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Gastrointestinal physiology > Digestion and absorption > Flashcards

Digestion and absorption Flashcards

1
Q

What nutrients enter the body through the small intestine?

A

Carbohydrates, proteins, fats and vitamins and minerals

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2
Q

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

A

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)

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3
Q

What does dietary carbohydrate consist of?

A

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

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4
Q

What initiates digestion of starch?

A

Salivary amylase

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5
Q

What completes digestion of starch to maltose?

A

Pancreatic amylase

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6
Q

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

A

Maltose, maltotriose and alpha limit dextrins

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7
Q

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

A

Membrane bound dissacharidase enzymes

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8
Q

What enzymes digest sucrose and lactose?

A

Membrane bound dissacharidase enzymes

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9
Q

What are monosaccharides?

A

Glucose, fructose and galactose

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10
Q

What transporter absorbs glucose and galactose?

A

SGLT1

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11
Q

What type of carrier is SGLT1?

A

Na+ dependent, secondary active symporter

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12
Q

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

A

GLUT2

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13
Q

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

A

When [glucose] increases

Facilitated diffusion

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14
Q

What is apical GLUT2 glucose absorption dependent on?

A

Ca2+

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15
Q

What absorbs fructose across the apical membrane?

A

Facilitated diffusion carrier GLUT5

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16
Q

How do monosaccharides leave the cell?

A

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

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17
Q

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

A

Mid-jejunum

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18
Q

What do endopeptidases do vs exopeptidases?

A

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

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

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19
Q

What is an oligopeptidase?

A

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

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20
Q

Where does protein digestion begin?

A

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

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21
Q

What type of protein digesting enzyme is pepsin?

A

Endopeptidase

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22
Q

What protein digesting enzymes does the pancreas secrete?

A

Endopeptidases (trypsin, chymotrypsin and elastase)

Exopeptidases (carboxypeptidases)

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23
Q

What transporters facilitate amino acid absorption?

A

Apical carriers, mostly Na+-dependent

Carriers for different classes (neutral, cationic, basic)

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24
Q

How do amino acids exit the cell?

A

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

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25
Q

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

A

Yes

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26
Q

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

A

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

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27
Q

What ion gradient is PepT-1 dependent on?

A

H+

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28
Q

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

A

Cytosolic peptidases then convert these absorbed peptides to amino acids

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29
Q

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

A

Maintains a high concentration gradient (peptides changed when they enter the cell)

More protein can pass as less selective.

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30
Q

What is the most common dietary lipid?

A

Triglycerides

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31
Q

What is the initial step of digestion of lipids?

A

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

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32
Q

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

A

End of jejunum

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33
Q

Where does most fat digestion occur?

A

Small intestine

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34
Q

What does pancreatic lipase do?

A

Digests triglycerides to monoglycerides and free fatty acids

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35
Q

What does co-lipase do?

A

Coordinates binding of pancreatic lipase to emulsion droplets

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36
Q

What is the effect of bile salts?

A

Detergents that decrease surface tension to make smaller oil droplets.

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37
Q

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

A

Micelles

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38
Q

Where do micelles move after they form?

A

Diffuse to unstirred acidic layer adjacent to the apical membrane

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39
Q

How do micelles move across the apical membrane?

A

They dissociate then diffuse across the membrane

40
Q

Where are constituates of lipids reassembled inside the absorptive cell?

41
Q

What do the lipid constituates form in the cell?

A

Chylomicrons

42
Q

How do the microscopic triglyceride particles form chylomicron?

A

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

43
Q

Where in the cell are chylomicrons modified?

44
Q

How do chylomicrons leave the cell?

A

Secreted through the basolateral membrane

45
Q

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

A

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

46
Q

How are water soluble vitamins absorbed?

A

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

47
Q

How are fat soluble vitamins absorbed?

A

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

48
Q

How much fluid enters the stomach and intestine each day?

49
Q

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

50
Q

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

51
Q

How much fluid is lost in stool per day?

52
Q

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

A

Small intestine ~8.5l

Colon ~400ml

53
Q

What type of epithelium is the small intestine?

A

Leaky epithelium

54
Q

What type of epithelium is the colon and especially rectum?

A

Tight epithelium

55
Q

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

A

Some

HCO3- from duodenal Brunner’s glands

Some isotonic NaCl solution

56
Q

How does water reabsorption occur in the bowel?

A

Through osmosis

Secondary consequence of solute (electrolyte or nonelectrolyte) reabsorption.

57
Q

What underpins water reabsorption in the bowel?

A

Absorption of osmolites

Establishment of a hypertonic interstitial fluid

Water drawn by osmosis from lumen

58
Q

What pathways of absorption can water take in the bowel?

A

Paracellularly through tight junctions

Transcellularly through membrane aquaporins

59
Q

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

60
Q

What are all pathways of ion absorption dependent on?

A

Basolateral Na/KATPase and the K+ leak channels.

61
Q

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

A

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.

62
Q

What causes chloride reabsorption in the proximal bowel?

A

Absorption of Na+ creates lumen negative interstitium positive gradient.

Cl- moves through the paracellular pathway to maintain electroneutrality.

63
Q

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

A

NHE (Na/H+ exchanger)

64
Q

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

A

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.

65
Q

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

A

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

66
Q

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

A

Tight junctions get tighter restricting paracellular movement

67
Q

How is Na+ absorbed in the colon and rectum?

A

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

68
Q

How is chloride mainly absorbed in the colon?

A

Mainly by AE (chloride bicarbonate exchanger)

69
Q

How does chloride exit the cell in the colon?

A

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

70
Q

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

A

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

71
Q

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

A

Passive absorption through paracellular route in jejunum, ileum

Active absorption by H+/K+ATPase in colon

72
Q

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

A

Passive secretion through paracellular route in colon

Active secretion through apical K+ channels in colon

73
Q

How does aldosterone stimulate K+ secretion?

A

Na+/K+ATPase activated

74
Q

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

A

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

75
Q

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

A

KCC and leak channels

76
Q

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

A

NBC1 transporter

77
Q

What does the NBC1 transporter transport and require?

A

HCO3- and Na+ into the cell

Requires inward Na+ gradient

78
Q

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

A

Enters cell via AE1

Exits cell on apical side via CFTR

79
Q

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

80
Q

What underpins the isoosmotic secretions in crypts?

A

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

81
Q

How is Cl- secreted in the isoosmotic secretions?

A

Through basolateral NKCC then apical CFTR

82
Q

What causes Na+ secretions from crypts (isoosmotic)?

A

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

83
Q

What is the recommended dietary intake of calcium per day?

84
Q

How much calcium do we absorb and excrete per day?

A
  1. 35g absorbed

0. 2g excreted renally

85
Q

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

A

Upper duodenum

86
Q

What transporters are involved in absorption of calcium?

A

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)

87
Q

What does calcium bind to inside cells?

88
Q

What upregulates ECaC and calbindin?

A

Vitamin D-derived hormone calcitriol

89
Q

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

A

Proton gradient driven DCT1

90
Q

Is there a separate transporter for haem and Fe2+?

91
Q

What is bile made from?

A

Cholesterol

92
Q

What can cause idiopathic pancreatitis?

A

Alcohol abuse, gallstones

93
Q

What drug can stimulate mucous production?

A

Prostaglandin analogue

94
Q

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

A

Rich in carbohydrate

95
Q

A main function of the colon is…

A

Net absorption of Water, Na+ and Cl-

96
Q

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

97
Q

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

Gastrointestinal physiology (5 decks)

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