Digestion and absorption Flashcards

(97 cards)

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
Can di/tripeptides be absorbed without prior digestion into amino acids?
Yes
26
How can di/tripeptides be directly absorbed across the apical membrane (transporter)?
H+ gradient dependent apical carrier protein, PepT-1.
27
What ion gradient is PepT-1 dependent on?
H+
28
Once inside the cell, what happens to absorbed di/tripeptidases?
Cytosolic peptidases then convert these absorbed peptides to amino acids
29
Why is the rate of amino acid absorption is faster in PepT1 than individual aa transporters, despite there being an extra step?
Maintains a high concentration gradient (peptides changed when they enter the cell) More protein can pass as less selective.
30
What is the most common dietary lipid?
Triglycerides
31
What is the initial step of digestion of lipids?
Muscular movements of stomach emulsify fats (transformation into emulsion of oil droplets in water), aided by lingual/gastric lipases
32
By what stage of the small bowel is protein digestion complete?
End of jejunum
33
Where does most fat digestion occur?
Small intestine
34
What does pancreatic lipase do?
Digests triglycerides to monoglycerides and free fatty acids
35
What does co-lipase do?
Coordinates binding of pancreatic lipase to emulsion droplets
36
What is the effect of bile salts?
Detergents that decrease surface tension to make smaller oil droplets.
37
What is formed through the action of pancreatic lipase, colipase and bile salts?
Micelles
38
Where do micelles move after they form?
Diffuse to unstirred acidic layer adjacent to the apical membrane
39
How do micelles move across the apical membrane?
They dissociate then diffuse across the membrane
40
Where are constituates of lipids reassembled inside the absorptive cell?
The SER
41
What do the lipid constituates form in the cell?
Chylomicrons
42
How do the microscopic triglyceride particles form chylomicron?
Reassembled in the SER then combine with apoproteins made in the RER
43
Where in the cell are chylomicrons modified?
Golgi
44
How do chylomicrons leave the cell?
Secreted through the basolateral membrane
45
Once the chylomicrons have left the cell, where do they go, what do they contribute to?
Into the lacteals, where they join lymph to become chyle.
46
How are water soluble vitamins absorbed?
Passive diffusion, specific Na+-coupled carrier proteins or binding to specific apical receptors (B12)
47
How are fat soluble vitamins absorbed?
Presented for absorption dissolved in bile micelles. In most cases, they exit the epithelial cell unmodified in chylomicrons.
48
How much fluid enters the stomach and intestine each day?
9l
49
How much fluid enters the GI tract each day from food and drink?
2l
50
How much fluid is in the GI tract per day from secretions alone?
7l
51
How much fluid is lost in stool per day?
~100ml
52
Roughly how much fluid does the small intestine and colon absorb per day?
Small intestine ~8.5l | Colon ~400ml
53
What type of epithelium is the small intestine?
Leaky epithelium
54
What type of epithelium is the colon and especially rectum?
Tight epithelium
55
Is there any secretion in the intestine (from the intestine)?
Some HCO3- from duodenal Brunner's glands Some isotonic NaCl solution
56
How does water reabsorption occur in the bowel?
Through osmosis Secondary consequence of solute (electrolyte or nonelectrolyte) reabsorption.
57
What underpins water reabsorption in the bowel?
Absorption of osmolites Establishment of a hypertonic interstitial fluid Water drawn by osmosis from lumen
58
What pathways of absorption can water take in the bowel?
Paracellularly through tight junctions Transcellularly through membrane aquaporins
59
True or false, processes and methods of ion absorption change along the length of the bowel?
True
60
What are all pathways of ion absorption dependent on?
Basolateral Na/KATPase and the K+ leak channels.
61
In the duodenum and jejunum what are the two key ways Na+ is absorbed?
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
What causes chloride reabsorption in the proximal bowel?
Absorption of Na+ creates lumen negative interstitium positive gradient. Cl- moves through the paracellular pathway to maintain electroneutrality.
63
What is the main method of Na+ absorption in the ileum and jejunum?
NHE (Na/H+ exchanger)
64
What THREE ways is chloride absorbeda apically in the ileum and jejunum?
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
How does chloride exit via the basolateral membrane in the ileum and jejunum?
Cl- moves out of the basolateral side via KCC1 alongside K+.
66
Why does the paracellular absorption of Cl- decrease along the bowel?
Tight junctions get tighter restricting paracellular movement
67
How is Na+ absorbed in the colon and rectum?
Na+ will only move through ENaC apically by simple diffusion.
68
How is chloride mainly absorbed in the colon?
Mainly by AE (chloride bicarbonate exchanger)
69
How does chloride exit the cell in the colon?
Cl- effluxes through the C1C-2 channels on the basolateral membrane.
70
How is H+ loss different in the colon than the small bowel?
H+ is lost by the basolateral NHE1 rather than the apical exchanger.
71
What are the two ways potassium can be absorbed, where and how?
Passive absorption through paracellular route in jejunum, ileum Active absorption by H+/K+ATPase in colon
72
What are the two ways potassium can be secreted, where and how?
Passive secretion through paracellular route in colon Active secretion through apical K+ channels in colon
73
How does aldosterone stimulate K+ secretion?
Na+/K+ATPase activated
74
Why is it not an issue for aldosterone to increase K+ secretion when dehydrated?
Most K+ will recycle across the basolateral membrane and the absolute flux of K+ to the lumen will be relatively low
75
How is K+ absorbed across the basolateral membrane in colon (2 transporters)?
KCC and leak channels
76
In secretion of HCO3- how can extra bicarbonate enter the cell at the basolateral side?
NBC1 transporter
77
What does the NBC1 transporter transport and require?
HCO3- and Na+ into the cell Requires inward Na+ gradient
78
In HCO3- secretion, what happens to chloride ( why isnt chloride absorbed)?
Enters cell via AE1 Exits cell on apical side via CFTR
79
How is H+ lost from the cell in bicarbonate secretion?
NHE1
80
What underpins the isoosmotic secretions in crypts?
Basolateral Na+/K+/2Cl- cotransport (driven by Na-KATPase) accumulates Cl- ions inside the cell. This creates a gradient across the apical membrane
81
How is Cl- secreted in the isoosmotic secretions?
Through basolateral NKCC then apical CFTR
82
What causes Na+ secretions from crypts (isoosmotic)?
Diffuses between cells through tight junctions (paracellular), along electrical gradient established by Cl- movement into the lumen
83
What is the recommended dietary intake of calcium per day?
1g
84
How much calcium do we absorb and excrete per day?
0. 35g absorbed | 0. 2g excreted renally
85
Where in the GI tract is Ca2+ absorption limited to?
Upper duodenum
86
What transporters are involved in absorption of calcium?
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
What does calcium bind to inside cells?
Calbindin
88
What upregulates ECaC and calbindin?
Vitamin D-derived hormone calcitriol
89
What apical transporter moves Fe2+ into cells? What drives it?
Proton gradient driven DCT1
90
Is there a separate transporter for haem and Fe2+?
Yes
91
What is bile made from?
Cholesterol
92
What can cause idiopathic pancreatitis?
Alcohol abuse, gallstones
93
What drug can stimulate mucous production?
Prostaglandin analogue
94
Food in the stomach is emptied faster into the duodenum when it contains chyme that is ....
Rich in carbohydrate
95
A main function of the colon is...
Net absorption of Water, Na+ and Cl-
96
All listed monosaccharides are well absorbed in the duodenum and upper jejunum except...
Mannose
97
Where along the GI tract are di/tripeptides absorbed across?
Duodenum