Digestive Physiology

Question 1

What are the intermediate steps and final steps of carbohydrate catabolism?

Answer 1

intermediate: polysaccharides —amylase—> oligosaccharides/disaccharides

final: disaccharides to monosaccharides

sucrose — sucrase—>glucose + fructose

maltose — maltase —> glucose + glucose

lactose —lactase—> glucose + galactose

Question 2

What are the intermediate and final steps of protein catabolism?

Answer 2

polypeptides —- pepsin, trypsin, chymotrypsin — > smaller/oligopeptides

oligo/dipeptides — Carboxypeptidase, amino peptidase, dipeptridase —> amino acids

Question 3

Where does the intermediate step of carbohydrate catabolism typically occur? What are the types amylase and where are they synthesized?

Answer 3

mostly occurs in the area of the duodenum that receives pancreatic amylase
- we do not chew food long enough for salivary amylase to have a greater effect than pancreatic amylase

salivary amylase - produced by salivary glands, released to mouth
pancreatic amylase - produced by pancreas, released to duodenum

Question 4

Where foes the final step of carbohydrate metabolism occur?

Answer 4

wall of the small intestine in the microvilli
- can be transported across the wall of small intestine

Question 5

Where does the intermediate step of protein synthesis occur?

Answer 5

pepsin - stomach HCl activates pepsinogen to pepsin

trypsin and chymotrypsin - work in small intestine

polypeptides to oligo/dipeptides

Question 6

Where dose the final steps of protein synthesis occur?

Answer 6

occurs in the brush border of the small intestine

Question 7

What kind of digestion and absorption occurs in the mouth?

Answer 7

no absorption, no protein digestion

salivary amylase for carbohydrate digestion

lingual lipase - active at pH 5-6
- remains alive in stomach and small intestine
- does not require bile acids
- minimal absorption

Question 8

What kind of digestion and absorption occurs in the stomach?

Answer 8

carbs
- no gastric enzyme, denatures salivary enzyme
- no absorption

proteins
- pepsinogen - activated by HCl into pepsin
- polypeptides - dipeptides
- no absorption

lipid
- gastric lipase
- does not require bile salts
- minimal absorption - absorbs alcohol and drugs

MAIN F(x) IS MECHANICAL DIGESTION

Question 9

What digestion and absorption occurs in the small intestine?

Answer 9

Carbs
- digestion via pancreatic amylase, maltase, lactase and sucrase

protein digestion
- trypsinogen to trypsin
- chymotrypsin
- carboxypeptidase, amino peptidase, dipeptidase

lipid digestion - pancreatic lipase

Question 10

What stimulates gastric activity?

Answer 10

distension - activates vagus nerve
cephalic - smelling/having food in mouth activates vagus
- preganglionic fibers activate stomach to increase pepsin, HCl, peristalsis

gastrin - G cells release gastrin - goes into blood and back to stomach to increase secretion of HCl, digestive enzymes and muscle motions

low acidity (high pH)

enterogastrin

Question 11

What inhibits gastric emptying?

Answer 11

sympathetic innervation - thoracic splanchnic nerves
- fight or flight, not rest/digest
- decrease muscles motility, HCl, and enzymes

secretin - from duodenum - inhibitory on stomach
cholecystokinin - from duodenum - inhibitory on stomach
gastric inhibiting peptide - from duodenum - inhibitory on stomach

stretching of the duodenal wall - chyme needs to be neutralized slowly

Question 12

Describe what effects these hormones that the duodenum release cause: gastric inhibitory peptide, cholecystokinin, secretin

Answer 12

gastric inhibitory peptide - inhibitory on stomach

cholecystokinin - inhibitory on stomach
- excitatory on pancreas - increase digestive enzymes: trypsin, chymotrypsin, pancreatic lipase and amylase
- excitatory on gall bladder - causes it to contract and sends bile salts to duodenum

secretin - inhibitory on stomach
- excitatory on pancreas - releases HCO3-
- excitatory on liver - increases biosynthesis of bile salts and increases bile salts in small intestine

Question 13

How is salt and water absorbed in the small intestine?

Answer 13

Na active transport by Na/K ATPase pumps at basolateral borders
Cl- passively follows
Water osmotically follows

Question 14

How are carbohydrates absorbed in the small intestine?

Answer 14

glucose and galactose enter via secondary transport with Na ions
fructose enters via facilitated diffusion
monosaccharides leave basal layer by facilitated diffusion

Question 15

How are proteins absorbed in the small intestine>?

Answer 15

AA - secondary transport with Na into the cell
amino acids leave by facilitated diffusion

Question 16

How are lipids absorbed in the small intestine?

Answer 16

formation of micelles which re lipid soluble and diffuse across PM

formation of chylomicrons within the epithelial cells

exocytosis across basolateal membrane into lacteals

Question 17

how are vitamins absorbed in the small intestine

Answer 17

fat-soluble vitamins (ADEK) absorbed in chylomicrons
water soluble vitamins absorbed by diffusion

Question 18

give the summary of protein digestion

Answer 18

mouth - no digestion
stomach:
- pepsin hydrolyzes certain peptide bonds - proteins broken into smaller polypeptides

Small intestine
- trypsin and chymotrypsin (from pancreas) hydrolyze other peptide bonds - break them into smaller oligopeptide
- carboxypeptidase removes 1 AA at at time form the carboxyl end of oligopeptide
- occurs mostly in jejunum - some in ileum

BRUSH BORDER (microvilli) OF SMALL INTESTINE
- carboxypeptidase present
- amino peptidase removes one AA at a time from the amino end
- dipeptide splits dipeptide into separate AA

Question 19

Give the summary of fat digestion and absorption

Answer 19

emulsification - lumen of duodenum
- fat globule broken up by lecithin and bile salts

fat hydrolysis - lumen of SI
- pancreatic lipase act on emulsified droplet
- hydrolyzes 1st and 3rd FA from triglyceride, mono with middle

micelle formation - lumen of SI
- lipids form micelles coated with bile acids

chylomicron formation - intestinal epithelial cells
- ribosomes make proteins for transport in blood
- proteins coat lipid components to form chylomicrons
- exit via exocytosis into lacteal - to heart

Question 20

Describe what occurs in the large intestine. What is its pH?

Answer 20

secretion - HCO3- and mucus

absorption - mostly occurs in jejunum, some ileum
- active absorption of sodium
- Cl- follows passively down its gradient
- H2O absorbed osmotically
- Vitamin K from E. coli