Chapter 65 Phys
Digestion of the Various Foods by Hydrolysis
 Almost all carbohydrates of diet are either large polysaccharides or disaccharides, which are combinations of
monosaccharides bound to one another by condensation (H+ has been removed from one of monosaccharides,
and OH- had been removed from the next one; add at site of removal and H2O is released)
o When carbs digested, above process is reversed, and carbs converted to monosaccharides
(R’’-R’ + H2O  R’’OH + R’H); specific enzymes in digestive juices of GI tract return H+ and OH- from
water to polysaccharides and thereby separate the monosaccharides (hydrolysis)
 Almost entire fat portion of diet consists of triglycerides (neutral fats); because its 3 fatty acids on a glycerol,
during condensation, 3 molecules H2O removed
o Digestion of triglycerides by hydrolysis consists of fat-digesting enzymes returning 3 H2O to triglyceride
molecule and splitting fatty acids away from glycerol
 Proteins formed by peptide linkages of amino acids; at each linkage, OH- removed from one amino acid and H+
removed from succeeding one (condensation) and digestion occurs by hydrolysis
o Proteolytic enzymes return H+ and OH- from water to protein molecules to split them into constituents
 All digestive enzymes are proteins
 Digestion of carbs – sucrose (disaccharide in cane sugar), lactose, and starches (large polysaccharides present in
almost all non-animal foods, particularly potatoes and different types of grain) are major sources of carbs in diet
o Minor carbs include amylose, glycogen, alcohol, lactic acid, pyruvic acid, pectins, dextrins, and minor
quantities of carb derivatives in meats; cellulose is carb, but not digested
o Saliva contains ptyalin (an α-amylase) secreted mainly by parotid glands; ptyalin hydrolyzes starch into
disaccharide maltose and other small polymers of glucose that contain 3-9 glucose molecules
 Not more than 5% of starches will have become hydrolyzed by time food is swallowed
 Starch digestion sometimes continues in body and fundus of stomach for as long as 1 hour
before food becomes mixed with stomach secretions; then activity of salivary amylase blocked
by acid of gastric secretions because amylase is essentially not active below pH of 4.0
 On average, before food and accompanying saliva become completely mixed with gastric
secretions, 30-40% of starches will be hydrolyzed mainly to maltose
o Pancreatic secretions contain large quantity of α-amylase (identical to that in saliva, but several times as
powerful); within 15-30 minutes after chyme empties from stomach into duodenum and mixes with
pancreatic juice, virtually all carbs will be digested
 Carbs almost totally converted to maltose and/or other small glucose polymers before passing
beyond duodenum or upper jejunum
o Enterocytes lining villi of small intestine contain lactase, sucrase, maltase, and α-dextrinase; capable of
splitting lactose, sucrose, and maltose, plus other small glucose polymers into constituent
monosaccharides
 Enzymes located in enterocytes covering intestinal microvilli brush border, so disaccharides
digested as they come in contact with enterocytes
 Lactose splits into galactose and glucose; sucrose splits into fructose and glucose; maltose and
other small glucose polymers split into multiple molecules of glucose
 Monosaccharides all water soluble and are absorbed immediately into portal blood
o In ordinary diet, which contains far more starches than all other carbs combined, glucose represents
more than 80% of final products of carb digestion, and galactose and fructose seldom more than 10%
 Digestion of proteins – pepsin (important peptic enzymes of stomach) most active at pH of 2.0-3.0 and is
inactive at pH above 5.0; average pH of stomach with all secretions and contents mixed is 2.0-3.0
o Pepsin able to digest collagen (albuminoid protein affected little by other digestive enzymes); collagen is
major constituent of intercellular CT of meats, so for digestive enzymes to penetrate meats and digest
other meat proteins, it is necessary that collagen fibers be digested
 Pepsin only initiates process of protein digestion (10-20% of total protein digestion) to convert
protein to proteoses, peptones, and a few polypeptides
 Splitting of proteins occurs as result of hydrolysis at peptide linkages between AAs
o Most protein digestion occurs in duodenum and jejunum under influence of proteolytic enzymes from
pancreatic secretion
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Immediately on entering small intestine, partial breakdown products of protein foods attached
by major proteolytic pancreatic enzymes (trypsin, chymotrypsin, carboxypolypeptidase, and
elastase)
 Both trypsin and chymotrypsin split protein molecules into small polypeptides
 Carboxypolypeptidase cleaves individual amino acids from carboxyl ends of polypeptides
 Elastase digests elastin fibers that partially hold meats together
 Only small percentage of proteins digested all the way to constituent amino acids by pancreatic
juices; most remain dipeptides and tripeptides
o In membrane of each microvilli are multiple peptidases that protrude through membranes to exterior,
where they come in contact with intestinal fluids
 Aminopolypeptidase and several dipeptidases split remaining larger polypeptides into
tripeptides and dipeptides and a few into amino acids
 All breakdown products easily transported through microvillar membrane to interior of
enterocyte
 Inside cytosol of enterocyte are multiple other peptidases that are specific for remaining types
of linkages between AAs; within minutes, virtually all dipeptides and tripeptides digested to final
stage to form single AAs, which pass on through to other side of enterocyte and into blood
o More than 99% of final protein digestive products that are absorbed are individual AAs, with only rare
absorption of peptides and very, very rare absorption of whole protein molecules
 Even few absorbed molecules of whole protein can sometimes cause serious allergic or
immunologic disturbances
Digestion of fats – by far, most abundant fats of diet are triglycerides; major constituent in food of animal origin,
but much, much less so in food of plant origin
o Small quantities of phospholipids, cholesterol, and cholesterol esters in diet; phospholipids and
cholesterol esters contain fatty acid and are considered fats; cholesterol is sterol compound that
contains no fatty acid, but does exhibit some physical and chemical characteristics of fats, and is derived
from fats and metabolized similarly to fats, so from a dietary point of view, it’s a fat
o Small amount of triglycerides digested in stomach by lingual lipase secreted by lingual glands in mouth
and swallowed with saliva; less than 10% of digestion
o Fat globules broken into small sizes so water-soluble digestive enzymes can act on globule surfaces
(emulsification); begins by agitation in stomach to mix fat with products of stomach digestion
 Most emulsification occurs in duodenum under influence of bile; lecithin especially important
for emulsification of fat
 Polar parts of bile slats and lecithin molecules highly soluble in water, and most remaining
portions of their molecules highly fat-soluble
 Fat-soluble portions of liver secretions dissolve in surface layer of fat globules, with polar
portions projecting; polar projections are soluble in surrounding watery fluids, which greatly
decreases interfacial tension of fat and makes it soluble as well
 When interfacial tension of globule of non-miscible fluid low, on agitation, fluid can be broken
up into many tiny particles far more easily than it can when interfacial tension is great
 Major function of bile salts and lecithin, especially lecithin, in bile is to make fat globules readily
fragmentable by agitation with water in small bowel; detergent action
o Each time diameters of fat globules significantly decreased as result of agitation in small intestine, total
surface area of fat increases manyfold; lipase enzymes are water-soluble compounds and can attack fat
globules only on surfaces, so detergent function of bile salts and lecithin important for digestion of fats
o By far, most important enzyme for digestion of triglycerides is pancreatic lipase, present in enormous
quantities in pancreatic juice, enough to digest all triglycerides it can reach in 1 minute
o Enterocytes of small intestine contain additional lipase (enteric lipase) that is usually not needed
o Most of triglycerides of diet split by pancreatic lipase into free fatty acids and 2-monoglycerides
fat + bile + agitation  emulsified fat  fatty acids + 2-monoglycerides
o Hydrolysis of triglycerides is highly reversible process; accumulation of monoglycerides and free fatty
acids in vicinity of digesting fats quickly blocks further digestion
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Bile salts remove monoglycerides and free fatty acids from vicinity of digesting fat globules
almost as rapidly as end products of digestion formed; bile salts have propensity to form
micelles (small spherical, cylindrical globules composed of bile salt) that develop because each
bile salt molecule composed of sterol nucleus that is highly fat-soluble and polar group that is
highly water-soluble; sterol nucleus encompasses fat digestate, forming small fat globule in
middle of resulting micelle, with polar groups of bile salts projecting outward to cover surface of
micelle; because polar groups negatively carged, they allow entire micelle globule to dissolve in
water of digestive fluids and remain in stable solution until fat absorbed into blood
o Bile salt micelles also act as transport medium to carry monoglycerides and free fatty acids to brush
borders of intestinal epithelial cells; monoglycerides and free fatty acids absorbed into blood, but bile
salts themselves released back into chyme to be used again for ferrying process
o Most cholesterol in diet in form of cholesterol esters (combination of free cholesterol and one molecule
of fatty acid)
o Phospholipids contain fatty acid in their molecules
o Both cholesterol esters and phospholipids hydrolyzed by 2 other lipases to pancreatic secretion that free
the fatty acids (cholesterol ester hydrolase hydrolyzes cholesterol ester, and phospholipase A2
hydrolyzes phospholipid)
o Bile salt micelles play same role in ferrying free cholesterol and phospholipid molecule digestates that
they do for monoglycerides and free fatty acids
Basic Principles of Gastrointestinal Absorption
 Stomach is poor absorptive area of GI tract because it lacks typical villus type of absorptive membrane and
because junctions between epithelial cells are tight junctions
o Only few highly lipid-soluble substances (alcohol and some drugs like aspirin) can be absorbed in small
quantities
 Almost all fluid absorbed in small intestine (6.5-7.5 L, leaving 1.5 L for large intestine)
 Absorptive surface of small intestinal mucosa has many folds (valvulae conniventes or folds of Kerckring) which
increase surface area of absorptive mucosa 3x; folds extend circularly most of way around intestine and are
especially well developed in duodenum and jejunum, where they protrude into lumen
o Each of the valvulae conniventes covered by small villi (distribution starts dense but becomes more
diffuse as you travel down small intestine); presence of villi increases total absorptive area another 10x
o Each epithelial cell on each villus characterized by brush border of microvilli, which increases surface
area at least another 20x
o Combination of folds of Kerckring, villi, and microvilli increases total absorptive area of mucosa 1000x
 Many small pinocytic vesicles in enterocyte membrane; small amounts of substances absorbed this way
 Extending from epithelial cell body into each microvillus are multiple actin filaments that contract rhythmically
to cause continual movement of microvilli, keeping them constantly exposed to new interstitial fluid
Absorption in the Small Intestine
 Water transported through intestinal membrane entirely by diffusion, obeying usual laws of osmosis
o When chyme dilute enough, water absorbed through intestinal mucosa into blood of villi
o Water can also be transported from plasma into chyme when hyperosmotic solutions discharged from
stomach into duodenum; within minutes, sufficient water usually transferred by osmosis to make chyme
isosmotic with plasma
 Na secreted in intestinal secretions; to prevent net loss of Na in feces, intestines must absorb Na
o Whenever significant amounts of intestinal secretions lost to exterior (as in extreme diarrhea), Na
reserves of body can be depleted to lethal level within hours
o Normally, less than 0.5% of intestinal Na lost in feces each day because it is rapidly absorbed through
intestinal mucosa
o Na+ plays important role in helping to absorb sugars and amino acids
o Motive power for Na+ absorption provided by active transport of Na+ from inside epithelial cells through
basal and lateral walls of cells into paracellular spaces; requires ATP
 Active transport of Na+ through basolateral membranes of cell reduces Na+ concentration inside
cell to low value, and because Na+ in chyme about equal to that in plasma, Na+ moves down
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electrochemical gradient from chyme through brush border of epithelial cell into epithelial cell
cytoplasm
o Part of Na+ absorbed along with Cl- (Cl- drags Na+ through because of charge)
o Na+ co-transported through brush border membrane by several specific carrier proteins including
sodium-glucose co-transporter, sodium-amino acid co-transporters, and Na+-H+ exchanger
 Transporters provide still more Na+ to be transported by epithelial cells into paracellular spaces
 Also provide secondary active absorption of glucose and amino acids, powered by active Na+-K+
ATPase pump on basolateral membrane
Osmosis of water – by transcellular and paracellular pathways; occurs because large osmotic gradient created by
elevated concentration of ions in paracellular space
o Much of osmosis occurs through tight junctions between apical borders of epithelial cells (paracellular
pathway), but also occurs through cells themselves (transcellular pathway)
o Osmotic movement of water creates flow of fluid into and through paracellular spaces and into
circulating blood of villus
When person becomes dehydrated, large amounts of aldosterone almost always secreted by cortices of adrenal
glands; in 1-3 hours, aldosterone causes increased activation of enzyme and transport mechanisms for all
aspects of Na absorption by intestinal epithelium
o Increased Na absorption causes secondary increases in absorption of Cl-, H2O, and some others
o Effect of aldosterone important in colon because it allows virtually no loss of NaCl in feces and little
water loss; function of aldosterone in intestinal tract is same achieved by aldosterone in renal tubules,
which also serves to conserve NaCl and water in body when person dehydrated
In upper part of small intestine, Cl- absorption rapid and occurs mainly by diffusion (absorption of Na+ through
epithelium creates electronegativity in chyme and electropositivity in paracellular spaces between epithelial
cells, so Cl- moves along electrical gradient)
o Cl- also absorbed across brush border membrane of parts of ileum and large intestine by brush border
membrane Cl--HCO3- exchanger
o Cl- exits cell on basolateral membrane through Cl- channels
Often large quantities of HCO3- must be reabsorbed from upper small intestine because large amounts secreted
into duodenum in both pancreatic secretion and bile
o HCO3- absorbed in indirect way – when Na+ absorbed, moderate amounts of H+ secreted into lumen of
gut in exchange for some of Na+; H+ combine with HCO3- to form H2CO3 which dissociates to form H2O
and CO2; H2O remains as part of chyme in intestines, but CO2 readily absorbed into blood and
subsequently expired through lungs
o Above called active absorption of HCO3- and is same mechanism that occurs in tubules of kidneys
Epithelial cells on surface of villi in ileum and large intestine capable of secreting HCO3- in exchange for
absorption of Cl-; important because it provides alkaline HCO3- that neutralize acid products formed by bacteria
in large intestine
Deep in spaces between intestinal epithelial folds, immature epithelial cells continually divide to form new
epithelial cells; new cells spread outward over luminal surfaces of intestines
o While still in folds, epithelial cells secrete NaCl and H2O into intestinal lumen; this is reabsorbed by older
epithelial cells outside folds, providing flow of water for absorbing intestinal digestates
o Toxins of cholera and some other types of diarrheal bacteria can stimulate epithelial fold secretion so
greatly that secretion often becomes much greater than can be reabsorbed; in 1-5 days, severely
affected patients die from loss of fluid alone
o Extreme diarrheal secretion initiated by entry of subunit of cholera toxin into epithelial cells that
stimulates formation of excess cAMP, which opens tremendous numbers of Cl- channels, allowing Cl- to
flow rapidly from inside cell into intestinal crypts
 Activates Na+ pump that pumps Na+ into crypts to go along with Cl Extra NaCl causes extreme osmosis of water from blood, providing rapid flow of fluid
o All excess fluid washes away most of bacteria and is of value in combating disease, but too much can be
lethal because of serious dehydration of whole body that might ensue
o In most instances, life of cholera victim can be saved by administration of tremendous amounts of NaCl
solution to make up for loss
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Ca2+ actively absorbed into blood, especially from duodenum, and amount of Ca2+ absorption is exactly
controlled to supply daily need of body for calcium
o PTH activates vitamin D, and activated vitamin D greatly enhances Ca2+ absorption
 Iron ions actively absorbed from small intestine
 K+, Mg2+, PO43-, and other ions can be actively absorbed through intestinal mucosa
o Monovalent ions absorbed with ease and in great quantities
o Bivalent ions normally absorbed in only small amounts, but only small quantities needed daily by body
 Essentially all carbs in food absorbed in form of monosaccharides; only small fraction absorbed as disaccharides
and almost none as larger carb compounds
o Virtually all monosaccharides absorbed by active transport process
o In absence of Na+ transport through intestinal membrane, virtually no glucose can be absorbed because
glucose absorption occurs in co-transport mode with active transport of Na+
 2 stages in transport of Na+ through intestinal membrane: active transport of Na+ through
basolateral membranes of intestinal epithelial cells into blood, thereby depleting Na+ inside
epithelial cells; decrease of Na+ inside cell causes Na+ from intestinal lumen to move through
brush border of epithelial cells to cell interiors by secondary active transport (Na+ combines with
transport protein, but transport protein will not transport Na+ to interior of cell until protein also
combines with some other appropriate substance such as glucose)
 Intestinal glucose combines simultaneously with same transport protein and both Na+ and
glucose molecule transported together to interior of cell
 Once inside epithelial cell, other transport proteins and enzymes cause facilitated diffusion of
glucose through cell’s basolateral membrane into paracellular space and from there into blood
o Galactose transported by almost exactly same mechanism as glucose
o Fructose transported by facilitated diffusion all the way through intestinal epithelium (not coupled)
 Much of fructose, on entering cell, becomes phosphorylated and converted to glucose, then
transported in form of glucose rest of way into blood
 Overall rate of transport is about ½ that of glucose or galactose
 Most proteins, after digestion, absorbed through luminal membranes of intestinal epithelial cells in form of
dipeptides, tripeptides, and few free AAs; energy for most of transport supplied by Na+ co-transporter
mechanism
o Most peptide or AA molecules bind in cell’s microvillus membrane with specific transport protein that
requires Na+ binding before transport can occur
o After binding, Na+ moves down electrochemical gradient to interior of cell and pulls AA or peptide along
with it
o Few AAs don’t require Na+ co-transport mechanism and are transported by membrane transport
proteins by facilitated diffusion
 Bile micelles carry monoglycerides and free fatty acids to surfaces of microvilli and penetrate into recesses
among moving, agitating microvilli
o Monoglycerides and fatty acids diffuse out of micelles into interior of epithelial cells; possible because
lipids soluble in epithelial PM
o In presence of abundance of bile micelles, about 97% of fat absorbed; without them, 40-50% absorbed
o After entering epithelial cell, fatty acids and monoglycerides taken up by cell’s sER, where they are
mainly used to form new triglycerides that are subsequently released as chylomicrons through base of
epithelial cell, to flow through thoracic lymph duct and empty into circulating blood
o Small quantities of short-chain and medium-chain fatty acids, such as those from butter, absorbed
directly into portal blood rather than being converted into triglycerides and absorbed by way of
lymphatics
 Short-chain fatty acids more water-soluble and mostly not reconverted into triglycerides by ER,
allowing for direct diffusion of short-chain fatty acids from intestinal epithelial cells directly into
capillary blood of intestinal villi
Absorption in the Large Intestine: Formation of Feces
 Most of water and electrolytes absorbed in colon; essentially all ions absorbed
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Most of absorption occurs in proximal ½ of colon (absorbing colon), whereas distal colon functions principally
for feces storage until propitious time for feces excretion (storage colon)
Mucosa of large intestine has high capability for active absorption of Na+, and electrical potential gradient
created by absorption of Na+ causes Cl- absorption as well
o Tight junctions between epithelial cells of large intestinal epithelium much tighter than those of small
intestine, preventing significant amounts of back-diffusion of ions through junctions, allowing large
intestinal mucosa to absorb Na+ far more completely than small intestine; especially true when large
quantities of aldosterone available because aldosterone greatly enhances sodium transport capability
o Mucosa of large intestine secretes HCO3- while simultaneously absorbing equal number of Cl- in
exchange transport process ; HCO3- helps neutralize acidic end products of bacterial action in large
intestine
o Absorption of Na+ and Cl- creates osmotic gradient across large intestinal mucosa, which in turn causes
absorption of water
When total quantity entering large intestine through ileocecal valve or by way of large intestine secretion
exceeds amount that can be absorbed, excess appears in feces as diarrhea
Numerous bacteria, especially colon bacilli, present even normally in absorbing colon; capable of digesting small
amounts of cellulose, providing few calories of extra nutrition for body
o Other substances formed as result of bacterial activity are vitamin K, vitamin B12, thiamine, riboflavin,
and various gases that contribute to flatus in colon (CO2, H2, and CH4)
o Bacteria-formed vitamin K especially important because amount of it in daily ingested foods normally
insufficient to maintain adequate blood coagulation
Feces normally ¾ water and ¼ solid matter composed of 30% dead bacteria, 10-20% fat, 10-20% inorganic
matter, 2-3% protein, and 30% undigested roughage from food and dried constituents of digestive juices, such
as bile pigment and sloughed epithelial cells
o Brown color of feces caused by stercobilin and urobilin (derivatives of bilirubin)
o Odor caused principally by products of bacterial action; products vary from one person to another,
depending on colonic bacterial flora and type of food eaten
o Actual odoriferous products include indole, skatole, mercaptans, and hydrogen sulfide