Carbohydrates PDF - Biochemistry & Molecular Biology

Carbohydrates Dr. Islam Sameh Ali Emara, Ph.D. Lecturer of Biochemistry & Molecular Biology, Basic Science Department, Delta University for Science and Technology Definition: They are biological macromolecules (Polymers) that are made up of many smaller molecules (monomers) cal...

Carbohydrates Dr. Islam Sameh Ali Emara, Ph.D. Lecturer of Biochemistry & Molecular Biology, Basic Science Department, Delta University for Science and Technology Definition: They are biological macromolecules (Polymers) that are made up of many smaller molecules (monomers) called monosaccharides (the simplest type of carbohydrates).  Classification of Carbohydrates: Carbohydrates are classified into four main categories as follows: 1- Monosaccharides: They are molecules having only one sugar group. They cannot be further hydrolyzed into smaller units. The monosaccharides are also called simple sugars and have the formula CnH2nOn. 2- Disaccharides: They are the carbohydrates which yield two molecules of similar or dissimilar monosaccharides upon hydrolysis. 3- Oligosaccharides: They are the carbohydrates which yield three to ten monosaccharide units upon hydrolysis. 4- Polysaccharides: They are the carbohydrates which yield more than ten monosaccharide units upon hydrolysis. Monosaccharides  Classification: Monosaccharides can be further subdivided on the basis of two criteria as follows: 1- On the basis of number of carbon atoms: Monosaccharides can be trioses, tetroses, pentoses, hexoses, and heptoses. 2- On the basis of functional group: Monosaccharides containing aldehyde (–CHO) group are called as “aldoses” and those containing ketone (–CO) group are called as “ketoses.” Disaccharides  Examples of Disaccharides: 1- Lactose: Lactose is called as “milk sugar”. It is composed of glucose and galactose. 2- Maltose: It is composed of two molecules of glucose. It is commonly called as malt sugar. 3- Sucrose: It is commonly called as table sugar or cane sugar. It is composed of glucose and fructose Polysaccharides  Starch: It is the reserve carbohydrate of plant kingdom. Starch is made up of amylose and amylopectin. Amylose represents about 20% of starch by weight. Amylose is a linear polymer of D- glucose residues. Amylopectin represents about 80% of starch by weight. Amylopectin is a greatly branched polymer of D-glucose residues.  Glycogen: Glycogen is the chief reserve food of animal kingdom. Glycogen is stored in the liver and skeletal muscles in humans. Glycogen is a highly branched polymer of D-glucose residues. It is more branched than amylopectin. Metabolism  Glycolysis: Glycolysis is defined as series of biochemical reactions involved in conversion of glucose into pyruvate (Aerobic glycolysis) or lactate (Anerobic glycolysis) with the release of energy. Glycolysis occurs in cytosol in almost all cells of the body. Aerobic glycolysis gives 8 ATP, while anaerobic glycolysis gives 2 ATP.  Krebs Cycle: Krebs cycle is defined as a series of biochemical reactions occurring inside mitochondrial matrix of eukaryotes after the aerobic glycolysis that are essential for synthesis of energy in the form of ATP. It gives 30 ATP, so the net energy gained from glucose catabolism under aerobic conditions is 38 ATP.  Glycogenesis & Glycogenolysis: Glycogenesis: Glycogenesis is the biochemical process of formation of glycogen from glucose in body tissues. It takes place mainly in the liver and skeletal tissues. Glycogenolysis: It is a biochemical process of breakdown of glycogen into either glucose-6-phosphate in muscle or free glucose in liver. Glycogenolysis occurs in the liver and skeletal muscles.  Gluconeogenesis: It is the process by which glucose molecules are produced from non- carbohydrate precursors. These include lactate, amino acids, glycerol and propionate. Gluconeogenesis occurs mainly in the liver. It takes place in the fasting state or on low carbohydrate diet. The pathway involves steps of TCA cycle and reversal of glycolysis. Regulation of Blood Glucose  Post-prandial Regulation: 1. Glucose is absorbed from the intestine and enters the blood leading to rise in the blood glucose level which stimulates the secretion of insulin by beta cells of islets of Langerhans of pancreas. 2. So, the uptake of glucose is increased by most extrahepatic tissues. Moreover, insulin helps in the storage of glucose as glycogen or its conversion to fat.  Regulation in Fasting State: Glycogenolysis will take care of the blood sugar level. Thereafter, gluconeogenesis will take charge of the situation. Glucagon hormone will keep the blood glucose level from falling. They are referred to as anti-insulin hormone.