6. Digestion & absorption

Question 1

name 2 features of the small intestine that promote absorption

Answer 1

1. large surface area created by PLICAE CIRCULARES (permanent folds), VILLI and MICROVILLI
2. slow mov. of contents

Question 2

how does the small intestine protect itself from bacteria

Answer 2

1. mucous production (mucous cells of crypts)

2. antimicrobial secretion (Paneth cells of crypts)

Question 3

how does the intestinal mucosa regenerate

Answer 3

Mucosa is constantly shed (3-6 days) and replaced by stem cells which migrate from base of crypts of Lieberkuhn

Question 4

where does carbohydrate breakdown and absorption occur

Answer 4

In brush border of jejenum by transmembrane hydrolases

Question 5

name the hydrolases required to breakdown common dietary carbs

Answer 5

• Lactose: cleaved by LACTASE into glucose + galactose
• Sucrose: cleaved by SUCRASE into glucose + fructose
• Starch - formed of:
  i. amylose (glucose chain with a-1,4 bonds): cleaved by AMYLASE into glucose or maltose (glucose x2, then cleaved by MALTASE)
  ii. amylopectin (glucose chain with a-1,4 and a-1,6 bonds) broken down into a-dextrins: a-1,6 bonds cleaved by ISOMALTASE

Question 6

how are monosaccharides transported from gut lumen into blood

Answer 6

Transported into enterocyte via:

• SGLT1 transporter - glucose/galactose co-transport with Na+ (requires basolateral Na/K ATPase)
• GLUT5 transporter - fructose (passive facilitated diffusion)

Transported into blood via:
- GLUT2 channel (diffuse down gradient)

Question 7

why do oral rehydratiion fluids contain Na+ and glucose?

Answer 7

Mix of Na+ and glucose stimulate max. water absorption:

• Na+ generates osmotic gradient followed by water
• glucose stimulated Na+ uptake (via SGLT1 co-transporter)

Question 8

how are proteins broken down in GI tract to allow absorption by enterocytes

Answer 8

1. In stomach (40% catabolism):
   CHIEF CELLS release pepsinogen… converted to PEPSIN by HCl… cleaves proteins to oligopeptides/amino acids… move to jejenum for absorption
2. ENTEROPEPTIDASE (released by duodenal cells) converts trypsinogen (released from pancreas) to TRYPSIN… activates other pancreatic proteases:
   • ENDOPEPTIDASES (trypsin, chymotrypsin, elastase): break bonds in middle of polypeptide
   • EXOPEPTIDASES (carboxypeptidase A and B): break bonds at end of polypeptide

Question 9

how are amino acids and di-/tri-peptides transported into enterocytes

Answer 9

amino acids: Na+/aa co-transporters

di-/tri-peptides: PepT1 (H+ co-transporter
then converted to aa by cytosolic peptidases)

Question 10

where and how is iron absorbed - why is this affected by PPIs

Answer 10

Absorbed in duodenum:

• as haem by haem transporter
• as Fe2+ by DMT1 transporter (H+ co-transporter so Fe absorption inhibited by PPIs)

Question 11

what happens to iron in the enterocyte

Answer 11

Converted to Fe3+…

• if low iron stores: Fe3+ binds transferrin and is transported to stores
• if high iron stores: Fe3+ contained in ferritin complexes and lost when enterocyte is replaced

Question 12

how is calcium absorbed when intake is high or low

Answer 12

Normal/high Ca2+ intake: passive paracellular absorption down conc. gradient

Low Ca2+ intake:

i. facilitated diffusion (via Ca channel) into enterocyte…
ii. basolateral Ca ATPase (PMCA) removes Ca2+ into blood - requires VitD and stimulated by PTH

Question 13

where and how is vitamin B12 absorbed

Answer 13

absorbed in terminal ileum bound to intrinsic factor secreted by gastric parietal cells

Question 14

why is much water present in duodenum

Answer 14

digestion of chyme creates isoactive molecules so much water secretion into duodenal lumen to render hypertonic chyme isotonic

Question 15

how is water reabsorbed further down GI tract

Answer 15

Na+ gradient needs to be created to reabsorb water: basolateral Na/K ATPase actively transports Na+ out of enterocyte to that Na+ and water can move across apical membrane into enterocyte:

• small intestine: Na+ is co-transported
• large intestine: Na+ channels induced by aldosterone

Question 16

describe the pathophysiology of Coeliac disease

Answer 16

Intolerance to gliadin fraction of gluten found in wheat, rye and barley, resulting in immune response that damages intestinal mucosa:

i. absence of intestinal villi
ii. lengthening of intestinal crypts
iii. lymphocytes infiltrate epithelium

Cause impaired digestion and malabsorption.

Question 17

describe the symptoms associated with coeliac disease and explain why these occur

Answer 17

1. diarrhoea - lack of protein and carb absorption so very osmotic gut contents
2. flatulence - bacterial fermentation of non-absorbed gut content
3. weight loss
4. sensory loss in hands - toxin malabsorption causing neurological side effects