1. Carbohydrate structures
Sucrose – glucose-fructose; not reducing
Maltose – glucose-glucose; reducing
Lactose – galactose-glucose; reducing
Trehalose – glucose-glucose; reducing
Amylose – glucose molecules linked by alpha-1,4-glycosidic bonds
Amylopectin – glucose molecules linked by alpha-1,6-glycosidic bonds
- these constitute starch
Glycogen – principal animal polysaccharide; similar to amylopectin, but more branch points à smaller proportion of 1,4- to 1,6- bonds
- glucosidases hydrolyze glycosidic bonds
2. Products of starch digestion by pancreatic alpha-amylase
glucose – when only terminal sugar is removed
maltose – a disaccharide of glucose
maltotriose – a trisaccharide of glucose
oligosaccharides – contain on average eight glucosyl units; oligosaccharides that are branched with one or two alpha-1,6-glycosidic bonds are called alpha-limit dextrins
3.
Sucrase-isolmaltase complex
- Sucrase hydrolyzes sucrose à fructose and glucose
- Maltase hydrolyzes maltose, maltotriose à glucose
- Isomaltase hydrolyzes alpha-limit dextrins (alpha-1,6 bond) and isomaltose à glucose
Glucoamylase complex
- gamma-(exosaccharidases) Amylase hydrolyzes oligosaccharides including the alpha-1,4 bonds in alpha-limit dextrins but not the alpha-1,6 bond
Lactase/beta-galactosidase complex
- lactase hydrolyzes lactose (at the beta-glycosidic bond) à galactose and glucose
Trehalase complex
- trehalase hydrolyzes Trehalose à glucose
4. Lactose intolerance
Primary
- genetic
- deficiency in lactase enzyme à attempted digestion of milk results in watery diarrhea (fluid secretion into intestinal lumen because of increased osmotic pressure from undigested lactose_
Late onset lactase deficiency
- Asians, native Americans, blacks have a prevalence of this disease
Secondary or acquired lactase deficiency
- disease that causes injury to the absorptive cells of the small intestines
5.
Tropical sprue
- destruction and flattening of the intestinal villi following a bacterial infection
Celiac sprue (celiac disease, non-tropical sprue, gluten-sensitive enteropathy)
- autoimmune destruction of intestinal villi caused by sensitivity to a protein, called gluten, found in wheat, rye, barley, and oats; severe malabsorption of nutrients because intestinal villi are destroyed and the flattened apical surface of the small intestine greatly loses its absorptive surface area; features of diarrhea, weight loss, and malnutrition
6. – two types of monosaccharide transporters to move monosaccharides from intestinal lumen into epithelial cell
1. Na-independent, facilitated diffusion type of transporter for fructose (mainly) and glucose
- GLUT-5
- hexose sugars enter via this transporter by virtue of the carbohydrate concentration gradient
- intracellular carbohydrate concentrations are kept low by transport out of the cytoplasm to capillaries via the Na-independent GLUT-2 transporter in the contraluminal plasma membrane.
- these monosaccharides then travel via the portal system to the liver where they are metabolized with glucose also continuing to other tissue for energy metabolism
2. Na-cotransporter that has high specificity for glucose and galactose; promotes active sugar absorption
- SGLT-1
- driving force is due to low inside Na by action of Na,K-ATPase
- intracellular carbohydrate concentrations are kept low by transport out of the cytoplasm to capillaries via the Na-independent GLUT-2 transporter in the contraluminal plasma membrane.