CARBOHYDRATES II

1. CARBOHYDRATES II

2. Digestion of Carbohydrates Digestion of Carbohydrates • Dietary carbohydrates principally consist of the polysaccharides: starch and glycogen. It also contains disaccharides: sucrose, lactose, maltose and in small amounts monosaccharides like fructose . Liquid food materials like milk, soup, fruit juice escape digestion in mouth as they are swallowed, but solid foodstuffs are masticated thoroughly before they are swallowed. • •

3. Digestion in Mouth Digestion in Mouth Digestion of carbohydrates starts at the mouth, where they come in contact with saliva during mastication. Saliva contains a carbohydrate splitting enzyme called salivary amylase (ptyalin). • • • • Action of ptyalin (salivary amylase Action of ptyalin (salivary amylase) It is α - amylase, requires Cl- ion for activation and optimum pH 6-7. The enzyme hydrolyzes (1,4) glycosidic linkage at random, from molecules like starch, glycogen and dextrins, producing smaller molecules maltose, glucose and disaccharides. Ptyalin action stops in stomach when pH falls to 3.0 • •

6. Digestion in Stomach Digestion in Stomach • No carbohydrate splitting enzymes are available in gastric juice. HCl may hydrolyze some dietary sucrose to equal amounts of glucose and fructose. •

7. Digestion in Duodenum Digestion in Duodenum • Food reaches the duodenum from stomach where it meets the pancreatic juice. Pancreatic juice contains a carbohydrate-splitting enzyme pancreatic amylase. It is also an α - amylase, optimum pH 7.1. Like ptyalin it also requires Cl- for activity. The enzyme hydrolyzes (1,4) glycosidic linkage situated well inside polysaccharide molecule. • • • • • Other criteria and end products of action are similar of ptyalin.

8. Lactase Lactase • It is agly cosidase, its pH range is 5.4 to 6.0. Lactose is hydrolyzed to glucose and galactose. •

9. Maltase Maltase • The enzyme hydrolyzes the (1,4) glycosidic linkage between glucose units in maltose molecule liberating two glucose molecules. Its pH range is 5.8 to 6.2. Sucrase Sucrase PH ranges 5.0 to 7.0. It hydrolyzes sucrose molecule to form glucose and fructose. • •

10. Absorption of Carbohydrates Absorption of Carbohydrates Products of digestion of dietary carbohydrates are practically completely absorbed almost entirely from the small intestine. • • Absorption from proximal jejunum is three times grater than that of distal ileum. It is also proved that some disaccharides, which escape digestion, may enter the cells of the intestinal lumen by “pinocytosis” and are hydrolyzed within these cells. No carbohydrates higher than the mono saccharides can be absorbed directly in to the blood stream. • •

11. Mechanism of Absorption Mechanism of Absorption Two mechanisms are involved: Glucose is transported through the absorptive cells of the intestine by facilitated diffusion Na-dependent facilitated transport • 1. 2. • Glucose, therefore, enters the absorptive cells by binding to transport proteins, membrane-spanning proteins that bind the glucose molecule on one side of the membrane and release it on the opposite side. Two types of glucose transport proteins are present in the intestinal absorptive cells: the Na-dependent glucose transporters and the facilitative glucose transporters •

12. NA-DEPENDENT TRANSPORTERS Na-dependent glucose transporters, which are located on the luminal side of the absorptive cells, enable these cells to concentrate glucose from the intestinal lumen. • • the transport of glucose from a low concentration in the lumen to a high concentration in the cell is promoted by the cotransport of Na from a high concentration in the lumen to a low concentration in the cell

13. FACILITATIVE GLUCOSE TRANSPORTERS Facilitative glucose transporters, which do not bind Na, are located on the cells. Glucose moves via the facilitative transporters from the high concentration inside the cell to the lower concentration in the blood without the expenditure of energy. In addition to the Na-dependent glucose transporters, facilitative transporters for glucose also exist on the luminal side of the absorptive cells. The various types of facilitative glucose transporters found in the plasma membranes of cells referred to as GLUT 1 to GLUT 5 One common structural theme to these proteins is that they all contain 12 membrane-spanning domains. Note that the sodium-linked transporter on the luminal side of the intestinal epithelial cell is not a member of the GLUT family. • • • • • •

15. Metabolism of Sugars by Colonic Metabolism of Sugars by Colonic Bacteria Bacteria digested in the small intestine Starches high in amylose, or less well hydrated (e.g., starch in dried beans), are resistant to digestion and enter the colon. Dietary fiber and undigested sugars also enter the colon. Here colonic bacteria rapidly metabolize the saccharides, forming gases, short-chain fatty acids, and lactate. The short-chain fatty acids are absorbed by the colonic mucosal cells and can provide a substantial source of energy for these cells. The major gases formed are hydrogen gas (H2), carbon dioxide (CO2), and methane (CH4).. Incomplete products of digestion in the intestines increase the retention of water in the colon, resulting in diarrhea. • Not all of the starch ingested as part of foods is normally • • • • •