Digestion and absorption Processes in the GI Tract Flashcards
cellular absorption
lumen to apical membrane to intestinal epithelial cells to basolateral membrane to blood
transporters in membranes
paracellular absorption
intestinal epithelial cells to lateral intercellular space to intestinal epithelial cells
tight junctions
cavital (luminal) digestion
refers to digestion resulting from the action of enzymes secreted by the salivary glands stomach and pancreas
membrane (contract) digestion
refers to hydrolysis by enzymes synthesized by epithelial cells
structure of the intestinal mucosa
kerckring - longitudinal folds
villi and microvilli increase surface area of the small intestine (longer in duodenum and shorter in ileum)
all increase the intestine total surface area by 600 fold
enterocytes
function in digestion, absorption and secretion
turnover rate cells are replaced 3-6 days
susceptible to irradiation and chemotherapy
goblet cells
mucus secreting cells
physical chemical and immunologic protection
paneth cells
part of the mucosal defense against infection
secrete agents that destruct bacteria or produce inflammatory responses
pinocytosis
at the base of the microvilli
major mechanism in the uptake of proteins
passive diffusion
particles move through pores in the cell membrane or through intercellular spaces
barriers crossed during absorption into enterocyte
unstirred layer of fluid glycocalyx apical membrane cytoplasm of the cell basolateral membrane basement membrane wall of blood capillary or lymphatic vessel
end products of carbohydrate digestion
only monosaccharides are absorbed by the enterocytes
glucose, glactose, and fructose
transport mechanisms in absorption of carbohydrates
co-transport and facilitated diffusion
SLGT1 - secondary active transport
lactose intolerance
failure to digest carbohydrates
brush border lactase is deficient or lacking
undigested lactose remains in the lumen and holds water and causes osmotic diarrhea
undigested and unabsorbed lactose is fermented into methane and H gas causing excess gas
Stomach proteases - endopeptidase
pepsin
trypsin
chymotrypsin
elastase
stomach proteases - exopeptidases
carbocypeptidase A
carboxypeptidase B
small intestine proteases
trypsin chymotrypsin elastase carboxy peptidase A carboxy peptidase B
transport mechanism in absorption of proteins
4 separate cotransporters - neutral, acidic, basic, and imino AA
4 separate facilitated diffusion mechanisms - one each for neutral acidic basic and imino AA
chronic pancreatitis and cystic fibrosis
deficiency of pancreatic enzymes
congenital trypsin absence
in the absence of trypsin it seems like all the pancreatic enzyme are gone
cystinuria
defect in or absence of Na amino acid cotransporters
transporter for di basic amino acids is absent from the SI and kidney
genetic disorder
as a result of the intestinal deficiency amino acids are secreted in feces
hartnup disease
cannot absorb neutral aa
recessive genetic disorder
symptoms resemble those caused by pellaga (deficiency of niacin)
symptoms - diarrhea, mood changes, neurologic problems, red, scaly skin rash, usually which skin is exposed to sunlight , photosensitivity, short stature, uncoordinated movements
urine same have high excretion of neutral aa
mutations in cystic fibrosis transmembrane conductance regulator
regulated anion channel that is located at the apical surface of the duct cells
loss of bicarb secretion
insolubility of lipids
lipids need to be solubilized in micelles and transported to the apical membrane of intestinal epithelial cells for absorption
stomach and lipid digestion
lingual and gastric lipases initiate lipid digestion in the stomach
CCK allows sufficient time for lipids to get digested properly by slowing gastric emptying
small intestine and lipid digestion
most lipid digestion occurs in the small intestine
bile salts emulsify lipids
pancreatic enzymes are secreted into the small intestine to complete the digestive work
pancreatic lipase
inactivated by bile slats colipase solves this problem
colipase
secreted as inactive form it is activated by trypsin
once activated it binds to pancreatic lipase displacing bile salts
cholesterol ester hydrolase
in addition to catalyze the production of cholesterol it also hydrolyzes triglycerides to produce glycerol
phospholipase A2
proenzyme is activated by trypsin
mechanism for processing lipids
solubilization by micelles diffusion of micellar contents across apical membrane reesterfication chylomicron formation exocytosis of chylomicrons
abnormalities of pancreatic enzyme secretion
failure to secrete adequate amounts of pancreatic enzymes
regulation of acidity of duodenal contents is critical for the integrity of pancreatic enzyme function
Zollinger-Ellison syndrome
Pancreatitis
deficiency of bile salts
interferes with the formation of micelles
ileal resection interrupts the enterohepatic circulation of bile salts
small intestinal bacterial over growth
decreased intestinal epithelial cells
tropical sprue - lipid absorption is impaired because the surface area for absorption is decreased
fat soluble vitamins (A, D, E and K)
same mechanism of absorption as lipids
water soluble vitamins (B1, B2, B3, B12, C, biotin, folic acid, nicotinic acid, and pantothenic acid)
absorbed via Na dependent cotransport mechanism in the small bowel
Vitamin B12
form complexes with other proteins in order to be absorbed
pernicious anemia
type of vitamin B12 anemia
stomach does not produce enough IF which decreases B12
due to atrophic gastritis and autoimmune conditions
absorption of vitamin D
D3 from diet
converted to 15 hydroxy D3 in liver
converted to 1,2 dihydroxy D3 and 24,25 dihydroxy D3 in liver
leads to increase gut Ca2 absorption, bone calcification, bone reabsorption
abnormalities of vitamin D absorption
inadequate Ca2 absorption - rickets and osteomalacia
no D3 and no 1,23 dihydroxy D3
