Intestinal Salts, water and nutrient digestion/absorption

Question 1

Where is the majority of the fluid (ingested/secreted) absorbed in the GI tract? Type of epithelium?

Answer 1

Small intestine - leaky

Question 2

Epithelium of colon and rectum?

Answer 2

Tight

Question 3

What two electrolytes are secreted in the SI/colon/rectum?

Answer 3

HCO3 in duodenum

NaCl in immature cells at base of villi

Question 4

What draws water in by osmosis? How does it move from lumen to interstitium?

Answer 4

Hypertonicity of the interstitium set up by the Na/K ATPase

Para- and transcellularly

Question 5

What is the SGLT channel?

Answer 5

Na/glucose cotransporter

Question 6

What hormone upregulates Na absorption in SI —> rectum?

Answer 6

Aldosterone

Question 7

Great diagram in lecture notes?

Answer 7

Look at it

Question 8

Where does calcium absorption take place?

Answer 8

Upper duodenum

Question 9

How is calcium absorbed? Regulation?

Answer 9

Enters apical through ECaC or through vesicles which can be exocytosed
Binds to calbindin
Exits basolateral via Ca ATPase and Na/Ca exchanger

Calcitriol increases ECaC and calbindin synthesis

Question 10

How is Fe absorbed into an intestinal cell? Which Fe is this?

Answer 10

DCT1 on apical membrane (divalent channel so only Fe2+) or through Haeme transporter
Fe2+ then —> Fe3+ by ferroxidase

Question 11

Two fates of absorbed Fe?

Answer 11

Absorption
— hephaestin and IREG1 complex on basolateral ferries iron into blood where it binds to transferrin

Storage
— ferritin binds iron and stalls absorption

Question 12

How is surface area of SI increased?

Answer 12

Folds in mucosa

Villi, cells have brush border

Question 13

What does pancreatic amylase complete carb digestion to?

Answer 13

maltose, maltotriose and alpha-limit dextrin (because 1,6 links are not degraded by amylose)

Question 14

What protein transports glucose and galactose?

Answer 14

SGLT1 (GLUT2 also but only for high luminal [glucose])

Question 15

What further digests maltose, sucrose and alpha-limit dextrins?

Answer 15

glucoamylase
Sucrase
Isomaltase

Question 16

How is fructose absorbed?

Answer 16

Through GLUT5

Question 17

How do monosaccharides exit basolateral enterocyte?

Answer 17

GLUT2

Question 18

Where is carb absorption completed?

Answer 18

Mid-jejunum

Question 19

Where does hydrolysis of polysaccharides occur?

Answer 19

Proteins on brush border of enterocytes

Question 20

What molecule must be present in the lumen for glucose absorption?

Answer 20

sodium

Question 21

Where does carb digestion start?

Answer 21

Mouth - salivary amylase

Question 22

Where does protein digestion start? What does this organ secrete?

Answer 22

Stomach - secretes pepsinogen which in acidic environment is truncated to pepsin

Question 23

What is the second organ involved in protein digestion? How far do the secretory products digest the protein to?

Answer 23

Pancreas - secreting pancreatic endo- and exopeptidases

Proteins broken down to 2-6 aa

Question 24

Where does further digestion after this point occur?

Answer 24

Brush border of SI

Question 25

How are amino acids absorbed?

Answer 25

Through Na dependent carriers

Question 26

What does PepT-1 do? How are the transport products dealt with in the cell

Answer 26

Absorb di- and tripeptides: H+ dependent transporter

Cytosolic peptidases then degrade to aas

Question 27

How do aa leave enterocyte?

Answer 27

Through basolateral, Na dependent carriers

Question 28

Where does aa and peptide absorption finish?

Answer 28

End of Jejunum

Question 29

How are fats emulsified?

Answer 29

Lingual lipases

Muscular stomach movements

Question 30

Where does most digestion of lipid occur and how?

Answer 30

SI

Pancreatic lipases digest tri- to monoglycerides and free fatty acids, coordinated by colipase (also secreted by pancreas)

Question 31

Function of micelles and what causes their formation?

Answer 31

Bile salts acts as detergents to decrease surface tension to make smaller oil droplets AND incorporate digestion products (like free fatty acids or monoglycerides)

Question 32

How do micelles dissociate?

Answer 32

Move into unstirred acidic layer near surface of enterocyte formed by Na/H exchanger - cause components to dissociate and dissolve in lipid membrane

Question 33

Where do glycerol and short chain fatty acids go? Into micelles yes or no?

Answer 33

NOT into micelles

Dissolve in lipid membrane

Question 34

What is the fate of lipid digestion products once in enterocyte cell?

Answer 34

Reassembled by SER to triglycerides

RER combine TAGs with apoproteins to form chylomicron

Golgi exports chylomicron into lymphatic capillaries

Question 35

Where does lymph from SI drain into?

Answer 35

Left subclavian vein via thoracic duct

Question 36

How are water-soluble vitamins absorbed?

Answer 36

Passive diffusion through paracellular pathway
Na coupled carrier proteins through transcellular pathway
Specific apical receptor (B12)

Question 37

Significance of intrinsic factor?

Answer 37

Aids in the receptor mediated endocytosis of Vitamin B12

Question 38

How are fat soluble vitamins absorbed? And exported from cells?

Answer 38

Dissolved in bile salt micelles, and dissociate like lipid digestion products in unmixed acid layer to dissolve across lipid membrane

They are exported through chylomicrons