Digestion and Absorption Flashcards
Digestion of Carbohydrates
salivary and pancreatic amylase
Amylase breaks down starches to
maltotriose, Dextrins, Maltose
Glycosidases include
maltase, lactase, sucrase, isomaltase
Glycosidases are located in
brush border
Glycosidases break down saccharides to
glucose, galactose, and fructose
Lactase
lactose –> glucose + galactose
Sucrase
sucrose –> glucose + fructose
GLUT-5
apical and basolateral membrane, transport of fructose
SGLT-1
apical membrane, transports glucose and galactose, Na-dependent, Na/K ATPase for gradient
GLUT-2
basolateral membrane, transports glucose and galactose
Peptide digestion
pepsin, pancreatic proteolytic enzymes, aminopeptidase, cytoplasmic peptidase
Pepsin
from chief cells activated by H (pH 1-2)
Pancreatic proteolytic enzymes
oligopeptides to amino acids, activated by enterokinase and trypsin (pH 7)
Brush border aminopeptidases
hydrolyze remaining oligopeptides, tri- and d-peptides to aa
Cytoplasmic peptidases
within enterocytes, try- and di- peptides are broken into AA
Pept-1
transports tri- and di-peptides across apical border with H+
M-cell Pinocytosis
intact absorption of tri- and di-peptides (bacteria, antibodies)
Triglyceride digestion
first broken down into 2-MG and FFA, 2-MG can form micelles
Digestion of phospholipids
phospholipase A2 releases FFA and lysophospholipids and long-chain FFA which form micelles
Cholesterol and long-chain FFA
form micelles
Short-chain and medium-chain FFA
<12 C; do not form micelles
Enzymatic cleavage of lipids must occur at
oil-water interface
Emulsification
break down of large globules of fat, mastication, retropulsion, peristalsis
Gastric Lipase
15% triglycerides to DAG and FFA
Colipase
activated by CCK –> trypsin cleavage; binds lipase to the oil-water interface
Lipase
inhibited by bile acids, activated by colipase
Micelle incorporation
LCFFA, 2-MG, lysophospholipids, cholesterol
Will not incorporate into micelle
SCFFA or MCFFA, triglyceride, or bilirubin
Absorption of lipid products
micelle break down at apical surface, and contents diffuse
SCFFA and MCFFA transport
diffusion
Once in enterocyte,
DAG, 2-MG, LGFFA are converted back to TG and packaged in chylomicrons
Chylomicrons have a protein coat
apoprotein B
Cholesterols are esterified to
cholesterol esters
Fat-soluble vitamine ADEK
are incorporated into micelles
Vitamin B12 absorption
binds R protein of saliva in gastric lumen, in small intestine it binds intrinsic factor, in ileum intrinsic factor binds receptor and facilitates absorption of B12
TRPV5/6
Ca transport protein
within enterocytes calcium binds
calbindin Ca-BP
2 requirements to extrude Ca at basolateral membrane
Ca-ATPase and Na/Ca exchanger
Vitamin D effect on Ca absorption
increases number of apical Ca-channels, number of cytosolic Ca-BP, and activity of Ca-ATPase
PPI and Ca absorption
less solubilized Ca for absorption due to higher pH
Na Absorption transport mechanism
co-transport with glucose and galactose and exchange for H+
IN COLON Na absorption is influenced by aldosterone
synthesis and insertion of ENaC
Divalent metal transporter 1
Fe3+ to Fe2+ and enters enterocyte through DMT1
Heme oxygenase
release Fe from heme in enterocyte
Fe inside enterocyte is stored as
ferritin and exported via ferooportin-1
Once ferritin is being exported
hephestin converts Fe2+ to Fe3+ and Fe3+ binds apotransferrin
Small intestine digests and absorbs almost all carbs, fats, and proteins but
colonic bateria may digest it is they reach the colon
Carbohydrate + colonic bacteria
SCFFA (reabsorbed or excreted) and gas
Protein + colonic bacteria
ammonia and cadavarine
Triglycerides + colonic bacteria
NOT digested = steatorrhea
Bile Acids + colonic bacteria
Secondary bile acids (lipid-soluble) absorbed by simple diffusion
Osmotic gradient and water flux
high accumulation of nutrients and electrolytes produced an osmotic gradient
Hydrostatic pressure and water flux
presence of water in ECS or lumen
Absorption of water
water follows osmoles into ECS, increasing hydrostatic pressure in ECS, causing flow from ECS into capillaries
Passive water flux decreases as we move
distally
How to treat diarrhea and dehydration
H2O, glucose, and Na SLGT-1 transporter
Osmotic diarrhea - pancreatic insufficiency (bile obstruction, ZES, CF, pancreatitis)
maldigestion, oncotic forces pull water into lumen causing diarrhea
Reduced luminal bile acid concentrations
impaired micelle formation causing diarrhea (locked duct, ileal disease, ZES, bacteria overgrowth)
Lactose intolerance
lactose remains in lumen drawing in water
Mucosal Malabsorption
destruction of intestinal mucosa (celiacs or IBD) decreased nutrient absorption = osmotic diarrhea
