Digestion and Absorption

Question 1

What must happen prior to the absorption of carbohydrates?

Answer 1

The must be digested into monosaccharides before being absorbed

Question 2

Describe the digestion of carbohydrates prior to absorption

Answer 2

Starches conveted to malt-ose -triose and dextrins via pancreatic/salivary amylases

Brush border membrane enzymes hydrolyze these oligosaccharides to glucose

Sucrose, lactose, and trehalose also cleaved by brush border enzymes

Question 3

What monosaccarhides are absorbed by the intestine?

Answer 3

Glucose

Galactose

Fructose

Question 4

How are glucose and galactose absorbed?

Answer 4

Common Na-dependent active transport system (SLGT-1)

Almost all glucose/galactose absorbed and subsequently transported to blood via facilitated diffusion

Question 5

How is fructose absorbed?

Answer 5

Exclusively by facilitated diffusion

Cannot go against concentration gradient

Question 6

What is lactose intolerance?

Answer 6

Deficiency in lactase, causing lactose to remain in GI tract as unabsorbed solute

Undigested lactose metabolized by colonic bacteria

Question 7

What is sucrase-isomaltase deficiency?

Answer 7

Inherited disorder resulting in an inability to digest sucrose and isomaltose

Question 8

What is glucose/galactose malabsorption?

Answer 8

Deficiency in SLGT-1

Question 9

What are the two sources of protein in the GI tract?

Answer 9

Endogenous - secretory proteins and cells shed into GI tract lumen

Exogenous - dietary protein

Question 10

What are endopeptidases, exopeptidases, and brush border peptidases

Answer 10

Endopeptidases - hydrolyze interior peptide bonds

Exopeptidases - hydrolyze one amino acid at a time from C terminus

BB Peptidases - cleave peptides produced by pancreatic proteases to oligopeptides and AAs

Question 11

Describe the process of pancreatic enzyme secretion and protein digestion

Answer 11

Pancreatic enzymes secreted as inactive precursors

Trypsinogen –> trypsin via enterokinase

Trypsin catalyzes conversion of trypsinogen, Chymotrypsinogen, and proelastase

Question 12

How are amino acids absorbed?

Answer 12

Na-dependent cotransport

Separate transporters for neutral, acidic, basic, and imino acids

Transported to blood via facilitated diffusion

Question 13

How are di- and tri-peptides absorbed?

Answer 13

H-dependent cotransporter peptide transporter 1 (PEPT1)

Following a meal, most protein is absorbed in form of di- and tri-peptides

Absorbed peptides are hydrolyzed in the enterocyte to AA’s

Question 14

What is cystinuria?

Answer 14

Affects uptake of basic amino acids

Question 15

What is Hartnup disease?

Answer 15

Affects uptake of neurtral amino acids

Question 16

What is familial iminoglycinuria?

Answer 16

Affects uptake of proline and hydroxyproline

Question 17

How are lipids digested in the stomach?

Answer 17

Mixing in stomach breaks lipids into droplets to increase total surface area

gastric lipase hydrolyzes triglycerides to diglycerides and FFA’s

CCK slows gastric emptying to allow sufficient time for digestion and absorption in duodenum

Question 18

What is the role of bile salts and lecithin in the small intestine?

Answer 18

Emulsify fats

Emulsification requires neutral or slightly alkaline environment

Question 19

What is the function of pancreatic lipase?

Answer 19

Cleaves fatty acids from 1 and 3 positions of triglycerides

Question 20

What is the function of cholesterol ester hydrolase?

Answer 20

Cleaves fatty acid from cholesterol esters

Produces free cholesterol and gatty acid

Question 21

What is the function of Phospholipase A2?

Answer 21

Releases fatty acids from 2 position of phospholipids

produces lysophospholipids and FFAs

Question 22

What is colipase?

Answer 22

Non-enzymatic protein secreted as inactive precursor by pancreas

Activated by trypsin

Prevents inhibition of pancreatic lipase by bile salts, which displace enzyme from surface of emulsion droplet

Question 23

How are lipids absorbed?

Answer 23

Micelles bring products of lipolysis through the unstirred water layer directly to the lumenal membrane

Lipid products diffuse directly into the enterocyte

Question 24

What is the monoglyceride acylation pathway?

Answer 24

Triglycerides synthesized from 2 monoglycerides and CoA-activated fatty acids

Long-chain reesterified in smooth ER

Medium chain enter bloodstream without resynthesis into triglyceride

Question 25

What is the phosphatidic acid pathway?

Answer 25

Formation of phospholipids from ingested fats

Question 26

How is cholesterol absorbed?

Answer 26

Absorbed in free form Significant portion re-esterified with fatty acids in enterocyte All transported in chylomicrons

Question 27

How are chylomicrons formed and transported?

Answer 27

Resynthesized triglycerides, cholesterol and phospholipids Transported out via exocytosis and enter lymphatic lacteals Reach bloodstream via thoracic duct

Question 28

What can cause lipid malabsorption?

Answer 28

Failure to digest fat - unsecreted or inactivated pancreatic enzymes Absence of bile salts Decrease number of absorbing cells Failure to synthesize apoproteins

Question 29

What can cause the absence of bile salts?

Answer 29

Liver disease Common bile duct obstruction Bacterial overgrowth of small intestine (leads to deconjugation) Increased duodenal acidity

Question 30

What is gluten enteropathy (celiac sprue)?

Answer 30

Allergic reaction to gluten in wheat products Leads to loss of villi in small intestine Also causes malabsorption of carbs and proteins

Question 31

What is abetalipoproteinemia?

Answer 31

Failure to synthesize ApoB Results in lack of formation of chylomicrons

Question 32

How is heme iron absorbed?

Answer 32

Most easily absorbed form Taken up by receptor-mediated endocytosis or transporter protein HCP1 Heme then broken down by oxygenase to release free iron

Question 33

How is free iron absorbed?

Answer 33

Gastric acid dissolves iron and permits it to form soluble complexes with ascorbic acid This reduces it to ferrous iron, which is more easily absorbed Ferrous iron transported across brush border by DMT1 Binds to apoferritin for storage

Question 34

What is the function of duodenal cytochrome b?

Answer 34

Dcytb reduces ferric to ferrous iron, allowing it to be absorbed

Question 35

What is ferroportin?

Answer 35

Protein that transports iron out of the enterocyte into plasma

Question 36

How is iron absorption regulated?

Answer 36

Hepcidin regulates entery of iron into plasma by binding to ferroportin This leads to internalization and degradation of ferroportin This blocks cellular iron export and reduces plasma iron

Question 37

What regulation occurs in low and high iron states?

Answer 37

Low iron - low hepcidin, causing increased iron absoprtion and elevated iron release from enterocytes High iron - high hepcidin, decrease iron export from enterocytes, enterocytes containing iron lost to intestine

Question 38

What is hemochromatosis?

Answer 38

Chronic absorption of too much iron Defect in HFE gene causes hepcidin levels to drop Excess iron collects in liver, leading to cirrhosis and cancer Can also lead to diabetes and coronary disease

Question 39

How does aging affect the mouth?

Answer 39

Lose taste buds, chewing weakens, may lose teeth

Question 40

How does aging affect the esophagus?

Answer 40

Swallowing becomes more difficult, LES function declines

Question 41

How does aging affect the stomach?

Answer 41

Lose parietal cells Increase in ulcers because increased use of NSAIDs

Question 42

How does aging affect the small intestine?

Answer 42

Decrease in motility Increase in malabsorption

Question 43

How does aging affect the colon?

Answer 43

Decrease in motility Increase in polyps