carbohydrate digestion and absorption.ppt
TRANSCRIPT
CARBOHYDRATE DIGESTION AND
ABSORPTIONLourdes L. Balcueva, M.D.
OBJECTIVES• 1. To be able to describe digestion of carbohydrates in the GIT 2. To discuss absorption of carbohydrates in the enterocytes 3. To describe carbohydrate transport from the enterocyte to the blood
Digestion- the change of food from a complex to a simple form; from an insoluble to a soluble state in the GIT
Mechanical digestion- involves chewing of food into smaller particles to increase surface area incontact with digestive enzymes;propelled through GIT by peristalsis
Chemical digestion-• - hydrolysis• - addition of water and digestive enzyme• - chemical breakdown of food particles
Metabolic digestion: - nutrients used at cellular level for basic life processes - liver plays a central role reassembling digested nutrients back into complex molecules
CHO Digestion:
• Steps in digestion and absorption: 1. conversion of polymers to simpler, soluble forms 2. transported across intestinal walls 3. delivery to tissues
CHO Digestion
1.Mouth – saliva contains lingual amylase; catalyzes a (14) glycoside bonds; inactivated by strong pH in the stomach 2. Stomach - acid hydrolysis – continues the degradation; chyme (mixture of food, saliva and gastric secretion) goes to small intestines
3. Small intestines – pancreatic amylase brush borders – disaccharidases ( sucrase, maltase and lactase)
Polysaccharide Digestion• - Hydration of polysaccharides thru mastication is essential for action of amylase. - Enzyme amylase is specific for internal a 14 glycosidic linkages; inactive to a 16 linkages and to 14 linkages of branching units of glycosyl residues - cleaved units are trisaccharides (maltotriose, disaccharide maltose and oligosaccharide w/ one or more a 16 branches called a-limit dextrin
Polysaccharide Digestion
in
- Sucrase-isomaltase complex-hydrolytic cleavage of a-16 linkage- Oligosaccharidases and glucosidases- act on other products of digestion of starch- Final product of digestion- glucose- Amylase – occurs free in the intestinal lumen-- a-glucosidase and isomaltase – attached to enterocyte mucosal membrane
Disaccharidases
• -Disaccharidases are attached to small intestinal brush-border membrane• All disaccharidases are inducible EXCEPT LACTASE- Rate limiting factor in absorption of disaccharides
(except lactase) is the transport of the resulting monomeric sugar - Rate limiting factor of lactose absorption is the
hydrolysis of lactose itself
Brush Border
Absorption of Carbohydrates• - Only monosaccharides are absorbed by the intestines - Absorption rate of galactose is more that of glucose; fructose is absorbed at lesser rate than glucose - Carbohydrates are polar- they cannot diffuse thru lipid bilayer of cell membrane
Transport Mechanism-• SGluT- ( Sodium Dependent Glucose Transporter) absorption from lumen to intestinal cell is by co- transport mechanism (secondary active transport – indirect
because mech only get the driving force from the pre-established gradient,in this case, Na gradient)
- glucose and galactose are transported from lower concentration to higher concentration; - coupled to movement of Na from higher concentration to lower conc.; drives glucose molecules
Glucose Transport
• - Sodium later expelled by sodium- potassium ATPase - Glucose Transporter Type 2 (GluT2)- not Na dependent; releases glucose into the blood; behaves as gated pore thru a process called ping-pong mechanism
Absorption of Other Sugars
• Galactose – absorbed the same way as glucose Fructose – enters and leaves epithelial cells by facilitated diffusion utilizing GluT5
Absorption of Glucose
Absorption of Glucose
Transporter Major Sitesof Expression Characteristics
SGLUT 1 Intestinal mucosa, kidney tubules
Cotransports one molecule of glucose or galactose along with two sodium ions. Does not transport fructose.
GLUT-1 Brain, erythrocyte, endothelial cells, fetal tissues
Transports glucose (high affinity) and galactose, not fructose. Expressed in many cells.
GLUT-2 Liver, pancreatic beta cell, small intestine, kidney.
Tranports glucose, galactose and fructose. A low affinity, high capacity glucose transporter; serves as a "glucose sensor" in pancreatic beta cells.
Glucose Transporters
GLUT-3 Brain, placenta and testes
Transports glucose (high affinity) and galactose, not fructose. The primary glucose transporter for neurons.
GLUT-4 Skeletal and cardiac muscle, adipocytes
The insulin-responsive glucose transporter. High affinity for glucose.
GLUT-5 Small intestine, sperm
Transports fructose, but not glucose or galactose. Present also in brain, kidney, adipocytes and muscle.
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