Absorption of Nutrients

1. General Features
1. transepithelial transport - nutrients must pass across the epithelial lining of the small intestine
2. active transport - most nutrients must be transported across membrane using ATP of the cells
2. Carbohydrate Absorption
• facilitated diffusion - glucose and galactose (coupled with active transport of Na+)
• "carrier molecule" has binding sites for both sugar and Na+; relies on Na+ gradient
3. Protein (Amino Acid) Absorption
• facilitated diffusion - amino acids and small peptides (coupled with Na+ active transport)
• "carrier molecule" has binding sites for both amino acid and Na+; relies on Na+ gradient
• food allergies - absorption of proteins in infant gut causes early immune reaction
4. Lipid Absorption
• micelles - tiny balls of fats that result from bile salt emulsification and "lecithin"
• contain cholesterol and fat-soluble vitamins
• diffuse through lipid bilayer of membrane
• chylomicrons - micelles combined with associated proteins within the cell; enter the lacteals of the lymphatic system
5. Nucleic Acid Absorption
• pentoses, nitrogen bases, phosphates - absorbed by similar processes as sugars and amino acids
6. Vitamin Absorption
• fat soluble - Vitamins A, D, E, K are absorbed by epithelial cells along with lipid micelles
• OLESTRA - will carry fat soluble vitamins out in feces with it
• water soluble - Vitamins B & C absorbed by diffusion
• Vitamin B12 - large and electrically charged, must bind with "intrinsic factor" before being taken into the cell by endocytosis
7. Electrolyte Absorption
• Fe and Ca - primarily absorbed in small intestine
• ferritin - sequesters Fe in intestinal cells
• transferrin - transfers Fe into circulation when need is present (menstruation)
• Vitamin D - facilitates Ca absorption
• Na - exchanged for sugars and amino acids
• Cl - absorbed into cells and exchanged for HCO3-
• K - absorbed into cells due to osmotic gradients
8. Water Absorption
• small intestine - 95% of water absorbed by small intestine following transport of solutes
• large intestine - absorbs remaining water before moving the chyme on to the rectum
9. Malabsorption of Nutrients
• impairment of bile or pancreatic juice release
• infections of the intestinal mucosa
• gluten enteropathy - "gluten" protein in grains damages the mucosa of the intestines

Category: Physiology Notes