Exam 4- Digestion and Absorption Flashcards
Final breakdown of all food substances occurs where in the GI tract?
Small intestines
Absorption involves what type of movement of digested products to the general circulation?
Transmural
What is the 2 step breakdown of complex sugars?
Complex sugars –> (a-amylase) –> oligosacchariades –> simple sugars (monosaccharide)
What enzymes break down 1:4 alpha linkages only?
Salivary and pancreatic amylase
How are the majority of carbs ingested?
Complex polysaccharides
Pancreatic amylase can’t handle what starch linkage?
1:6 linkage
All oligosaccharides must be broken down into one of what 3 products to be absorbed?
Glucose, galactose, fructose
What type of proteins are synthesized by epithelial cells and are needed for the membrane digestion of oligosaccharides?
Intrinsic membrane proteins
What are the two main groups of the intrinsic membrane proteins?
Galactosidases and alpha glucosidases
Lactase is what type of intrinsic membrane protein?
Galactosidase
Sucrase, maltase, and alpha dextrinase are what type of intrinsic membrane proteins?
Alpha glucosidases
The majority (80%) of ingested carbs are digested to what?
Glucose
Glucose transport is primary or secondary active transport?
Secondary active (since plasma glucose concentration is high)
What transporter is Na-dependent entry at the apical membrane and facilitates the movement of glucose and galactose into the cell?
SGLT-1 transporters
What transporter on the basolateral membrane is involved with transport of glucose and galactose to the interstitium via facilitated diffusion?
GLUT2
What transports fructose (w/o energy) on the apical membrane into the cell?
GLUT 5
Most disorders of carbohydrate digestion and absorption are due to what?
Deficiencies that prevent breakdown of carbs to an absorbable form
What breaks down lactose? What are the end products?
Lactose → glucose and galactose, via lactase
What breaks down sucrose? What are the end products?
Sucrose → glucose and fructose, via sucrase
What breaks down a-dextrins? What is the end product?
Alpha-dextrins → glucose, via maltase and alpha-dextrinase
What breaks down maltotroise? What is the end product?
Maltotriose → glucose, via maltase, sucrase, alpha-dextrinase
What breaks down maltose? What is the end product?
Maltose → glucose, via maltase, sucrase, alpha-dextrinase
Unabsorbed carbs hold water osmotically and cause what clinical condition?
Osmotic diarrhea (lumen is isosmotic to interstitium)
What are the 3 clincal sx of unabsorbed lactose?
Osmotic diarrhea, cramps, abd distension
What is a potential consequence of long-term intestinal disease?
Lactase deficiency (membrane lactase dysconfigured)
What is the primary enzyme in luminal digestion?
Amylase
What are the products of protein digestion? (2)
Amino acids and oligopeptides
Where are the proenzymes of protein digestion stored?
Pancreas
What happens if the proenzymes of protein digestion become active before being released from the pancreas?
Pancreatitis
What enzyme is required to activate the proenzymes of protein digestion?
H+ to activate pepsinogen to pepsin
Enterokinase to activate trypsinogen to trypsin
Proteins make up 20-35% of caloric intake with a significant amount coming from where?
Endogenous sources (desquamated cells and gastric secretions)
The final breakdown of proteins occurs where?
Brush border via peptidases
Amino acids are co-transported into the cell with what?
Sodium gradient via 2˚ active transport
How are di and tripeptides co-transported in the cell?
Coupled w/ H+ transport via 2˚ active trasport
In addition to peptidase breakdown in the brush border layer, how else are proteins digested? (2)
Amino acid carriers and endocytosis (neonates)
Decreased protein absorption as a result of pancreatic insuffiency will lead to what?
Malnutrition
Pancreatic insufficiency will have what effect on the amount of trypsin that reaches the intestines?
Decreased/insufficienct amounts of trypsin
What condition is a congential defect in transport of neutral amino acids?
Hartnup disease (absorption of di- and tripeptides ok)
In what condition is the transporter for dibasic amino acids absent from the gut and kidney resulting in the loss of dibasic amino acids in urine and feces?
Cystinuria
What motion is required for the digestion of fats due to their insolubility and tendency to coalesce?
Churning and bile emulsification
What are the 4 key events in fat digestion?
Secretion of bile and lipases, emulsification of fats by bile, enzymatic hydrolysis of ester linkages, solubilizing digestion products within micelles for delivery to enterocytes
What enzyme is primary responsible for the digestion of fats?
Pancreatic lipase (lingual lipase plays small role)
Pancreatic lipase requires assistance from what other enzyme?
Colipase (prevents bile salt inhibition of this enzyme)
What eznymes removes fatty acid moiety from cholesterol, vitamins A, D, E, and glycerides and can remove all three ester linkages?
Cholesterol ester hydrolase
What enzyme hydrolyzes phospholipids at 2 position in fat digestion?
Phospholipase A2
Products of fat digestion are removed quickly by the formation of what?
Micelles
Once triglycerides are broken down, they combine with bile acids to form micelles. How is the micelle absorbed?
Diffuse across the unstirred layer to the mucosal surface of the apical membrane of the enterocyte where it breaka down and its contents absorbed by simple diffusion
What are the components of a micelle? (breaken down into these components for simple diffusion) (4)
Long-chain FFA, 2-MG, phospholipids, cholesterol
How much of ingested fat is removed by the small intestine?
95%
What happens to the components of micelles once they pass into the enterocyte via simple diffusion?
Reassemble TG and phospholipids
What structure has a TG and cholesterol core is surrounded by phospholipids and apoproteins?
Chylomicron
How does a chylomiron re-enter blood circulation? (3 steps)
Exocytosed and enters lacteal → lacteal transports chylomicron into general lymph → into blood stream
Chylomicrons carry triglycerides to what tissues? (3)
Adipose tissue, skeletal muscle, cardiac muscle
What makes up the majority of a chylomicron?
Triglycerides (> cholesterol, PLs, apoproteins)
Zollinger Ellison syndrome results in abnormal lipid digestion or lipid absorption?
Lipid digestion
What syndrome causes dysfunctional lipolysis as a result of a gastrin secreting tumor in duodenum or pancreas that creates an acidic small intestine and dysfunctional pancreatic lipase?
Zollinger Ellison syndrome
What rare disease causes steatorrhea due to abnormal lipid absorption caused by generalized lymphatic dystrophy?
Whipple’s disease
Whipple’s disease results in abnormal lipid digestion or lipid absorption?
Lipid absorption
What enzymes are produced in the pancreas? (8)
Amylase, trypsin, chymotrypsin, carboxypeptidase, elastase, lipase-colipase, phospholipase A, cholesterol esterase nonspecific lipase
What enzymes are produced in the brush border? (8)
Enterokinase, disaccharidases, maltase, sucrase, lactase, peptidases, aminooligopeptidase, dipeptidase
What dictates the vitamin absorption mechanism?
Solubility
What are the fat soluble vitamins? (4)
A, D, E, K
What are the water soluble vitamins? (8)
B1, B2, B6, B12, C, biotin, folate, pantothenic acid
How are fat soluble vitamins solubilized/ transported and how do they enter circulation?
Micelles, chylomicrons
How are most water soluble vitamins absorbed?
Via Na+ dependent co-transport (exception is B12)
How is vitamin B12 absorbed?
Binds to gastric intrinsic factors which protects it from digestion, then absorbed in ileum
What is the main MOA for calcium reuptake in the intestines?
Upregulation of calcium binding protein (CaBP = rate limiting) present in enterocytes
Ca passivley enters the enterocyte and is transported via what to the basolateral membrane?
CaBP (binding protein)
What are the 2 mechanisms for movement of calcium on the basolateral membrane of the intestines?
Ca-ATPase (active) and NaCa exchanger (passive)
Does PTH have a direct effect on Ca absorption in the enterocyte?
No
What is calcium absorption regulated by?
PTH and vitamin D
How much iron is ingested vs absorbed?
Amount ingested >> amount absorbed
What promotes intestinal absorption and solubilization of iron?
Gastric acid (reduces Fe3+ to Fe2+)
What binding protein is required for plasma transport of iron?
Transferrin
What is the preferred form of iron within the body?
Ferrous (Fe2+)
What is the intracellular storage depot for iron that allows for the release of iron in a controlled fashion?
Ferritin (iron binding protein)
When is ferritin lost?
When enterocyte is sloughed off
What is the single biggest source of ingested and secreted fluids?
Pancreatic juice and bile (diet contributes ~2/10 L)
What is the primary site of water and electrolyte absorption?
Small intestine (then colon)
What organ is impermeable to water?
Stomach
What is the driving force for water movement?
Solute transport (Na)
What is the 1st site for Na+ absorption?
Small intestine (mechanism similar to proximal tubule, Na/H exchagner maintained by Na-K-ATPase)
What part of the intestines has a net reabsorbtion of Na and HCO3 (+sugar, and AA)?
Jejunum
What are the effects of Na+/H+ exchange in the jejunum? (2)
Acidifies lumen and neutralizes lumenal HCO3-
What drives paracellular Cl- absorption?
Voltage gradient
What portion of the intestines does the following occur:
Both H+ and HCO3 are secreted into the lumen (resulting in NO pH change) to drive transport of Na and Ca in the cell
Ileum
What is the net reabsorption activity of the ileum?
Recovery of sodium and chloride (+ sugar and amino acids)
What transporters contribute to chloride being reabsorbed into cells in the ileum? (2)
Apical chloride-HCO3 antiporter and Na/H exchanger
Passive Na and K channels in the apical membrane of the colon responsible for what?
Na absorption (into the cell) and K secretion (into the lumen of the colon)
ENaC is induced by what in the colon?
Aldosterone
What is K secretion dependent on in the colon?
“Flow dependent”
What is the effect of diarrhea on the colon?
Increase K+ loss due to increased flow of chyme through the colon
Diarrhea is often associated with loss of what 3 things?
Fluid, HCO3- (causes metabolic acidosis), loss of K+ (hypokalemia)
Diarrhea will result in electrolyte imbalances that manifest as what?
Metabolic acidosis and hypokalemia
What is the result of decreased/ impaired surface area of the small intestine? (ex Crohns)
Diarrhea
What is the result of osmotic diuresis due to unreabsorbable osmolytes? (ex. lactase enzyme deficiency)
Diarrhea
What is a common cause of secretory diarrhea?
Cholera
What can occur if chloride secretion in the colon is increased?
Secretory diarrhea
What is the MOA for cholera? (4 steps)
- Cholera toxin binds to apical membrane of crypt cells
- Irreversible activation of adenylate cyclase
- Increased cAMP production/ activity which opens Cl- channels
- Increased secretion of Cl- on the apical membrane
What is the treatment for secretory diarrhea?
Oral rehydration therapy (and control presence of vibrio cholerae)
What maintains the Na and Cl concentration gradients within the epithelials cells of the colon?
NaKCC1 transporter (Na, K, 2 Cl) maintained by Na/K-ATPase
