Physiology Ch 65 p789-798
Digestion and Absorption in the GI Tract
Digestion of Various Foods by Hydrolysis
1. Hydrolysis of Carbs – almost all carbs of diet are large polysaccharides or disaccharides composed of monosaccharides held together by condensation. When digested, carbs are converted to monosaccharides to form water and break up the polysaccharides, called hydrolysis
R’’-R’ + H2O –digestive enzyme  R’’OH + R’H
2. Hydrolysis of Fats – entire fat portion of diet consists of triacylglycerols composed of 3 fatty acids and one glycerol, and during condensation, 3 water molecules are removed
-digestion of fats returns 3 molecules of H2O to triglyceride to split triacylglycerides from glycerol
3. Hydrolysis of Proteins – proteins held together by peptide bonds, where at each linkage, an
OH ion is removed from amino acid and hydrogen has been removed from the other AA
-water is removed from proteins in hydrolysis to break up the peptide
Digestion of Carbs – only 3 major sources of carbs exist in diet: sucrose, lactose, and starches
(large polysaccharides); cellulose exists but we cannot digest it with our enzymes
-when food is chewed, it is mixed with saliva containing ptyalin (α-amylase) secreted by parotid glands, which hydrolyzes starch  maltose and glucose (only 5% gets digested in mouth)
-starch digestion occurs up to 1 hour further in stomach before mixed with gastric secretions
-30-40% of starches are hydrolyzed to maltose by the time it finishes working in stomach
-Digestion in small intestine – pancreas secretes α-amylase, but it is more powerful than parotid amylase, and 15min after chyme reaches small intestine, all carbs are digested into maltose or glucose polymers
-enterocytes lining villi of small intestine contain the enzymes sucrose, lactase, maltase and α-
dextrinase which are capable of digesting lactose, sucrose, maltose, and other glucose polymers
-disaccharides are digested as they come in contact with intestinal brush border
-maltose is split into multiple molecules of glucose; final product of digestion=monosaccharides
-water-soluble products are absorbed directly into portal blood
Digestion of Proteins – dietary proteins are linked AA bound by peptide bonds
-pepsin is most active at pH of 2.0-3.0 and inactive above pH 5; gastric glands secrete HCl from oxyntic (parietal) cells in glands
-pepsin helps digest collagen that is not affected by other enzymes
-pepsin only initiates digestion (10-20% of total protein digestion)
-most protein digestion occurs from pancreatic proteolytic enzymes in small intestine such as trypsin, chymotrypsin carboxypeptidase, and proelastase
-elastase breaks down elastin, carboxypeptidase cuts from carboxy end of protein
-most proteins remain as undigested dimers or trimersd
-last digestive stages of protein happens by enterocytes that line villi of small intestine in duodenum and jejunum
-brush border with microvilli containing peptidases protruding to exterior exist
-most important are aminopolypeptidase and several dipeptidases
-amino acids/dipeptides easily cross microvillar membrane to interior of enterocyte
-inside cytosol, multiple peptidases exist to complete the digestion
-more than 99% of final protein digestive products are individual amino acids

Digestion of Fats – triglycerides are the most abundant fat in diet, composed of glycerol and 3 fatty acids; also phospholipids, cholesterol, and cholesterol esters exist
-small amount of triglycerides digested in stomach by lingual lipase that is secreted by lingual glands in the mouth and swallowed with saliva (10% digested)
-first step in digestion is emulsification by bile acids and lecithin – agitation in stomach to mix fat with stomach products is the first step in emulsification
-emulsification mostly in duodenum under influence of bile from liver
-bile contains bile salts and lecithin, important in emulsification
-polar parts of bile salts and lecithin are highly H2O soluble whereas most of remaining portions of molecules are highly soluble in fat, so the amphipathic molecules bind to fat and make them soluble in the hydrophilic environment
-when interfacial tension of a globule of immiscible liquid is low, on agitation it breaks up into tiny particles when interfacial tension is great.
-major function of bile salts and lecithin is to make fat globules readily fragmentable by agitation with H2O in small bowel
-lipase enzymes are H2O soluble and can attack fat globules only on their surfaces; bile helps
Triglycerides are digested by Pancreatic Lipase – present in pancreatic juice, and enough to digest all triglycerides in 1 minute
-enteric lipase exists in enterocytes
-End products of fat digestion are free fatty acids – triglycerides are split by pancreatic lipase to free fatty acids and 2-monoglycerides
-Bile salts form micelles that accelerate fat digestion – bile salts remove fatty acids and monoglycerides from vicinity of digesting fat globules rapidly by forming micelles (bile salts contain a sterol nucleus which allow it to form micelles)
-micelles act as transport medium to carry monoglycerides and fatty acids to brush borders of intestinal epithelial cells and later absorbed in blood
Digestion of cholesterol esters and phospholipids – most cholesterol is in form of cholesterol ester, containing free cholesterol + 1 fatty acid. Phospholipids also contain fatty acids
-both are hydrolyzed by lipases from pancreas: cholesterol ester hydrolase and
phospholipase A2 respectively
-bile salts ferry free cholesterol and phospholipids
Basic Principles of GI Absorption – total quantity of fluid absorbed each day is equal to ingested fluid + secreted from various GI secretions (1.5L is not absorbed and passes to colon)
-stomach is poor absorptive area because it lacks villus type membrane and it has tight junctions
-only aspirin and alcohol can pass through stomach
Folds of Kerckring, villi, and microvilli increase absorptive area by 1000x – on each intestinal cell exists a brush border containing 1000 microvilli to expose surface area even more
-there is an advantageous arrangement of vascular system for absorption of fluid into portal blood and the arrangement of central lacteal lymph vessel for absorption into the lymph
-small substances are absorbed by pinocytosis
Absorption in Small Intestine – several hundred g of carbs, 100 more grams fat, 40-100g AA, 50-
100 grams ions, and 7-8L H2O
-absorptive capacity is greater in small intestine; large intestine can still absorb water
Absorption of H2O by Osmosis – H2O is transported through intestinal membrane by diffusion, obeying the laws of osmosis; when chyme is dilute enough, water is absorbed through intestinal

mucosa into blood entirely by osmosis; it can be reversed if hyperosmotic solutions are discharged from stomach to duodenum
Absorption of Ions – Sodium is actively transported through the intestinal membrane
-sodium plays an important role in helping absorb sugars and AA
-motive power for Na absorption takes place from inside epithelial cells through basolateral walls of cells into paracellular spaces
-it requires energy through ATP enzymes on cell membrane, and part of the sodium is absorbed with chloride
-active transport of Na through basolateral membranes reduces cell Na concentration inside the cell to a low value (50mEq/L)
-because Na concentration in chyme is normally 142mEq/L, Na moves down steep electrochemical gradient from chyme through brush border into epithelial cell cytoplasm
-Na is also cotransported by carrier proteins such as (1) Na/glucose cotransporter, (2)
Na/amino acid cotransporter, (3) Na/H+ antiporter
-active transport of Na out of the cell on the basolateral membrane can use Na/K
ATPase
Osmosis of H2O – large osmotic gradient created between elevated concentration of ions in paracellular space. Much of the osmosis occurs through tight junctions between the apical borders of epithelial cells
-osmotic movement of H2O creates flow of fluid into and thorugh paracellular spaces and into circulating blood
Aldosterone enhances Na absorption – during dehydration, aldosterone is secreted by adrenal gland cortex to cause activation and transport mechanisms for all sodium absorption in intestine, and increased Na absorption  H2O, Cl absorption
-especially important in colon
Absorption of Chloride in Small Intestine – chloride absorption occurs rapidly by diffusion and move through electrical gradient to follow sodium ions
-also exchanges in brush border of ileum and large intestine by Cl/HCO3 exchanger
Absorption of HCO3 in duodenum and jejunum – when Na is absorbed, H+ is secreted into lumen in exchange, and combines with HCO3  H2CO3 which dissociates to H2O + CO2
Secretion of HCO3 in Ileum and Large Intestine – Simultaneous absorption of chloride ions – in ileum and large intestine, bicarb can be secreted in exchange for Cl to neutralize acid formed by bacteria in large intestine
Extreme secretion of Cl, Na, and H2O from Large intestine by diarrhea – immature epithelial cells in folds of intestinal epithelium that continue to divide exist. In those folds, epithelium secretes NaCl and H2O into lumen of intestine and reabsorbed by older cells outside folds to provide flow of H2O for absorption
-toxins of cholera and diarrheal bacteria can stimulate these secretions so much that it cannot be reabsorbed enough to cause diarrhea
Active absorption of Ca, Fe, K, Mg, PO4 – Ca are actively absorbed into blood from duodenum and just enough is absorbed for used by body; controlled by parathyroid hormone and vitamin D
-Fe is absorbed from small intestine

-K, Mg, PO4 can be absorbed through small intestine mucosa with ease
Absorption of nutrients –
-Carbs are absorbed as monosaccharides – glucose is the most abundant carb absorbed because it is the final digestion product of carbs, the remaining 20% are galactose and fructose
-glucose is transported by Na/Cotransport mechanism – without sodium transport, no glucose can be absorbed, and occurs in 2 stages.
1. active transport of Na through basolateral membrane of intestinal epithelium to blood, depleting Na inside cells
2. decrease of Na inside cells cause Na from intestinal lumen to move through brush border of epithelial cells to cell interiors by process of secondary active transport
-Na combines with transport protein, but will not be transported unless combined with another substance like glucose
-low concentration of Na inside cell DRAGS Na to interior of the cell along with glucose, called facilitated diffusion
Absorption of other monosaccharides – galactose is transported by almost the same way as glucose
-fructose is transported by facilitated diffusion all the way through intestinal epithelium by phosphorylation inside enterocyte and conversion into glucose
-rate of fructose absorption is ½ that of glucose or galactose
Absorption of proteins as dipeptides – energy for transport supplied by sodium co-transporter same way as glucose is transported, this is called co-transport, or secondary active transport of amino acids
Absorption of Fats – absorbed with the help of bile micelles which make fat soluble in chyme and are carried to surfaces of microvilli on intestinal brush border where monoglycerides and fatty acids diffuse immediately out of micelles and into enterocytes, leaving bile micelles in intestine to keep functioning.
-inside epithelial cells, triglycerides are reformed and released in chylomicrons through basal side of enterocyte and through lymph blood into circulating blood
Direct absorption of fatty acids into portal blood – small short- and medium- chain fatty acids can be directly absorbed into portal blood without being converted into triglycerides and being carried by lymphatics
-short chain fatty acids are more water soluble
Absorption in Large Intestine, formation of feces – 1.5L of chyme pass through ileocecal valve every day, and H2O is absorbed through colon leaving less than 100mL of fluid excreted in feces
-much of the absorption occurs in proximal portion whereas distal colon functions as storage
Absorption and secretion of electrolytes and H2O – in mucosa of large intestine, active absorption of Na occurs to cause electrical gradient
-tight junctions in colon are tighter than small intestine, preventing backflow, allowing more complete Na absorption and more true with more aldosterone
-HCO3 in distal small intestine helps neutralize acid in large intestine
Maximal absorption capacity of large intestine – maximum of 5-8L of fluid each day; excess of which would pass through to cause diarrhea

Bacterial action in colon – colon bacilli are present in absorbing colon which are capable of digesting small amounts of cellulose, but also vitamin K, B12, thiamine, riboflavin, and gases that contribute to flatus in colon – especially CO2, H2 and CH4 produced
Composition of Feces – 3/4 th H2O and ¼ solid matter composed of 30% dead bacteria, 10-20% fat, 10-20% inorganic matter, protein, undigested roughage
-brown color of feces is caused by stercobilin and urobilin, derivatives of bilirubin
-odor is caused by products of bacterial action depending on person’s colonic flora
-actual odors come from indoles, skatoles, mercaptans, and hydrogen sulfide