Digestive Physiology Flashcards
Process that mostly occurs in the small intestine
Digestion
◦Enzymes secreted to the lumen hydrolyze (break
down by reacting with water) larger molecules into monomers
Absorption of molecules in the intestine involves:
◦Entering epithelial cells across apical membrane
from the intestinal lumen
◦Movement across the basolateral membrane of
epithelial cells into the intestinal fluid
◦Diffusion into capillaries or lacteals
Digestion begins in the mouth with
Salivary amylase splitting starch into oligosaccharides (chains of 3–10 simple sugars)
Digestion continues in the intestine with
◦Pancreatic amylase breaking down starch &
glycogen into oligosaccharides & disaccharides
(chains of 2 simple sugars)
◦Brush border enzymes breaking oligosaccharides & disaccharides into monosaccharides (single simple
sugars)
monosaccharides (glucose, galactose, fructose) are moved across apical membrane of absorptive cells via
Transporters
Monosaccharides glucose & galactose move across the apical membrane via
Na+ dependent secondary active transport (SGLT1) ◦ Na+K+ ATPase pumps Na+ across basolateral membrane to maintain Na+ gradient (maintains low Na+ inside of cell) necessary for SGLT1 to function
Fructose is moved across the apical membrane via
facilitated diffusion (GLUT5)
Monosaccharides move into the interstitial fluid across basolateral membrane via
facilitated diffusion (GLUT2) & then enter capillaries via intercellular clefts
Percentage of adults that can’t digest lactose
60-70%
Lactose is hydrolyzed into glucose & galactose by
Lactase
Producing lactase throughout your life is
Lactase persistence
Deficient amounts of lactase is
Lactose intolerance
◦ Microbial metabolism of undigested solutes
produces large amounts of gas (H 2, CO2 & CH4) → bloating, flatulence, & cramping pain
◦ Undigested lactose creates an osmotic gradient in
intestine that prevents water from being absorbed → diarrhea
In the stomach hydrolysis of denatured proteins begins via
Pepsin (secreted by chief cells)
In the small intestine hydrolysis of proteins continues via
◦Pancreatic proteases breaking down proteins &
protein fragments into smaller pieces & some
amino acids (Trypsin, chymotrypsin, carboxypeptidase B)
◦Brush border peptidases break oligopeptides
into di- & tripeptides, & amino acids
◦Intracellular peptidases hydrolyze di- & tr- peptides
Proteins are absorbed via
Transporters moving amino acids or small peptides across apical membrane of absorptive cells (active transport)
◦ Amino acids use primary active transport and Na+-dependent secondary active transport (like monosaccharides)
◦ Di- & tri-peptides use H+-dependent secondary active transport
In protein digestion amino acids move into interstitial fluid across basolateral membrane via
Facilitated diffusion & then enter capillaries via intercellular clefts
Allergies result from
Intact proteins being taken up by intestinal epithelial cells via endocytosis & being released into body where the immune system detects them as foreign antigens
The primary site for lipid digestion is
The small intestine
Lipid digestion begins with
The emulsification of lipids with bile salts
◦ ↑ SA exposed to water-soluble lipase enzymes
2nd step of lipid digestion is
Pancreatic lipases breaking down triglycerides to monoglycerides & free fatty acids
3rd step of lipid digestion is
Monoglycerides & fatty acids grouping with bile salts to form micelles
◦Helps transport lipids to absorptive surfaces of
intestinal cells
◦Also combine with fat-soluble vitamins (A, D, E,
K) & cholesterol to aid with their absorption
At plasma membrane, using simple diffusion, fatty acids & monoglycerides
Dissociate from micelles & move into epithelial cells
Within epithelial cells monoglycerides & fatty acids are converted back to
Triglycerides
Chylomicrons are
Protein-coated combinations of triglycerides with cholesterol & other lipids that move into interstitial fluid via exocytosis & are taken into lacteals for transport in lymph (too large to enter blood capillaries)
Pancreatic nucleases hydrolyze nucleic acids (DNA & RNA) into
Nucleotide monomers
Nucleosidases & phosphatases (brush border enzymes) break down nucleotides into
Free nitrogenous bases, pentose sugars, & phosphate ions; which are then actively transported by special carriers in the epithelium of the villi
Vitamins are
Organic cofactors that help with metabolism (mostly absorbed in the small intestine)
Vitamins mostly absorbed by specific active or passive transporters
Water-soluble vitamins (B’s & C)
◦Vitamin B 12 – a large polar molecule, is bound by intrinsic factor (produced by stomach) & absorbed in ileum by endocytosis
Vitamins carried by micelles of lipids & absorbed by simple diffusion
Fat-soluble vitamins (A, D, E & K) – Vitamins K & B 7 – made available through bacterial metabolism & absorbed by large intestine
Most electrolytes are absorbed via
Active transport along the entire length of the small intestine
◦Calcium & iron – mainly absorbed in duodenum
The most abundant substance in chyme is
Water
90% of the water in chyme is
Reabsorbed via osmosis in the small intestine (remaining 10% in the large intestine)
