Ch.23 Digestive System

23.10 Mechs of Digestion & Absorption:

o Enzymatic Hydrolysis:

• Digestion: catabolic process that breaks down macromolecules into monomers

  • intrinsic & accessory gland enzymes are involved

  • enzymes carry out hydrolysis, where water is added to break chem bonds

o Mechs of Absorption:

• Absorption: process of moving substances from lumen into body

• tight junctions ensure molecules pass thru epithelial cell rather than bw them

  • materials enter cell thru apical membrane & exit thru basolateral membrane

• lipid molecules can be absorbed passively thru membrane but other polar molecules use active transport

• most nutrients are absorbed b4 chyme reaches ileum

23.11 Processing of Nurtients:

o Carbohydrates Digestion

• only monosaccharides can be absorbed

• starch & disaccharides are broken down to oligosaccharides & disaccharides

  • begins in mouth w/ salivary amylase

• further broken down into lactose, maltose, & sucrose. final breakdown into monosaccharides (glucose, fructose, galactose)

• Starch digestion in intestine:

  • pancreatic amylase breaks down starch/glycogen into oligosaccharides & disaccharides

  • brush border enzymes dextrinase, lactase, glucoamylase, maltase, & sucrase further breaks them down into lactose, maltose, & sucrose & then into monosaccharides

  • monosaccharides are cotransported across apical membrane by secondary active transport w/ Na+ & lastly exit basolateral membrane by facilitated diffusion

• Proteins digestion:

  • not only dietary, but also includes digestive enzymes & proteins from breakdown of mucosal cells

  • broken into large polypeptides, small polypeptides/peptides, and finally into amino acid monomers w/ some dipeptides & tripeptides

  • digestions begins in stomach when pepsinogen becomes pepsin @ pH 1.5-2.5 (inactive in duodenum)

  • in the intestines:

    • pancreatic proteases trypsin & chymotrypsin cleave protein into smaller peptides, while carboxypeptidase takes 1 amino acid @ end at a time

    • brush border enzymes aminopeptidase, carboxypeptidases, & dipeptidases break oligopeptides & dipeptides into amino acids

    • amino acids are cotransported across apical membrane via secondary active transport carriers Na+/H+ & exot across basolateral membrane via facilitated diffusion

• Lipids digestion:

  • emulsification: triglycerides & their breakdown products are insoluble in H2O & need pre-treatment w/ bile salts that break large fat globules into smaller

  • digestion: pancreatic lipases break down fat into monoglyceride + 2 free fatty acids

  • micelle formation: products from digestion become coated w/ bile salts & lecithin

  • diffusion: lipid products leave micelles & cross apical membrane via diffusion

  • chylomicron formation: lipid products go back into triglycerides & packed w/ licethin & lipoproteins

  • chylomicron transport: are exocytosed from basolateral side & enter lymphatic lacteal, eventually emptied into venous blood @ thoracic duct

    • once in blood, chylomicrons are broken into free fatty acids & glycerol by lipoprotein lipase so cells can use

      • short-chain fatty acids can diffuse directly into blood

• Nucleic acid digestion:

  • pancreatic nucleases hydrolyze nucleic acid to nucleotide monomers

  • enzymes nucleosidases & phosphatases break nucleotides down into free nitrogenous bases, pentose sugars, & phosphate ions

  • breakdown products are actively transported by special carriers in epithelium of villi

o Absorption of Vitamins, Electrolytes, & H2O:

• Of Vitamins:

  • in small intestine:

    • fat-soluble vits A, D, E, & K are carried by micelles & diffuse into absorptive cells

    • H2O-soluble vits C & B are absorber by diffusion/passive/active transporters

    • vit B12, a large & charged molecule, binds w/ intrinsic factor & is absorbed by endocytosis

  • in large intestine, vits K & B from bacterial metabolism are absorbed

• Of electrolytes:

  • most ions actively transported along length of small intestine

    • Fe & Ca absorbed in duodenum

      • absorption is related to need

        • ionic Fe stored in mucosal cells w/ ferritin

        • when needed, transported in blood by transferrin

    • Na+ absorption coupled w/ active absorption of glucose & amino acids

    • Cl- transported actively

    • K+ diffuses in response to osmotic gradients; lost of H2O absorption is poor

    • Ca2+ absorption regulated by vit D & PTH

• Of H2O:

  • 9L H2O, most from GI secretions, enter small intestine

    • 95% absorbed; most of rest absorbed in large intestine

  • net osmosis occurs if conc. gradient is established by active transport of solutes

  • H2O uptake coupled w/ solute uptake