Digestion and absorption

Question 1

What nutrients enter the body through the small intestine?

Answer 1

Carbohydrates, proteins, fats and vitamins and minerals

Question 2

What are 5 different pathways of nutrient digestion and absorption (give an example for each)?

Answer 2

Some nutrients are absorbed directly from the lumen without prior digestion (such as glucose).

Hydrolysis in the lumen, followed by absorption across the cell (starch, protein)

Hydrolysis on the apical membrane, followed by absorption across the cell (disaccharides)

Absorption into, and hydrolysis within the cell (di, tripeptides)

Following luminal hydrolysis, substances may be taken up and resynthesised within the cell (triglycerides)

Question 3

What does dietary carbohydrate consist of?

Answer 3

Starch (amylose, amylopectin) and sugars (sucrose, lactose)

Question 4

What initiates digestion of starch?

Answer 4

Salivary amylase

Question 5

What completes digestion of starch to maltose?

Answer 5

Pancreatic amylase

Question 6

What does starch get digested into (first step, 3 things)?

Answer 6

Maltose, maltotriose and alpha limit dextrins

Question 7

What enzymes digest products of amylase digestion, where are they found?

Answer 7

Membrane bound dissacharidase enzymes

Question 8

What enzymes digest sucrose and lactose?

Answer 8

Membrane bound dissacharidase enzymes

Question 9

What are monosaccharides?

Answer 9

Glucose, fructose and galactose

Question 10

What transporter absorbs glucose and galactose?

Answer 10

SGLT1

Question 11

What type of carrier is SGLT1?

Answer 11

Na+ dependent, secondary active symporter

Question 12

What other transporter (other than SGLT1) can absorb glucose across the apical membrane?

Answer 12

GLUT2

Question 13

When is GLUT2 inserted apically, what sort of transporter is it?

Answer 13

When [glucose] increases

Facilitated diffusion

Question 14

What is apical GLUT2 glucose absorption dependent on?

Answer 14

Ca2+

Question 15

What absorbs fructose across the apical membrane?

Answer 15

Facilitated diffusion carrier GLUT5

Question 16

How do monosaccharides leave the cell?

Answer 16

Exit the cell at the basolateral membrane on GLUT2 by facilitated diffusion

Question 17

At what stage of the small bowel is absorption of carbohydrates complete?

Answer 17

Mid-jejunum

Question 18

What do endopeptidases do vs exopeptidases?

Answer 18

Endopeptidase break peptide bonds of nonterminal amino acids (i.e. within the molecule)

Exopeptidases break peptide bonds from end-pieces of terminal amino acids.

Question 19

What is an oligopeptidase?

Answer 19

A type of endopeptidase where substrate is an oligopeptide not a protein

Question 20

Where does protein digestion begin?

Answer 20

The acidic environment of the stomach, where pepsin is secreted and activated.

Question 21

What type of protein digesting enzyme is pepsin?

Answer 21

Endopeptidase

Question 22

What protein digesting enzymes does the pancreas secrete?

Answer 22

Endopeptidases (trypsin, chymotrypsin and elastase)

Exopeptidases (carboxypeptidases)

Question 23

What transporters facilitate amino acid absorption?

Answer 23

Apical carriers, mostly Na+-dependent

Carriers for different classes (neutral, cationic, basic)

Question 24

How do amino acids exit the cell?

Answer 24

Across basolateral membrane by facilitated diffusion on Na+-independent carriers

Question 25

Can di/tripeptides be absorbed without prior digestion into amino acids?

Answer 25

Yes

Question 26

How can di/tripeptides be directly absorbed across the apical membrane (transporter)?

Answer 26

H+ gradient dependent apical carrier protein, PepT-1.

Question 27

What ion gradient is PepT-1 dependent on?

Answer 27

H+

Question 28

Once inside the cell, what happens to absorbed di/tripeptidases?

Answer 28

Cytosolic peptidases then convert these absorbed peptides to amino acids

Question 29

Why is the rate of amino acid absorption is faster in PepT1 than individual aa transporters, despite there being an extra step?

Answer 29

Maintains a high concentration gradient (peptides changed when they enter the cell) More protein can pass as less selective.

Question 30

What is the most common dietary lipid?

Answer 30

Triglycerides

Question 31

What is the initial step of digestion of lipids?

Answer 31

Muscular movements of stomach emulsify fats (transformation into emulsion of oil droplets in water), aided by lingual/gastric lipases

Question 32

By what stage of the small bowel is protein digestion complete?

Answer 32

End of jejunum

Question 33

Where does most fat digestion occur?

Answer 33

Small intestine

Question 34

What does pancreatic lipase do?

Answer 34

Digests triglycerides to monoglycerides and free fatty acids

Question 35

What does co-lipase do?

Answer 35

Coordinates binding of pancreatic lipase to emulsion droplets

Question 36

What is the effect of bile salts?

Answer 36

Detergents that decrease surface tension to make smaller oil droplets.

Question 37

What is formed through the action of pancreatic lipase, colipase and bile salts?

Answer 37

Micelles

Question 38

Where do micelles move after they form?

Answer 38

Diffuse to unstirred acidic layer adjacent to the apical membrane

Question 39

How do micelles move across the apical membrane?

Answer 39

They dissociate then diffuse across the membrane

Question 40

Where are constituates of lipids reassembled inside the absorptive cell?

Answer 40

The SER

Question 41

What do the lipid constituates form in the cell?

Answer 41

Chylomicrons

Question 42

How do the microscopic triglyceride particles form chylomicron?

Answer 42

Reassembled in the SER then combine with apoproteins made in the RER

Question 43

Where in the cell are chylomicrons modified?

Answer 43

Golgi

Question 44

How do chylomicrons leave the cell?

Answer 44

Secreted through the basolateral membrane

Question 45

Once the chylomicrons have left the cell, where do they go, what do they contribute to?

Answer 45

Into the lacteals, where they join lymph to become chyle.

Question 46

How are water soluble vitamins absorbed?

Answer 46

Passive diffusion, specific Na+-coupled carrier proteins or binding to specific apical receptors (B12)

Question 47

How are fat soluble vitamins absorbed?

Answer 47

Presented for absorption dissolved in bile micelles. In most cases, they exit the epithelial cell unmodified in chylomicrons.

Question 48

How much fluid enters the stomach and intestine each day?

Answer 48

9l

Question 49

How much fluid enters the GI tract each day from food and drink?

Answer 49

2l

Question 50

How much fluid is in the GI tract per day from secretions alone?

Answer 50

7l

Question 51

How much fluid is lost in stool per day?

Answer 51

~100ml

Question 52

Roughly how much fluid does the small intestine and colon absorb per day?

Answer 52

Small intestine ~8.5l | Colon ~400ml

Question 53

What type of epithelium is the small intestine?

Answer 53

Leaky epithelium

Question 54

What type of epithelium is the colon and especially rectum?

Answer 54

Tight epithelium

Question 55

Is there any secretion in the intestine (from the intestine)?

Answer 55

Some HCO3- from duodenal Brunner's glands Some isotonic NaCl solution

Question 56

How does water reabsorption occur in the bowel?

Answer 56

Through osmosis Secondary consequence of solute (electrolyte or nonelectrolyte) reabsorption.

Question 57

What underpins water reabsorption in the bowel?

Answer 57

Absorption of osmolites Establishment of a hypertonic interstitial fluid Water drawn by osmosis from lumen

Question 58

What pathways of absorption can water take in the bowel?

Answer 58

Paracellularly through tight junctions Transcellularly through membrane aquaporins

Question 59

True or false, processes and methods of ion absorption change along the length of the bowel?

Answer 59

True

Question 60

What are all pathways of ion absorption dependent on?

Answer 60

Basolateral Na/KATPase and the K+ leak channels.

Question 61

In the duodenum and jejunum what are the two key ways Na+ is absorbed?

Answer 61

1) Absorbed with SGLT or specific amino acid transporters. Na+ moves into the interstitium by Na/KATPase. 2) Na+/H+ exchange occurs by the NHE3 isoform exchanger.

Question 62

What causes chloride reabsorption in the proximal bowel?

Answer 62

Absorption of Na+ creates lumen negative interstitium positive gradient. Cl- moves through the paracellular pathway to maintain electroneutrality.

Question 63

What is the main method of Na+ absorption in the ileum and jejunum?

Answer 63

NHE (Na/H+ exchanger)

Question 64

What THREE ways is chloride absorbeda apically in the ileum and jejunum?

Answer 64

The decreased luminal [Na+] causes a lumen negative interstitium positive gradient. Chloride moves into the cell AE (bicarbonate moves out, chloride in) Direct Na+/Cl- symport. (NCC) Cl- moves through the paracellular pathway to maintain electroneutrality.

Question 65

How does chloride exit via the basolateral membrane in the ileum and jejunum?

Answer 65

Cl- moves out of the basolateral side via KCC1 alongside K+.

Question 66

Why does the paracellular absorption of Cl- decrease along the bowel?

Answer 66

Tight junctions get tighter restricting paracellular movement

Question 67

How is Na+ absorbed in the colon and rectum?

Answer 67

Na+ will only move through ENaC apically by simple diffusion.

Question 68

How is chloride mainly absorbed in the colon?

Answer 68

Mainly by AE (chloride bicarbonate exchanger)

Question 69

How does chloride exit the cell in the colon?

Answer 69

Cl- effluxes through the C1C-2 channels on the basolateral membrane.

Question 70

How is H+ loss different in the colon than the small bowel?

Answer 70

H+ is lost by the basolateral NHE1 rather than the apical exchanger.

Question 71

What are the two ways potassium can be absorbed, where and how?

Answer 71

Passive absorption through paracellular route in jejunum, ileum Active absorption by H+/K+ATPase in colon

Question 72

What are the two ways potassium can be secreted, where and how?

Answer 72

Passive secretion through paracellular route in colon Active secretion through apical K+ channels in colon

Question 73

How does aldosterone stimulate K+ secretion?

Answer 73

Na+/K+ATPase activated

Question 74

Why is it not an issue for aldosterone to increase K+ secretion when dehydrated?

Answer 74

Most K+ will recycle across the basolateral membrane and the absolute flux of K+ to the lumen will be relatively low

Question 75

How is K+ absorbed across the basolateral membrane in colon (2 transporters)?

Answer 75

KCC and leak channels

Question 76

In secretion of HCO3- how can extra bicarbonate enter the cell at the basolateral side?

Answer 76

NBC1 transporter

Question 77

What does the NBC1 transporter transport and require?

Answer 77

HCO3- and Na+ into the cell Requires inward Na+ gradient

Question 78

In HCO3- secretion, what happens to chloride ( why isnt chloride absorbed)?

Answer 78

Enters cell via AE1 Exits cell on apical side via CFTR

Question 79

How is H+ lost from the cell in bicarbonate secretion?

Answer 79

NHE1

Question 80

What underpins the isoosmotic secretions in crypts?

Answer 80

Basolateral Na+/K+/2Cl- cotransport (driven by Na-KATPase) accumulates Cl- ions inside the cell. This creates a gradient across the apical membrane

Question 81

How is Cl- secreted in the isoosmotic secretions?

Answer 81

Through basolateral NKCC then apical CFTR

Question 82

What causes Na+ secretions from crypts (isoosmotic)?

Answer 82

Diffuses between cells through tight junctions (paracellular), along electrical gradient established by Cl- movement into the lumen

Question 83

What is the recommended dietary intake of calcium per day?

Answer 83

1g

Question 84

How much calcium do we absorb and excrete per day?

Answer 84

0. 35g absorbed | 0. 2g excreted renally

Question 85

Where in the GI tract is Ca2+ absorption limited to?

Answer 85

Upper duodenum

Question 86

What transporters are involved in absorption of calcium?

Answer 86

Ca2+ entry across the apical membrane via specific channels - ECaC / TRPV5/6 Ca2+ exit on the basolateral membrane via a Ca2+ATPase (and NCX - less important)

Question 87

What does calcium bind to inside cells?

Answer 87

Calbindin

Question 88

What upregulates ECaC and calbindin?

Answer 88

Vitamin D-derived hormone calcitriol

Question 89

What apical transporter moves Fe2+ into cells? What drives it?

Answer 89

Proton gradient driven DCT1

Question 90

Is there a separate transporter for haem and Fe2+?

Answer 90

Yes

Question 91

What is bile made from?

Answer 91

Cholesterol

Question 92

What can cause idiopathic pancreatitis?

Answer 92

Alcohol abuse, gallstones

Question 93

What drug can stimulate mucous production?

Answer 93

Prostaglandin analogue

Question 94

Food in the stomach is emptied faster into the duodenum when it contains chyme that is ....

Answer 94

Rich in carbohydrate

Question 95

A main function of the colon is...

Answer 95

Net absorption of Water, Na+ and Cl-

Question 96

All listed monosaccharides are well absorbed in the duodenum and upper jejunum except...

Answer 96

Mannose

Question 97

Where along the GI tract are di/tripeptides absorbed across?

Answer 97

Duodenum