D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
DIGESTIVE SYSTEM
Lecture 24: Digestion & Absorption
• hydrolysis: breakdown of complex foodstuffs entails hydrolysis - addition of
water to cleave chemical bonds of polymers (e.g. polysaccharides, proteins) and also of smaller food molecules (e.g. triglycerides, dipeptides, disaccharides, etc.); enzymes
that catalyze these reactions are hydrolases
• carbohydrate digestion (fig. 65 – 1 & ppt. 1): main forms are sucrose & lactose
(disaccharides) and starches (polysaccharides)
- starches are hydrolyzed by salivary amylase (ptyalin) in oral cavity & temporarily in stomach (until inactivation of amylase by gastric juice); digestion is
continued by pancreatic amylase in small intestine; main product is maltose (a disaccharide)
- disaccharides are hydrolyzed in small intestine by disaccharidases located in
brush border (microvilli) of intestinal epithelial cells; sucrose yields glucose & fructose, lactose yields galactose & glucose and maltose yields glucose
• protein digestion (fig. 65 – 2 & ppt. 2): proteins are polymers of amino acids linked together by peptide bonds
- digestion begins in stomach where pepsin breaks down longer chains of amino acids to shorter chains; process is continued in small intestine where pancreatic enzymes – trypsin, chymotrypsin & carboxypolypeptidase – produce mainly dipeptides & tripeptides
- digestion to amino acids is completed by peptidases
(aminopolypeptidase & dipeptidases) located in brush border of enterocytes

Bio 659 lec. 24 - p. 2 -
• fat digestion (figs. 65 – 3, 65 – 4 & ppts. 3 & 4): mainly triglycerides; also
cholesterol and phospholipids
- emulsification – dispersal of fats into microdroplets; accomplished by bile
salts & lecithin (a phospholipid) from liver; greatly increases accessibility for fat-
digesting enzymes
- triglycerides digested to fatty acids and monoglycerides mainly by pancreatic
lipase in small intestine; products are sequestered in micelles by bile salts
- cholesterol esters and phospholipids digested by cholesterol esterase and phospholipases, respectively, from pancreas; products also sequestered in micelles
• surface area: small intestine, site of most absorption, has several features that extend surface area of absorbing epithelium (ppt. 5):
- circular folds of submucosa & mucosa (fig. 65 – 5 & ppt. 6)
- finger-like folds of mucosa (villi) with core of blood & lymphatic supply (fig.
65 – 6 & ppt. 7)
- finger-like folds of apical membrane of absorbing epithelium (microvilli) (fig. 65 –
7 & ppt. 8)
- surface area is extended approx. 1000-fold
• water absorption: occurs by osmosis; any solute absorption that creates an osmotic gradient promotes water absorption; small intestine absorbs 7 – 8 liters/day
• ion absorption (fig. 65 – 8 & ppt. 9): sodium is absorbed via active transport on basal and side walls of epithelial cells; this maintains a low sodium concentration in absorbing cell to facilitate diffusion from intestinal lumen through apical border
- sodium pumps are stimulated by aldosterone (adrenal corticosteroid)
- sodium absorption provides osmotic gradient to promote water absorption
- chloride ions are concurrently absorbed by electrochemical ‘drag’ effect of sodium absorption
- bicarbonate ions combine with hydrogen ions (exchanged for absorbed sodium) to
form carbonic acid that splits to CO
2
(absorbed into bloodstream) & H
2
O
- other ions are absorbed in much smaller amounts (by active transport)

Bio 659 lec. 24 - p. 3 -
• carbohydrate absorption: absorbed as monosaccharides, mainly glucose; active transport of sodium establishes a sodium concentration gradient from intestinal lumen to absorbing cell cytoplasm & sodium enters by facilitated diffusion; the
sodium gradient provides motive force for glucose transport via the same transport protein
(cotransport or secondary active transport)
- galactose is absorbed in essentially the same manner as glucose
- fructose transport is by facilitated diffusion (fructose transport protein)
- monosaccharides pass by facilitated diffusion from absorbing cell to blood
• protein absorption: absorbed as dipeptides, tripeptides or amino acids
- many amino acids and peptides cotransport with sodium (similar to glucose)
- other amino acids enter by facilitated diffusion (variety of specific transport proteins involved) (ppt. 10)
• fat absorption: absorbed as fatty acids & monoglycerides; micelles ‘ferry’ these materials to the luminal border of the absorbing cell where the fatty acids and monoglycerides penetrate the lipid bilayer
- new triglycerides formed within absorbing cell from freshly absorbed fatty acids and monoglycerides; these are bundled with small amount of protein to form chylomicrons (extremely low density lipoproteins) that are absorbed into the lymphatic vessel (lacteal) (fig. 65 – 6 & ppts. 7 & 11)