Ch. 23(5) Digestive System: Physiology of Digestion and Absorption

process of digestion/absorption of lipids

1. emulsification bile salts in the duodenum break large fat globules into smaller fat droplets, increasing the surface area available to lipase enzymes 2. digestion pancreatic lipases hydrolyze triglycerides, yielding monoglycerides and free fatty acids 3. micelle formation micelles (made of fatty acids, monoglycerides, and bile salts) ferry their contents to epithelial cells 4. diffusion fatty acids and monoglycerides diffuse from micelles into epithelial cells 5. chylomicron formation fatty acids and monoglycerides are recombined and packaged with other fatty substances and proteins to form chylomicrons 6. chylomicron transport chylomicrons are extruded from the epithelial cells by exocytosis, enter lacteals, and are carried away from the intestine in lymph

process of digestion/absorption of proteins

1. proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxypeptidase), and by brush border enzymes (carboxypeptidase, amino peptidase, and dipeptidase) of mucosal cells 2. the amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side 3. the amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts

process of absorption of carbohydrates

1. the NaK pump stores energy that drives glucose and galactose uptake by creating a *steep concentration gradient for Na* entry into intestinal cells NaK pump on other side of cell is constantly pumping as much Na to create a conc gradient - *lower amount of Na in the cell higher conc in lumen and moves to lower conc inside the cell* 2. as Na moves across the membrane through a membrane *cotransporter protein* (in this case SGLT), it drives glucose against its concentration gradient into the cells 3. *fructose enters the cell by facilitated diffusion* 4. all 3 monosaccharides exit across the basolateral membrane via *facilitated diffusion* on the GLUT2 sugar transport

absorption of water

9 L water, most from GI tract secretions, enter small intestine 95% absorbed in the small intestine by osmosis most of rest absorbed in large intestine net osmosis occurs if concentration gradient established by active transport of solutes water uptake coupled with solute uptake lumen: solute dec, water inc blood: solute inc, water dec

absorption of electrolytes: Cl

transported *actively*

pancreatic proteases (digestion of proteins)

trypsin, chymotrypsin, and carboxypeptidase

absorption of vitamins in large intestine

vitamin K and B vitamins from bacterial metabolism are absorbed

hydrolysis

water is added to break bonds

homeostatic imbalance of proteins

whole proteins not usually absorbed can be taken up by endocytosis/exocytosis most common in newborns -> food allergies usually disappear with mucosa maturation - mucosal epithelium isn't completely developed - cells don't realize they should be taking in large molecules; body knows its only supposed to absorb monomers allows IgA antibodies in breast milk to reach infant's bloodstream -> passive immunity

absorption of electrolytes: Ca

absorption regulated by *vitamin D* and parathyroid hormone (*PTH*)

absorption

all food 80% electrolytes most water absorbed in small intestine most prior to ileum - ileum reclaims bile salts most absorbed by active transport -> blood exception - lipids - go into lymphatic system

proteins ->

amino acids

absorption of protein

amino acids transported by several types of carriers - most coupled to active transport of Na dipeptides and tripeptides actively absorbed by H - dependent cotransport; digested to amino acids within epithelial cells enter capillary blood by diffusion

brush border enzymes (digestion of proteins)

amino peptidases, carboxypeptidases, and dipeptidases dextrinase, glucoamylase, lactase, maltase, sucrase

enzyme for carbohydrate -> monosaccharides

amylase

causes of malabsorption of nutrients

anything that *interferes with delivery of bile or pancreatic juice* *damaged intestinal mucosa* (bacterial infection; some antibiotics)

absorption of vitamins in small intestine: *fat-soluble vitamins*

carried by *micelles* diffuse into absorptive cells A,D,E, and K in small intestine

digestion

catabolic macromolecules -> monomers small enough for absorption enzymes: intrinsic and accessory gland enzymes break down food

absorption of lipids: absorption of *monoglycerides and fatty acids*

cluster with bile salts and lecithin to *form micelles* released by micelles to diffuse* into epithelial cells* combined with lecithin, phospholipids, cholesterol, and coated with proteins to form *chylomicrons* *enter lacteals; transported to systemic circulation* *hydrolyzed* to free fatty acids and glycerol by lipoprotein lipase of capillary endothelium - *cells can use for energy or stored fat*

absorption of electrolytes: Na

coupled with active absorption of glucose and amino acids

enzyme for lipids (fats) -> monoglycerides and fatty acids

lipases

chylomicrons

micelles combined with lecithin, phospholipids, cholesterol, and coated with proteins (lipids) *allow lipids to go to lymphatic system through lacteals*

lipids (fats) ->

monoglycerides and fatty acids

carbohydrates ->

monosaccharides

absorption of electrolytes

most ions actively along length of small intestine iron and calcium are absorbed in duodenum Na coupled with active absorption of glucose and amino acids Cl- transported actively K diffuses in response to osmotic gradients; lost if poor water absorption usually amount in intestine is amount absorbed iron and calcium absorption related to need ionic iron stored in mucosal cells with ferritin when needed, transported in blood by transferrin Ca2+ absorption regulated by vitamin K and parathyroid hormone (PTH)

passive immunity

mother makes antibodies and infant is receiving immunity because they can cross mucosal barrier

enzyme for nucleic acids (DNA/RNA) ->free bases, pentose sugars, phosphate ions

nucleases

digestion of carbohydrates

only *monosaccharides* can be absorbed monosaccharides absorbed as ingested - *glucose, fructose, galactose*

digestion of nucleic acids: enzymes

pancreatic ribonuclease and deoxyribonuclease -> nucleotide monomers brush border enzyme nucleosides and phosphates -> free bases, pentose sugars, phosphate ions

enzyme for proteins -> amino acids

proteases

starch digestion (digestion of carbohydrates)

salivary amylase (saliva) oligosaccharides at pH 6.75-7 pancreatic amylase (small intestine) -> breaks down any that escaped salivary amylase -> oligosaccharides brush border enzymes (dextrine, glucoamylase, lactase, maltase, sucrase) -> oligosaccharides -> monosaccharides

digestive enzymes for digestion of carbohydrates

salivary amylase, pancreatic amylase, and brush border enzymes (dextrine, glucoamylase, lactase, maltase, and sucrase) break down disaccharides sucrose, lactose, maltose; polysaccharides glycogen and starch

absorption of nucleic acids

*active transport* across epithelium into bloodstream

absorption of vitamins in small intestine: *vitamin B12*

*binds with intrinsic factor*, and is *absorbed by endocytosis* large, charged molecule in small intestine

digestion of lipids

*exception* pre-treatment: *emulsification by bile salts* - does not break bonds enzymes - pancreatic lipases - fatty acids and monoglycerides bile salts (in bile made by liver) break apart lipids which allows enzymes to have a *greater surface area*

absorption of carbohydrates: *glucose and galactose*

*secondary active transport (cotransport)* with Na -> epithelial cells move out of epithelial cells by *facilitated diffusion* -> capillary beds in villi

absorption of vitamins in small intestine: *water-soluble vitamins*

absorbed by *diffusion or by passive or active transporters* vitamin C and B vitamins in small intestine

absorption of electrolytes: iron and calcium

absorbed in duodenum related to need ionic iron stored in mucosal cells with *ferritin* - keeps iron in the cell when needed, transported in blood by *transferrin*

brush border enzymes

dextrine, glucoamylase, lactase, maltase, and sucrase

absorption of lipids: absorption of *short chain fatty acids*

diffuse into *portal blood* for distribution

absorption of electrolytes: K

diffuses in response to osmotic gradients lost if poor water absorption *poor water absorption = poor K absorption*

absorption of carbohydrates: fructose

facilitated diffusion to enter and exit cells

absorption of vitamins in small intestine

fat-soluble vitamins (A, D, E, and K) carried by micelles; diffuse into absorptive cells water-soluble vitamins (vitamin C and B vitamins) absobed by diffusion or by passive or active transporters vitamin B12 (large, charged molecule) binds with intrinsic factor, and is absorbed by endocytosis

nucleic acids (DNA/RNA) ->

free bases pentose sugars phosphate ions

gluten-sensitive enteropathy (celiac disease)

immune reaction to gluten gluten causes immune cell damage to intestinal villi and brush border treated by eliminating gluten from diet (all grains but rice and corn)

digestion of proteins

source is dietary, digestive enzymes, mucosal cells; *digested to amino acid monomers* beings with *pepsin* in stomach at pH 1.5-2.5 - inactive in high pH of duodenum; made in pancreas, but site of action is small intestine *large proteins -> peptides -> amino acids*