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metabolism > digestion and absorption > Flashcards

digestion and absorption Flashcards

1
Q

what are the two major tasks of the intestinal epithelial barrier?

A

absorption of nutrients

controls passage of pathogens/toxins

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2
Q

what regulates the IEB?

A

components of its
outer microenvironment - microflora and chyme
inner microenvironment - immune cells, fibroblasts or ENS

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3
Q

what specialised cytoskeletal proteins make up enterocytes?

A

polarity complexes
brush border
apical and lateral junctions
ECM and collagen

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4
Q

what is the function of polarity complexes?

A

orientates the enterocyte - tells cell where lumen and inside of the body is

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5
Q

what is the function of the bush border?

A

increases surface area

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6
Q

what is the brush border made up of?

A

myosin and actin

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7
Q

what is the function of the apical and lateral junctions of the enterocytes?

A

to act as a barrier

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8
Q

how much of nutrient absorption occurs in the small intestine?

A

95%

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9
Q

what is the surface area of the small intestine?

A

30-40m2

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10
Q

what kind of stem cells are found at the base of crypts and what do they do?

A

LGR5 - replicate to form daughter cells which form enterocytes and goblet cells

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11
Q

what is the process which removes cells from the top of the villus?

A

extrusion

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12
Q

why are cells removed from the top of the villus?

A

cannot survive in the harsh environment of the gut

might mutate to cope and turn cancerous

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13
Q

how is the small intestine structurally different to the colon?

A

colon is much flatter

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14
Q

where is water absorbed?

A

everywhere in the digestive tract

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15
Q

what does the ileum absorb?

A

B12
intrinsic factors
bile acids

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16
Q

what does the colon absorb?

A

electrolytes

water

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17
Q

what are the phases of digestion and absorption and what occurs in each?

A

luminal - breaks down ingested food
mucosal -brush border takes up nutrients
post-absorptive - absorbed nutrients transported to the whole body

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18
Q

what breaks down food in the luminal phase?

A
  • acid in the stomach
  • alkali in the small intestine
  • substrate specific enzymes secreted by gastric and small bowel mucosa and pancreas
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19
Q

how are nutrients transported to the whole body in the post-absorptive phase?

A

via lymphatics and portal circulation

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20
Q

why is water needed in the gut?

A

hydrolysis reactions of digestion
facilitation of absorption
facilitation of propulsion of gut contents
Combination with mucin granules to make mucus

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21
Q

how do secretary cells of the crypt absorb water?

A

Cl- pumped into the cell from ISF using co-transport
CFTR channels open and Cl- is pumped into the lumen
Na+ attracted to the Cl-
Na+ travels through paracellular pathway into the lumen
water follows

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22
Q

explain what happens to carbohydrates in the luminal phase?

A

split into disaccharides and limit dextrins by salivary and pancreatic enzymes

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23
Q

why are salivary enzymes trivial in the luminal phase of carbohydrate absorption?

A

destroyed quickly after being swallows and doesn’t work at the low pH of the stomach

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24
Q

what enzymes are involved in the mucosal phase of carbohydrate absorption?

A

sucrase, lactase, maltase, limit dextrine, glucoamylase

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25
Q

how do glucose and galactose enter epithelial cells?

A

via sodium-linked secondary active transport across the apical membrane

26
Q

how does fructose enter epithelial cells?

A

facilitated diffusion

27
Q

what happens to the broken down carbohydrates in the post absorptive phase?

A

sugars exit the cell across the basolateral membrane by facilitated diffusion into the portal vein

28
Q

what happens in the luminal phase of lipid digestion?

A

lipids are broken down by enzymes

starts in the mouth - lingual enzymes
stomach adds gastric lipases
pancreatic enzymes help digest triglycerides

29
Q

are lipids hydrophillic/phobic?

A

hydrophobic

30
Q

are lipase hydrophilic/phobic?

A

hydrophillic

31
Q

how do bile salts help lipase?

A

break down lipid globules to increase surface area upon which lipase can act

32
Q

where are bile salts absorbed?

A

the ileum

33
Q

where are bile salts secreted?

A

duodenum

34
Q

what are lipids broken down into in the luminal phase?

A

monoglycerides and free fatty acids

35
Q

what happens in the mucosal phase of lipid digestion?

A

monoglycerides and fatty acids enter enterocytes by diffusion

molecules reassembled into triglycerides inside enterocytes

packaged into chylomicrons

36
Q

what happens in the post-absorptive phase of lipid digestion?

A

chylomicrons pass across the basolateral membrane by exocytosis into the lymphatic circulation then into the blood

37
Q

explain what happens in the luminal phase of protein digestion

A

hydrochloric acid secreted by parietal cells activated pepsin
pepsin acts on pepsinogen to make more pepsin
pepsin helps in protein digestion in the small intestine

38
Q

explain what happens in the mucosal phase of protein digestion

A

enterokinase causes trypsinogen –> trypsin
trypsin activates;
chymotrypsinogen -> chymotrypsin
procarboxypeptidase -> carboxypeptidase

amino acids enter epithelial cells

39
Q

how do amino acids enter the epithelial cells during the mucosal phase of protein digestion

A

sodium-linked secondary active transport across the apical membrane down a sodium concentration gradient

40
Q

what happens in the post-absorptive phase of protein digestion?

A

amino acids transported across basolateral membrane by facilitated diffusion

enter portal vein and go straight to the liver

41
Q

how are vitamins obtained?

A

from food

body can’t make them

42
Q

what are the two types of vitamins and give examples of each

A

fat soluble - absorbed with lipids
vitamins A, D, E, K
water soluble - follow flux of water from gut lumen through the mucosa
vitamins B and C

43
Q

explain the absorption pathway of B12

A

Binds to Haptocorrin made in salivary glands

Complexed with intrinsic factor

moves into portal circulation

transferred to plasma transporter – transcolbalamin II (TC-II/B12)

Absorbed into terminal ileum

44
Q

what’s another name for vitamin B12?

A

cobalamin

45
Q

what type of bacteria is vibrio cholera?

A

gram negative

46
Q

what are the parts of the vibrio cholera toxin?

A

a and b

47
Q

explain how the toxins secreted by vibrio cholera lead to secretory diarrhoea

A

alpha part of toxin binds to crypt receptor
receptor complex enters cells
binds and activates adenylyl cyclase –> makes cAMP
causes CFTR channel to remain constantly open
another channel which captures Na+ and Cl- closes
Na+ attracted to the lumen paracellularly to maintain electric neutrality
water follows
causes secretory diarrhoea

48
Q

what is oral rehydration therapy?

A

solution of glucose and electrolyte - used to prevent and treat dehydration and diarrhoea

49
Q

which 2 receptors in the small intestine are not affected by cAMP?

A

sodium-glucose transporters

amino acid-sodium transporters

50
Q

explain how sodium absorption occurs in the small intestine

A

1 - cotransport with glucose via the SGLT1 symporter via secondary active transport

2 - enterocytes pump Na+ into the extracellular space via active transport via sodium-potassium pump on the basolateral membrane

51
Q

what is the cause of general malabsorption?

A

small intestine disease

can also be from pancreaticc disease or disease of other organs

52
Q

give examples of small intestine diseases

A

coeliac disease
post-infectious malabsorption
Crohn’s disease

53
Q

what is the most common cause of malabsorption in the economically developed world?

A

coeliac disease

54
Q

what is coeliac disease and what causes it?

A

autoimmune response of the body to gluten

caused by the destruction of the villi so everything becomes flat and nutrients cannot be absorbed

55
Q

in the developing world, what is the main cause of malabsorption?

A

acute or chronic infection caused by viral infection or chronic bacterial infection of the upper GI tract

56
Q

what is specific malabsorption?

A

when one class or type of nutrient isn’t absorbed

57
Q

give examples of specific malabsorption diseases and explain each

A

lactase deficiency - genetic or acquired to absorb disaccharide sugars
pernicious anaemia - failure to absorb B12

58
Q

what can cause the lack of lactase in lactase deficiency?

A

mutation in the LCT gene

enterocytes in the brush border not producing lactase

59
Q

what causes glucose-galactose malabsorption?

A

mutation in the SLC5A1 gene

60
Q

what does the SLC5A1 gene code for?

A

SGLT1 symporter

61
Q

what are the consequences of lactase deficiency?

A

Unabsorbed sugars reach colon
Add to osmotic load and cause watery diarrhoea
Fermentation of sugars in intestine causes gaseous distension (and adds to diarrhoea)

metabolism (23 decks)

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