digestion and absorption Flashcards
what are the two major tasks of the intestinal epithelial barrier?
absorption of nutrients
controls passage of pathogens/toxins
what regulates the IEB?
components of its
outer microenvironment - microflora and chyme
inner microenvironment - immune cells, fibroblasts or ENS
what specialised cytoskeletal proteins make up enterocytes?
polarity complexes
brush border
apical and lateral junctions
ECM and collagen
what is the function of polarity complexes?
orientates the enterocyte - tells cell where lumen and inside of the body is
what is the function of the bush border?
increases surface area
what is the brush border made up of?
myosin and actin
what is the function of the apical and lateral junctions of the enterocytes?
to act as a barrier
how much of nutrient absorption occurs in the small intestine?
95%
what is the surface area of the small intestine?
30-40m2
what kind of stem cells are found at the base of crypts and what do they do?
LGR5 - replicate to form daughter cells which form enterocytes and goblet cells
what is the process which removes cells from the top of the villus?
extrusion
why are cells removed from the top of the villus?
cannot survive in the harsh environment of the gut
might mutate to cope and turn cancerous
how is the small intestine structurally different to the colon?
colon is much flatter
where is water absorbed?
everywhere in the digestive tract
what does the ileum absorb?
B12
intrinsic factors
bile acids
what does the colon absorb?
electrolytes
water
what are the phases of digestion and absorption and what occurs in each?
luminal - breaks down ingested food
mucosal -brush border takes up nutrients
post-absorptive - absorbed nutrients transported to the whole body
what breaks down food in the luminal phase?
- acid in the stomach
- alkali in the small intestine
- substrate specific enzymes secreted by gastric and small bowel mucosa and pancreas
how are nutrients transported to the whole body in the post-absorptive phase?
via lymphatics and portal circulation
why is water needed in the gut?
hydrolysis reactions of digestion
facilitation of absorption
facilitation of propulsion of gut contents
Combination with mucin granules to make mucus
how do secretary cells of the crypt absorb water?
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
explain what happens to carbohydrates in the luminal phase?
split into disaccharides and limit dextrins by salivary and pancreatic enzymes
why are salivary enzymes trivial in the luminal phase of carbohydrate absorption?
destroyed quickly after being swallows and doesn’t work at the low pH of the stomach
what enzymes are involved in the mucosal phase of carbohydrate absorption?
sucrase, lactase, maltase, limit dextrine, glucoamylase
how do glucose and galactose enter epithelial cells?
via sodium-linked secondary active transport across the apical membrane
how does fructose enter epithelial cells?
facilitated diffusion
what happens to the broken down carbohydrates in the post absorptive phase?
sugars exit the cell across the basolateral membrane by facilitated diffusion into the portal vein
what happens in the luminal phase of lipid digestion?
lipids are broken down by enzymes
starts in the mouth - lingual enzymes
stomach adds gastric lipases
pancreatic enzymes help digest triglycerides
are lipids hydrophillic/phobic?
hydrophobic
are lipase hydrophilic/phobic?
hydrophillic
how do bile salts help lipase?
break down lipid globules to increase surface area upon which lipase can act
where are bile salts absorbed?
the ileum
where are bile salts secreted?
duodenum
what are lipids broken down into in the luminal phase?
monoglycerides and free fatty acids
what happens in the mucosal phase of lipid digestion?
monoglycerides and fatty acids enter enterocytes by diffusion
molecules reassembled into triglycerides inside enterocytes
packaged into chylomicrons
what happens in the post-absorptive phase of lipid digestion?
chylomicrons pass across the basolateral membrane by exocytosis into the lymphatic circulation then into the blood
explain what happens in the luminal phase of protein digestion
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
explain what happens in the mucosal phase of protein digestion
enterokinase causes trypsinogen –> trypsin
trypsin activates;
chymotrypsinogen -> chymotrypsin
procarboxypeptidase -> carboxypeptidase
amino acids enter epithelial cells
how do amino acids enter the epithelial cells during the mucosal phase of protein digestion
sodium-linked secondary active transport across the apical membrane down a sodium concentration gradient
what happens in the post-absorptive phase of protein digestion?
amino acids transported across basolateral membrane by facilitated diffusion
enter portal vein and go straight to the liver
how are vitamins obtained?
from food
body can’t make them
what are the two types of vitamins and give examples of each
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
explain the absorption pathway of B12
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
what’s another name for vitamin B12?
cobalamin
what type of bacteria is vibrio cholera?
gram negative
what are the parts of the vibrio cholera toxin?
a and b
explain how the toxins secreted by vibrio cholera lead to secretory diarrhoea
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
what is oral rehydration therapy?
solution of glucose and electrolyte - used to prevent and treat dehydration and diarrhoea
which 2 receptors in the small intestine are not affected by cAMP?
sodium-glucose transporters
amino acid-sodium transporters
explain how sodium absorption occurs in the small intestine
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
what is the cause of general malabsorption?
small intestine disease
can also be from pancreaticc disease or disease of other organs
give examples of small intestine diseases
coeliac disease
post-infectious malabsorption
Crohn’s disease
what is the most common cause of malabsorption in the economically developed world?
coeliac disease
what is coeliac disease and what causes it?
autoimmune response of the body to gluten
caused by the destruction of the villi so everything becomes flat and nutrients cannot be absorbed
in the developing world, what is the main cause of malabsorption?
acute or chronic infection caused by viral infection or chronic bacterial infection of the upper GI tract
what is specific malabsorption?
when one class or type of nutrient isn’t absorbed
give examples of specific malabsorption diseases and explain each
lactase deficiency - genetic or acquired to absorb disaccharide sugars
pernicious anaemia - failure to absorb B12
what can cause the lack of lactase in lactase deficiency?
mutation in the LCT gene
enterocytes in the brush border not producing lactase
what causes glucose-galactose malabsorption?
mutation in the SLC5A1 gene
what does the SLC5A1 gene code for?
SGLT1 symporter
what are the consequences of lactase deficiency?
Unabsorbed sugars reach colon
Add to osmotic load and cause watery diarrhoea
Fermentation of sugars in intestine causes gaseous distension (and adds to diarrhoea)
