Digestion, Absorption & Transport of CHO Lecture Notes PDF

Digestion, absorption & transport of CHO By the end of this lecture you must be able to - explain digestion of CHO starting from the buccal cavity & ending up in the small intestine - Describe the transport of monosaccharides (products of digestion) from the intestinal lumen to the bloo...

Digestion, absorption & transport of CHO By the end of this lecture you must be able to - explain digestion of CHO starting from the buccal cavity & ending up in the small intestine - Describe the transport of monosaccharides (products of digestion) from the intestinal lumen to the blood stream - Describe the transport of glucose, fructose & galactose from blood into different tissues  Metabolism= catabolism and anabolism Catabolic = degradative Anabolic = synthetic  The three stages of catabolism Stage I Stage II Stage III Stage I: Hydrolysis of their complex molecules to their component building blocks Stage II: Conversion of building blocks to Acetyl CoA Stage III: Oxidation of Acetyl CoA (Oxidative phos.)  Metabolism of carbohydrates Digestion: Starts in the oral cavity. Saliva contains α – amylase that hydrolyses starch giving maltose + oligosaccharides (short chains of sugars formed of 3-10 glucose molecules). Salivary amylase is inactivated at pH 4.0.  Digestion of carbohydrates continues in the small intestine by: Pancreatic amylase: (finally gives maltose + some glucose + isomaltose, 2 glucose molecules attached by 1 – 6 linkage). Intestinal disaccharidases: include: Maltase Sucrase Lactase isomaltase.Lactose intolerance: Develops due to deficiency of lactase enzyme and causes osmotic diarrhea. Over 70% of adults suffer lactose intolerance world wide. The mechanism by which loss of lactase by age is not known  Final products of carbohydrates digestion are: Glucose (main) Fructose, galactose, pentoses..etc.  Carbohydrates are absorbed as monosaccharides by: Active transport (against concentration gradient) Facilitative transport (diffusion) Glucose and galactose fulfil the necessary configuration for active transport: - OH on carbon 2 - A pyranose ring - A methyl group at C5 Fructose is absorbed slowly by active transport. Active absorption of glucose is powered by the sodium pump This system acts in the small intestine as well as in the kidney tubules. Phlorhizin is an inhibitor of sodium pump and hence of glucose reabsorption in the kidney.  Glucose transporters GLUT1: ubiquitously distributed; exhibits constitutive transport activity (responsible for the low-level of basal glucose uptake required to sustain respiration in all cells.) GLUT2: present in gut, liver, and pancreatic islets GLUT3: present in the central nervous system and brain GLUT4: present in insulin-responsive tissues, skeletal muscle, adipose tissue, and heart  GLUT 1 Is widely distributed in fetal tissues. In the adult, it is expressed at highest levels in erythrocytes and also in the endothelial cells of barrier tissues such as the blood brain barriers. However, it is responsible for the low- level of basal glucose uptake required to sustain respiration in all cells.  GLUT 2 Is a bidirectional transporter, allowing glucose to flow in 2 directions. Is expressed by renal tubular cells, small intestinal epithelial cells, liver cells and pancreatic beta cells.  Bidirectionality is required in liver cells to uptake glucose for glycolysis, and release of glucose during gluconeogenesis. In pancreatic beta cells, free flowing glucose is required so that the intracellular environment of these cells can accurately gauge the serum glucose levels. All three monosaccharides (glucose, galactose and fructose) are transported from the intestinal mucosal cell into the portal circulation by GLUT2  GLUT 3 Expressed mostly in neurons (where it is believed to be the main glucose transporter isoform), and in the placenta  GLUT 4 Present in insulin-responsive tissues, skeletal muscle, adipose tissue, and heart Found in adipose tissue and striated muscles (skeletal muscle and cardiac muscle).  GLUT 5 The GLUT 5 Na+-independent facilitative transporter allows fructose as well as glucose and galactose to be transported with their concentration gradients.  Transport of glucose, fructose, and galactose across the intestinal epithelium. The SGLT 1 (Sodium – glucose transporter) coupled to the Na+-K+ pump, allowing glucose and galactose to be transported against their concentration gradients. The GLUT 5 Na+-independent facilitative transporter allows fructose as well as glucose and galactose to be transported with their concentration gradients. Exit from the cell for all the sugars is via the GLUT 2 facilitative transporter. Cotransport