Lec 22 Digestion and Absorption II Flashcards
Where does digestion occur?
- in lumen or on brush-border membrane
- pre-mucosal
What is definition of digestion?
- breaking down food to small, absorbable components
What is definition of absorption?
- transporting breakdown products of digestion across intestinal epithelium
Where does absorption occur?
- mucosal
What 2 digestive enzymes in salivary glands?
- salivary amylase
- lingual lipase
What 2 digestive enzymes in stomach?
- pepsin [pepsinogen]
- gastric lipase
What 4 types of disaccharides in small intestine?
- maltase
- isomaltase
- lactase
- sucrase
What types of peptidases in small intestine?
- on brush border: aminopeptidase
- intracellular peptidase
What 2 general types of digestive enzymes in small intestine?
- disaccharidases
- peptidases
What general types of digestive enzymes in pancreas?
- proteases
- lipases
- glyosidases
- nucleases
What 4 types of proteases in pancreas?
- trypsin
- chymotripsin
- carboxypeptidase [break down polypeptide from C terminus]
- elastase
Where are disaccharidases and peptidases located?
in brush border of small intestine
Where is bulk of lipase made?
pancreas
Is lipase made in active or inactive form? why or why not?
- active form
- doesn’t actually act unless has coenzyme co-lipase
- co-lipase does get made in inactive form [pro Co-lipase]
What type of macronutrient is most of what we take in every day [carb, protein, or lipid]?
carbohydrates
Where is iron primarily absorbed?
duodenum
What 3 things are primarily absorbed in duodenum
iron, calcium, magnesium
What 2 things are primarily absorbed in ileum?
- vit B12 [cobalamin]
- bile acids
Where are carbohydrates absorbed in small bowel?
everywhere - D, J, and I
What does low pH in duodenum trigger?
secretin release which tells pancreas to produce bicarbonate
What do fatty acids and amino acids in duodenum trigger?
CCK release which tells gallbladder to contract/empty, tells pancreas acinar cells to secrete enzymes
What is breakdown of types of carbohydrates in diet [starch vs sucrose vs lactose]?
starch: 50%
sucrose: 30%
lactose: 10%
How is starch digested?
amylase
- primarily from pancreas, some from saliva
- converts starch –> maltose
maltase
- from intestine brush border membrane
- converts maltose –> glucose units
What is starch made of?
number of glucose units together
How is sucrose digested?
sucrase-isomaltase
- from intestine brush border membrane
- converts sucrose –> glucose + fructose
How is lactose digested?
lactase
- from intestine brush border membrane
- converts lactase –> glucose + galactose
How are carbohydrates absorbed?
breakdown products glucose/galactose are actively transported into cell then diffuse into portal vein
What happens to unabsorbed carbohydrate [CHO] in GI tract?
- gets converted to short chain fatty acids [SCFA] in colon
what do scfa do?
keep colonic cells healthy
does pancreas make disaccharidases?
no, only small intestine brush border does
how does glucose get into cell for absorption usually?
Na-glucose cotransporter
What does starch need to break down into for absorption?
monosaccharides
What does protein breakdown into before absorption?
oligopeptides or AA
How is protein digested in stomach?
- HCl [acid] denatures protein
- pepsin hydrolyzes protein into polypeptides
What is path of pancreatic enzyme activation in duodenum? What 4 proteases does pancreas release that get activated this way?
- pancreas releases 4 proteases [trypsin, chymotrypsin, elastase, carboxypeptidase]
- enterokinase [enteropeptidase] in duodenal brush border membrane activates trypsin
- trypsin then activates other pancreatic pro-enzymes
How is protein digested in small intestine?
- brush border membrane oligopeptidases hydrolyze polypeptides –> smaller oligopeptides and AA
What activates proteases from pancreas?
- enterokinase [enteropeptidase] in duodenal brush border membrane activates trypsinogen –> trypsin
- trypsin activates other proenzymes
What 4 things activate chief cells pepsinogen release?
- gastrin
- histamine
- acetylcholine
What causes conversion pepsinogen –> pepsin?
low pH in stomach
How much of protein breakdown occurs in stomach?
10-15%
What activates chymotrypsinogen?
- active trypsin
What happens to di/tripeptides that are absorbed into epithelial cell?
they get broken down in cell into AA before being absorbed into capillaries to portal vein
What are two ways that protein breakdown products get absorbed
- via di/tripeptide transporter
- via AA transporter
Where do absorbed protein breakdown products go?
diffuse into villus capillaries and then into portal vein
What is path of 3 main steps of lipid break down for absorption?
- emulsified [get them into solution]
- cleaved by lipolysis
- packaged into micelles
What happens to lipids in stomach?
- churns fat [mostly triglycerides] into unstable emulsion
- gastric and lingual lipase break down a little [pretty minor]
What happens to lipids in duodenum and small intestine?
- fat emulsion stabilized by dietary phospholipids and bile salts from liver
- dietary fat causes CCK release from duodenal cells –> tells pancreas acinar to release lipase gallbladder to release bile [salts]
- pancreatic lipase + co-lipase convert: TG –> monoglycerides [MG] + FFA [free fatty acids]
What does pancreatic lipase do to triglycerides?
- uses coenzyme co-lipase
- converts TG –> MG [monoglycerides] + FFA [free fatty acids]
What 2 things stabilize lipid emulsion?
- phospholipids [from diet]
- bile salts [from liver]
What do bile salts do to lipid digestion products?
- package MG and FFA into mixed micelles with bile salts on outside
What gets absorbed into intestinal epithelial cell in lipid digestion? Active or passive?
- micelles diffuse passively across brush border membrane
What happens to micelles once they enter epithelial cells in intestine?
- get resynthesized as chylomicrons and VLDL
Where do lipid breakdown products end up after absorption into epithelial cell?
- get exported into lacteals [lymphatics] as chylomicrons and VLDL
What happens to long chain TG vs medium chain TG in digestion?
- long chain TG get digested into micelles and then to chylomicrons and into lacteals/lymphatics
- medium chain TG absorbed directly into portal vein
What is chemical composition of dietary fat [TG vs phospholipids vs cholesterol esters]?
95% of fat intake is tryglycerides
phospholipids - 2-8g per day
cholesterol esters - 300-400 mg per day
What is structure of triglyceride
- glycerol connected to 3 fatty acids [usually long chain] via ester bonds
What bond does lipase hydrolyze?
ester bond between glycerol and fatty acid
What percentage of lipid digestion is by gastric lipase?
20-30%
What 3 malabosprtion deficiencies can cause anemia?
- Vit B12
- Folate
- Iron
What is clinical feature of vit K deficiency?
easy bruising
What is clinical feature of vit E deficiency?
neuropathy
What are 2 clinical features of vit D deficiency?
- if vit D deficiency can’t get Ca absorption
- get tetany [muscle spasm]
- ostemolacia [bone thinning]
What is clinical feature of vit A deficiency?
- night blindness
What are clinical features of carb deficiency?
- diarrhea
- bloating
- gas
What are 2 clinical features of fat deficiency?
- steatorrhea [fatty diarrhea]
- weight loss
What is clinical feature of protein deficiency?
edema
How is heme iron absorbed? vs nonheme iron?
heme: can get absorbed into epithelial cell and extracted to Fe2+ in cell
nonheme: converted to Fe2+ in lumen then absorbed via DMT [divalent metal transporter] into cell
How does iron get from epithelial cell to blood stream?
via ferroportin
With what does iron circulate in blood stream?
transferrin
Where are Ca, Mg, and Fe primarily absorbed in duodenum?
duodenum
What form do Ca/Mg/Fe need to be in to be absorbed?
divalent [+2] form
How does Ca get absorbed?
- Ca2+ transported into epithelial cell
- Ca2+ binds binding protein
- Ca-ATPase pumps Ca out into blood stream
What is role of vit D in Ca absorption?
- Ca binding protein made in response to vit D
- need to have healthy vit D in order to get Ca secretion from epithelial cell into blood stream
Path of B12 absorption?
- B12 comes in food to stomach
- B12 binds R factor [haptocorrin, from saliva] in stomach
- in duodenum, pancreatic juices break apart B12 and R
- B12 binds IF in duodenum
- B12 reabsorbed in ileum
4 things that could lead to B12 deficiency
- No R factor [salivary disease]
- No IF [disease of parietal cell, pernicious anemia, autoimmune gastritis]
- pancreatic disease [pancreatic juice not available to break up B12 and R]
- ileal disease or cut out ileum [B12 doesn’t get reabsorbed
What vitamins are fat soluble?
A, D, E K
What is abetalipoproteinemia?
- normal brush border
- no beta lipoprotein so can’t package fats that enter epithelial cells
- epithelial cells get packed with FFA and monoglycerides but can’t get out of epithelial cells into lymph
- have malabsorption of long chain fatty acids
What is possible treatment of abetaproteinemia?
- medium chain triglycerides that can get absorbed diretly into portal vein
What is role of lipoproteins in fat absorption?
- necessary for packaging fat breakdown products into chylomicrons
