Lecture 8: Digestion and Absorption Flashcards by B 32

What is the difference between cellular and paracellular transportation?

CELLULAR: through the cell
PARACELLULAR: substances travel between cells via tight junctions

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Carbohydrates must be digested to ____________ to be absorbed through the intestine

monosaccharides

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Starch is a mixture of both ____ and _________ polymers of glucose

straight; branched

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What are the straight chain polymers?

Amylose

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What are the branched polymers?

Amylopectin

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What are the 3 disarccharides?

1) Trehalose (2 glucose)
2) Sucrose (glucose and fructose)
3) Lactose (glucose and galactose)

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What are the 3 monosaccharides?

1) Glucose
2) Fructose
3) Galactose

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Which crucial linkage in cellulose cannot be broken down by the body?

beta 1,4 linkage

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What enzyme digests polysaccharides in the saliva and pancreas?

alpha amylase

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Digestion of starch begins with which enzyme?

a amylase

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Which bonds do pancreatic amylase digest?

alpha 1,4 bonds

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What 3 disaccharides does amylase digestion produce?

1) a limit dextrins
2) maltose
3) maltotriose

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The 3 dissarchide products from starch digestion (a limit dextrins, maltose, and maltotriose) are broken down into monosaccharides by which 3 intestinal BRUSH BORDER ENZYMES?

1) a dextrinase
2) maltase
3) sucrase

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What carbohydrates do NOT require amylase digestion?

Disaccharides (trehalose, sucrose, lactose)

they have their own enzymes named after them but with ‘ase’ ending

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What transporter carries GLUCOSE and GALACTOSE into the cell and what is special about it?

SGLT1

It is sodium dependent and ACTIVE (both sugars and sodium are traveling up their concentration gradient)

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Where is SGLT1 located?

apical membrane of small intestine

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Which transporter carries fructose into the cell?

GLUT5

ffffffructose fffffive

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What transporter allows all 3 monosaccharides to pass via facilitated diffusion across the basolateral membrane?

GLUT2

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In lactose intolerant patients, what is missing?

Lactase enzyme from the brush border

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In lactase deficient patients, lactose remains undigested and unabsorbed in the intestinal lumen. This retains water and leads to ________ _________

osmotic diarrhea

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Where is lactase missing in congenital lactose intolerance?

jejunum

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What are the consequences of a mutation in SGLT1?

glucose-galactose mal absorption; severe diarrhea (fructose diet only)

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What is the meaning of “essential” amino acids?

cannot be synthesized by body, need to be obtained by diet

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Where does protein digestion start?

stomach (pepsin)

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Where does protein digestion end?

small intestine (pancreatic and brush-border proteases)

What are the 2 classes of proteases?

1) endopeptidase (hydrolyze interior peptide bonds of proteins) 2) exopeptidase (hydrolyze one AA at a time from the C-terminal end)

What enzyme cleaves trypsinogen into its active form, trypsin?

enterokinase

What inactivates pepsin?

pancreatic HCO3

True or false: the partial protein digestion by pepsin is essential

FALSE (patients without a stomach do just fine)

What are the 5 major pancreatic proteases secreted as inactive precursors?

1) Typsinogen 2) Chymotrypsinogen 3) proelastase 4) Procarboxypeptidase A 5) Procarboxypeptidase B

Where is enterokinase located?

brush border epithelium

Which pancreatic enzyme is capable of autocatalysis?

Trypsin

What is the digestable/absorbable products of protein digestion?

amino acids, di and tri peptides

True or false: oligo peptides are absorbable

FALSE; they need to by further hydrolyzed by brush-border proteases

What do amino acids transporters commonly symport with?

Sodium

What do peptide transporters symport with?

H+

Which transporters are responsible for the uptake of drugs?

peptide transporters

How do amino acids exit the cell?

facilitated diffusion

What are 3 disorders of protein digestion and absorption?

1) Single AA absorption (solution: absorb di or tri peptides that have AA hydrolyzed by enzymes in cytoplasm) 2) Trypsinogen deficiency (solution: eat diet of partially hydrolyzed proteins) 3) Cystinuria - transporter for dibasic AAs absent in small intestine/kidney

What are the 4 categories of lipid in the diet?

1) Triglycerides (MAJOR SOURCE) 2) Phospholipids 3) Cholesterol 4) Vitamins A, D, E, and K (fat soluble)

What is the functional benefit of emulsification in terms of digestion?

mixing of the stomach churns the dietary lipids into a suspension of fine droplets increasing surface area for digestive enzymes

What does the emulsifying in the stomach?

dietary proteins

What does the emulsifying in the small intestine?

bile acids

Where does lipid digestion begin?

Stomach (gastric lipase secreted by gastric chief cells hydrolyzes 10% of dietary triglycerides to glycerol and free fatty acids)

What is the biggest contribution of the stomach to lipid digestion?

Timing the emptying to allow enough time for pancreatic enzymatic action

Where does the majority of lipid digestion occur?

small intestine

What are the 3 important lipolytic enzymes in pancreatic juice that work at neutral pH?

1) pancreatic lipase 2) phospholipase A2 3) cholesterol ester hydrolase

What lipid substrate does pancreatic lipase work on? And what critical cofactor does it rely on for efficacy?

Triglycerides (hydrolyzes them to monoglycerides and fatty acids) needs COFACTOR COLIPASE to anchor lipase to fat droplet so that bile acids won't inactive lipase

What lipid substrate does phospholipase A2 work on?

Phospholipids (secreted as proenzyme and activated by trypsin)

What lipid substrate does cholesterol ester hydrolase work on?

cholesterol - frees cholesterol and fatty acids

What are the products of lipid digestion? How are the solubilized?

1) monoglycerides 2) fatty acids 3) cholesterol 4) lysolecithin micelles

What is the only lipid digestion byproduct that is water soluble?

Glycerol

True or false: the lipid-containing micelles do not enter the cells

TRUE; the micelles drop off their contents at the apical membrane at the brush border where the lipids diffuse down their concentration gradient

Where are bile salts reabsorbed?

ILEUM

What happens to the lipid digestion products inside the cell?

they are re-esterified with FFA on the smooth ER to make triglycerides, cholesterol, and phospholipids again

How are lipids exported from the cell after digestion?

Chylomicrons (triglycerides and cholesterol at the core and phospholipids and apoproteins on the outside)

Where do chylomicrons travel?

lymphatic capillaries to the thoracic duct where they empty into the bloodstream

What leads to disorders of lipid digestion?

1) Pancreatic insufficiency: defective enzyme secretion 2) Acidity of duodenal contents (ex: Zollinger-Ellison): pancreatic enzymes don't function at optimal pH 3) deficiency of bile salts: no micelle formation 4) Abetalipoproteinemia: inability to form chylomicrons

Where is a majority of the water absorbed in the GI tract?

Small intestine (remainder absorbed in colon)

The tight junctions of the small intestine are _______ permitting a lot of paracellular movement while the tight junctions of the colon are _______ and do NOT permit paracellular movement

leaky; tight

Where is the major site of Na+ absorption?

small intestine

sodium enters the jejunal epithelial cells on the apical side via which 3 transporters?

1) Na-monosaccharide cotransporters 2) Na amino acid cotransporter 3) Na H exchanger

In the Na/H exchanger, which ion enters the cell and which leaves it?

H leaves; Na enters

What additional transporter is found in the ileum?

Cl/HCO3 exchanger in the apical and a Cl transporter in the basolateral

In the ILEUM there is net absorption of ______ and in the JEJUNUM there is net absorption of ________

NaCl; NaHCO3

Cl diffuses into the lumen through _________ while Na diffuses passively through ______ ______

channels; tight junctions

Cl channels on apical membrane are usually ________ (open/closed)

closed

What activates the Cl channels on apical membrane?

neurotransmitters (Ach, VIP)

What is the secondary messenger which opens the Cl- channels?

cAMP

What is osmotic diarrhea?

caused by the presence of non-absorbable solutes in the intestinal lumen (ex: lactase deficiency so lactose hangs out in the lumen and pulls water with it)

What is secretory diarrhea?

caused by excessive secretion of fluid by crypt cells (ex: overgrowth of enterohepatic bacteria)

What is necessary for calcium absorption in the small intestine?

vitamin D

What is the path of calcium absorption in the small intestine?

Ca diffuses down electrochemical gradient --> bound to calbindin --> pumped across basolateral membrane by Ca-ATPase

What does calcium malabsorption lead to?

rickets and osetomalacia

Where does vitamin B12 bind when it is released from food by pepsin?

R proteins

When does Vitamin B12 associate with Intrinsic Factor?

when it travels to duodenum and pancreatic proteases degrade R proteins

What secretes IF?

gastric parietal cells (NEED THESE or get pernicious anemia)

Where is B12 absorbed?

ileum