Biochemistry

CHAPTER 11. CARBOHYDRATE METABOLISM 11.1 THE BREAKDOWN OF CARBOHYDRATES IN THE HUMAN DIGESTIVE TRACT A person consumes 450-800 g of carbohydrates per day. 35% of them are monosaccharides and disaccharides, 65% are polysaccharides. The latter are predominantly starch. The amount of glycogen in food is small: 4-15 g/day. The main sources of carbohydrate foods are plant products: bread, cereals, vegetables, fruits, fruits and berries. All carbohydrates, except fiber and pectin, undergo hydrolytic digestion of enzymes in the gastrointestinal tract. Enzymes that catalyze the hydrolysis of starch and disaccharides have an optimum action during a weakly alkaline reaction and are inactive in an acidic medium. The exception is salivary amylase, which exhibits maximum activity at pH 6,8. Starch and glycogen begin to be digested in the oral cavity under the action of a-salivary amylase, with the formation of "fragments" of molecules - dextrins and a small amount of maltose, which splits into glucose of salivarymaltase. Dextrinshave reducing properties, and they increase with decreasing molecular weight, their color under the action of iodine changes. There are amylodextrins, which are painted with iodine in violet-blue color; erythrodextrins , which give a red-brown color with iodine; achrodextrins and maltodextrins do not change color in reaction with iodine. In the stomach, there are no enzymes catalyzing the hydrolysis of carbohydrates, which stops in the acidic environment of the stomach. Further breakdown of starch and dextrins occurs in the duodenum under the influence of enzymes of pancreatic juice: a-amylase, maltase, lactase, sucrase. The final breakdown of polysaccharides to monosaccharides (glucose) occurs in the jejunum and ileum. Hydrolysis of disaccharides does not occur in the lumen of the small intestine, but on the membrane of cells of its mucosa under the action of the corresponding enzymes localized in the brush border of the epithelium. At the same time, maltose is split into Avo glucose molecules, sucrose into glucose and fructose, and lactose into glucose and galactose. Particular interest has the breakdown of fiber (cellulose/ In the human digestive juices there is no cellulase enzyme. In a small amount, cellulose is broken down in the large intestine under the influence of microflora. Enzymes of microorganisms - cellulose and cellobiasis hydrolyze cellulose to glucose, which undergoes various types of fermentation to form H 2 , CO 2 , CH 4 , alcohols, and 211