allowed to grow on sucrose media. - It is used as plasma substitute, to increase the blood volume in condition such as a severe haemorrhage. 4- Glycogen - It is the main storage form of carbohydrate in the animal tissues (named animal starch). - Its structure is similar to the amylopectin of the starch, i.e. highly branched polymer formed of glucose units joined together via 1-4 link (straight chain) and 1-6 link (at the branch point). - Glycogen stored in muscular tissues which utilized as energy source for muscular contraction (glycogen ® Lactic acid). - Clycogen stored in liver which functions as a source of blood glucose maintenance during starvation and in between meals (by process named glycogenolysis). 6 7 Biomedical Importasance of carbohydrates: Carbohydrates are important energy source of 70% or more. Glucose that represent the most important member sof carbohydrate can be source of: 1. Ribose and deoxyribose sugars form part of the structural framework of RNA and DNA. Pentoses which essential in synthesis of nucleoprotein (DNA & RNA), coenzymes and ATP. 2.. Can be converted into galactose, important for lactose synthesis (milk sugar). 3. Combines with complex lipids forming proteoglycans and glycoprotein. 4.. Glucose can be stored in the liver in the form of glycogen which utilized during starvation and between meals. 5.. Surplus amounts of glucose can be converted into fat (lipids). 6. Glucose is only energy source for red blood cells (RBCs) and fetus. 7. Immediate energy source for muscle muscle contraction. 8. Energy source for brain and nerve tissue. Therefore, most dietary carbohydrate is absorbed into the blood stream as glucose, and other sugars are converted into glucose in the liver. 8