Digestion and Absorption Flashcards
what anchors mucosal secretion to the intestinal microvilli?
glycocalyx
what are the diffusion barriers to absorption? (4)
- unstirred water layer (UWL)
- luminal (apical) cell membrane
- basal cell membrane
- ECF
what pump accounts for 33% of the total resting cell energy budget of the GI tract?
Na-K ATPase
how is secondary active transport typically done?
couple movement of Na with a sugar or amino acid
example of uniport, symport, and antiport?
uniport: Ca-ATPase
symport: Na-glucose, Na-amino acid
antiport: Na-K ATPase, H-K ATPase
pepsin:
- what secretes it
- what activates it
- action
- from stomach
- activated by low pH
- acts on aromatic aa’s and some others
trypsinogen:
- what secretes it
- what activates it
- action
- from pancreas
- activated by mucosal enteropeptidase (enterokinase)
- cleaves peptide bonds nest to characteristic aa’s
procarboxypeptidase:
- what secretes it
- what activates it
- action
- from pancreas
- activated by trypsin
- cleaves C-terminal aa’s
what types of nutrients are absorbed via secondary active symport?
- free aa’s
- dipeptides
- tripeptides
what nutrients are absorbed via facilitated diffusion?
- Glu
- Asp
how do nutrients get through the basal membrane and into the blood/lymph?
basal membrane -> facilitated diffusion -> then simple diffusion to blood/lymph
describe the breakdown of total digested protein in small intestine in terms of where it is from
50% - food
25% digestive juices
25% sloughed mucosal cells
how much total protein escapes digestion and absorption?
2-5%
principal carb sources
- polysaccharides (starches)
- disaccharides (sucrose, lactose)
- monosaccharides (glucose, fructose)
animal starch vs. plant starches
- glycogen (animal): 1,4a-linkages, 1,6a-branches
- amylopectin (plant): similar but less 1,6a-branches
- amylose (plant): only 1,4a-linkages
action of salivary and pancreatic alpha-amylases
dk
action of salivary and pancreatic alpha-amylases
- attack straight chain 1,4a-linkages
- spare 1,6a-links, terminal 1,4a-links, and 1,4a-links next to branch points
products of salivary and pancreatic alpha-amylases
- maltose (disaccharide)
- maltotriose (trisaccharide)
- a-limit dextrins
where does the final digestion of carbs occur?
ileum via membrane-bound oligosaccharidases in microvilli
what causes lactose intolerance?
deficiency of lactase and/or sucrase -> retain disaccharides -> retain water -> diarrhea, bloating, gas
describe levels of lactase throughout lifetime
highest in infants, declines during childhood and adulthood
what are the principal absorbed monosaccharides and how is each absorbed through apical then basal membrane?
- glucose (Na-dependent symport ->facilitated diffusion)
- galactose (Na-dependent symport ->facilitated diffusion)
- fructose (facilitated diffusion -> facilitated diffusion)
what are non-digestible carb components of food?
- cellulose
- pectins
- lignins
impact of non-digestible carb components of food
add bulk, retain water -> enhance stood transit time by distention related events (natural laxative)
what emulsifies lipids and where?
small intestine - bile salts (glycocholate), lecithin, monoglycerides
what is the major lipid digestive enzyme?
pancreatic lipase - hydrolyzes fatty acid ester linkages
what is required for lipase to function?
colipase - binds TG surface, displacing emulsifier molecule and anchoring lipase
what is required for lipase to function?
colipase - binds TG surface, displacing emulsifier molecule and anchoring lipase
describe structure of micelles
- outer layer of emulsifier
- inner layer of cholesterol, lysolecithin, MG, fa’s
describe intracellular fate of absorbed lipids of different sizes
directly pass to portal blood
>10-12 C -> re-esterified
how much fat is normally in stool?
how much cholesterol is absorbed from lumen and why?
almost insoluble in water -> only 50% absorbed
describe formation of chylomicrons and how they help absorption of fat
glycophospholipids + free TGs -> Golgi -> lipid droplets -> add lipoproteins, phospholipids -> chylomicron -> secretory vesicle into extracellular space -> lymph -> thoracic duct -> circulation
describe absorption of water-soluble vitamins
- B’s, niacin, folate, ascorbate
- diffusion, cotransport, and active transport
which water-soluble vitamin has different absorption?
B12 - needs intrinsic factor, absorbed in ileum
describe absorption of fat-soluble vitamins
- A,D,E,K
- like water-soluble but in proximal small intestine primarily
how much of Na entering each portion of gut is absorbed?
duodenum and jejunum - 55%
ileum - 25%
large intestine - 20%
describe Na absorption in jejunum
-passively absorbed at tight junctions and lateral spaces in response to osmotic forces
describe Na absorption in ileum
- mainly electrogenic - diffusion down gradient via specific membrane protein channels
- also some by neutral Na/Cl exchange
- indirectly dependent on Na/K ATPase
describe Na absorption in colon
purely electrogenic
describe Na absorption in colon
purely electrogenic
how much of the Ca ingested is absorbed and where?
50% - mostly in upper small intestine
describe Ca absorption
needs Ca-binding protein (upregulated by vit D), then basal Ca ATPase (2 Ca: 1 ATP)
how is iron transported in blood?
bound to transferrin
how is iron stored?
complexed with ferritin
describe iron absorption
- as free iron: combines w/ DMT-1 in duodenum
- as iron heme complex: directly into intestinal mucosal cells
what iron carrier releases iron to the plasma?
ferroportin
what regulates iron uptake into mucosal cells?
intracellular [ferritin]
where does water absorption primarily occur?
lower small intestine and large intestine - net water movement occurs in small intestine until osmolality of lumen equals that of cytoplasm-interstitium-plasma
describe water absorption in colon
solely follows electrogneic movement of Na - largely through tight junctions between epithelial cells
