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Updated: 12/26/06 02:17 PM |
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| HOME | HEAL | EDUCATE | RESEARCH | DIRECTORY | OUTREACH |
Authors: D.R. Saunders, C.E. Rubin, and J.D. Ostrow
Download Chapter ♦ Download Lecture Slides & Notes
Download Chapter ♦ Download Lecture Slides & Notes
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O. Normal Carbohydrate Absorption
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Carbohydrates make up more than half of the dietary weight but less than half of the dietary calories (4 cal/gram). The major sources are the polysaccharides in plant starch or in animal glycogen and the disaccharides. Starches and glycogen are glucose polymers. The common disaccharides are sucrose (table sugar) - a glucose-fructose dimer; lactose (milk sugar) - a glucose-galactose dimer; and maltose (a starch derivative) - a glucose-glucose dimer.
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Glucose in starch is released by amylases, and mucosal enzymes Starch is usually completely digested in the upper ½ of small gut |
In the gut lumen, the dietary polysaccharides are broken down by salivary and pancreatic amylases mostly to oligosaccharides containing 5, 3 or 2 glucose molecules. Only monosaccharides can enter the absorptive cell. The oligosaccharides and the disaccharides must be hydrolyzed to monosaccharides by oligosaccharidases which are located on the microvillous membrane. There are three oligosaccharidases: lactase which hydrolyzes lactose; glucoamylase which hydrolyzes maltose and larger malto-oligosaccharides and sucrase- isomaltase which hydrolyzes sucrose and the branched oligosaccharides containing 5 glucose molecules (alpha-dextrins). The brush border enzymes glucoamylase and sucrase-isomaltase insure rapid hydrolysis, but lactase acts relatively slowly and disappears at age 7 to 10 years in 60 90% of most ethnic groups that do not come from northern or central Europe.
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| Na+/K+ ATPase provides energy for glucose absorption |
The hexoses, glucose and galactose, are absorbed actively via a carrier which also transports Na+ across the cell membrane. Fructose absorption occurs by facilitated diffusion which utilizes an apical transport protein (GLUT5) and the basolateral transporter (GLUT2).
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Next Section (P): Carbohydrate Malabsorption » |
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