Nutrient Assimilation

Question 1

What is the approximate total amount of daily fluid that is handled by the GI?

Answer 1

8-10 L

Question 2

What is the site at which 90% of water is reabsorbed?

Answer 2

Small Intestines

Question 3

What are the factors that affect fluid movement? What influences those factors?

Answer 3

Absorption (surface area), and motility (transit time)

Question 4

By what mechanism does water move within the GI?

Answer 4

Diffusion affected by electrolyte gradients

Question 5

What are all of the possible substrates that can pass through GLUT2 depending on the site of the transporter in the GI?

Answer 5

Glucose, fructose, and galactose

Question 6

What is the mechanism of water secretion in the small intestines?

Answer 6

K+ leaks out through the BL membrane to power the NKCC1 which creates a high intracellular chloride concentration. Chloride then diffuses into the lumen through the apical membrane through CFTR and also stimulates the movement of water with it

Question 7

What is the mechanism by which water is absorbed in the small intestines?

Answer 7

Lumenal Na+ is transported into epithelial cells via NHE3 and then into the interstitial space through the Na/K pump in the BL membrane. Water follows

Question 8

What are all of the possible mechanisms by which sodium is absorbed by the GI? Which are most important in the small intestines? In the colon?

Answer 8

ENaC (most important in colon), NHE3, SGLT1, Na+/Amino acid transporters (these three important in SI); Na+/K+ ATPase on the BL membrane

Question 9

What is the primary ion secreted by the GI tract?

Answer 9

Chloride

Question 10

What are the mechanisms by which chloride is brought into the cell in the cell?

Answer 10

DRA (Cl/HCO3), NKCC1, KCC1

Question 11

What is the process by which chloride is secreted?

Answer 11

Cl- is taken from ISF through NKCC1, KCC1, and it is secreted into the lumen through CFTR channels

Question 12

Which factors regulate the activity of the CFTR channel?

Answer 12

VIP, PGE2, cAMP, Histamine, Insulin

Question 13

Where is K+ absorbed in the GI? Secreted? Through what mechanisms?

Answer 13

Absorbed in the small intestine through passive, paracellular diffusion; Actively secreted in the colon upon stimulation by aldosterone

Question 14

In general, in the GI tract, where does secretion and absorption occur?

Answer 14

Secretion in the Crypts of Lieberkuhn, absorption at the tips of villi

Question 15

What are the 2 pathophysiologic mechanisms of Diarrhea?

Answer 15

Decreased absorption of fluid and electrolytes or increased secretion of fluid and electrolytes

Question 16

How does cholera cause diarrhea?

Answer 16

Produces a toxin that binds to a GM1-ganglioside receptor, specifically to adenylate cyclase, increasing cAMP concentration which increased phosphorylation of CFTR channels causing increased anion secretion. leading to diarrhea

Question 17

What colonic electrolyte exchangers are under hormonal regulators? What are the major and minor regulators?

Answer 17

NHERF1 (Na/H ex.), ENaC, and BCA1 (K secretion); primarily regulated by aldosterone and PYY but also estrogen and prolactin

Question 18

What is the structure through which water absorption in the colon is regulated? What are the hormonal regulators?

Answer 18

AQP3- regulated by ANP and BNP (also VIP, ADH?, and PYY)

Question 19

Where does fermentation occur? By what structures?

Answer 19

Large intestine– by the microbiome

Question 20

Within the colon, where does the most fermentation occur?

Answer 20

More fermentation in the cecum and ascending colon, less in the transverse, and even less in the descending and colon

Question 21

Roughly what percent of poop is made of dead bacteria?

Answer 21

60%

Question 22

What is the function of lingual lipase?

Answer 22

Conversion of solid fat into oils

Question 23

How does the stomach participate in chemical digestion?

Answer 23

The addition of acid protenates food leading to fragmentation of protein and activates pepsinogen; pepsin cleaves polypeptides into oligopeptides; gastric llipase converts triglycerides into DAG and FFA

Question 24

What percent of triglycerides are hydrolyzed in the stomach?

Answer 24

~10%

Question 25

What stimulus from the stomach determines how much CCK is produced?

Answer 25

The 10% of triglycerides

Question 26

What is the process of B12 absorption?

Answer 26

B12 binds to salivary haptocorrins. ONce haptocorrrin is cleaved in the duodenum, intrinsic factor produced by the stomach binds to B12 and the complex is taken up in the terminal ileum

Question 27

What enzyme cleaves trypsinogen? Where is it produced?

Answer 27

Enterokinase from enterocytes

Question 28

What are the three phases of digestion in the gut? What is each?

Answer 28

Luminal- mixing of chyme with enzymes; brush boarder- specific enzymes on the luminal surface of enterocytes; cytosolic/intracellular- intracellular digestion in the enterocytes

Question 29

What phases of digestion are used on each of the 3 types of nutrients?

Answer 29

Protein- all three; Carbs- only luminal and brush border; fat- all digestion is luminal

Question 30

What are the pancreatic digestive enzymes? What are their products?

Answer 30

Amylase- disaccharides; proteases- oligopeptides and amino acid; lipase- glycerol and FFA; Nucleases- nucleotides

Question 31

What are the general brush boarder enzymes and what are their products?

Answer 31

Disaccharidases-> simple sugars; Peptidases–> dipeptides, tripeptides, amino acids; Alkaline phosphatase-> nucleosides

Question 32

What enzyme(s) break down large peptides into peptides with C-terminal neutral amino acids? What enzyme subsequently breaks the small peptides into short peptides?

Answer 32

Chymotrypsin and/ or elastase; Carboxypeptidase A

Question 33

What results from the cleavage of large peptides by trypsin? Which enzyme further breaks down the products?

Answer 33

Peptides with C-terminal basic amino acid– further digested by carboxypeptidase B

Question 34

How are amino acids, dipeptides and tripeptides absorbed by the GI?

Answer 34

Simple aa are absorbed by simple sodium cotransport; di- and tri-peptides are taken up by an H+ cotransporter

Question 35

How are amylose and amylopectin digested by the GI?

Answer 35

They are broken down into disaccharides, trisaccharides and oligosaccharides which are further broken down by specific enzymes

Question 36

How are simple sugars absorbed by the GI?

Answer 36

Glucose enters the enterocyte via SGLT1, fructose enters through GLUT5, and all simple sugars exit through the BL via GLUT2

Question 37

How are fats emulsified?

Answer 37

Addition of bile, containing bile salts and phospholipids

Question 38

What is the only form fat can be absorbed as?

Answer 38

Free fatty acids or monoglycerides

Question 39

What form into micelles?

Answer 39

Monoglycerides and FFA along with bile salts

Question 40

What is the role of colipase?

Answer 40

Normally lipase binds to lipid but is displaced by bile acids, which inhibits the breakdown of the fat. Colipase binds to both lipase and bile acids to stabilize the lipase

Question 41

What are chylomicrons and how are they fomed?

Answer 41

FFAs and monoglycerides are reformed into triglycerides inside the enterocytes, packaged with cholesterol and apo-lipoprotein molecules to form chylomicrons

Question 42

How are cholesterol and other sterols transported across the brush border? How does it work?

Answer 42

NPC1L1- undergoes clathrin-mediated internalization after sterol binding

Question 43

What sorts of signals can be mediated by LCFA?

Answer 43

Fat taste perception, satiety effects, inhibited glucose production by the liver