small bowel and large intestine Flashcards
digestive epithelium: explain how the digestive epithelium is specialised for its role in absorption, and describe the source and migration route of newly formed gut epithelia
cells of small intestinal mucosa
lining mucosa: enterocytes, goblet cells, enteroendocrine cells; in crypts: paneth cells, stem cells
what is the most abundant gut cell
enterocytes
shape of enterocytes
tall columnar, with microvilli on apical surface (brush border)
where is the nucleus, mitochondria and rough ER located in enterocytes
nucleus and mitochondria towards basolateral membrane, rough ER towards apical region
what are enterocytes connected to each other by
tight junctions
lifespan of enterocyte
1-6 days
how many microvilli per enterocyte
2000, about 1um in length
in enterocytes, what is the glycocalyx and where is it located
rich network of carbohydrates in apical membrane covering enterocytes
2 functions of glycocalyx in enterocytes
trap layer of water, mucus and enzymes on surface of cell to protect it from luminal contents, especially enzymes which may damage cell/membrane; regulate digestion and absorption
what is the glycocalyx layer referred to as
unstirred layer
second most abundant cell in gut
goblet cells
where is the nucleus, rough ER , Golgi appartus and mucin granules located in goblet cells
nucleus and rough ER towards basolateral membrane, Gogli apparatus in middle, mucin granules towards apical region
what distorts goblet cell shape
mucin granules
contents and function of mucous, and subsequent abundance of goblet cells along intestine
water and glycoproteins, acting as lubricant to facilitate gut passage (useful as water absorbed, so number of goblet cells increase along length of intestine)
what are enteroendocrine cells and where are they
hormone secreting epithelial cells, found in bottom of crypts
shape of enteroendocrine cells
columnar
where are sensory apparatus located in enteroendocrine cells
apical portion
where are collections of manufactured hormones kept in enteroendocrine cells
near basolateral membrane ready to secrete into local blood supply
4 examples of enteroendocrine cells and what they secrete
G-cells secrete gastrin, I-cells secrete cholecystokinin (CCK), S-cells secrete secretin, D-cells secrete somatostatin
what are closed enteroendocrine cells
ones which are not exposed to gut lumen at all
what are paneth cells and where are they located
immunological cells located at bottom of crypts near stem cells
what do paneth cells contain
acidophilic granules
what 3 things do acidophilic granules contain
lysozyme (antibacterial enzyme), glycoproteins (protect local cells from enzymes), zins (cofactor for lysozymes)
why are paneth cells located near stem cells
help protect progenitor cells rather than other cells
what do paneth cells do on top of lysozyme granules
engulf bacteria, regulate intestinal flora
life span of enterocytes and goblet cells
36 hours
significance of stem cell nucleus
large, near basolateral membrane; show some form of memory so can’t replace all cells (e.g. can’t go into colon)
journey of energy-intensive pluripotent stem cell (continuous escalator)
proliferate in crypts → move up sides towards villus tip → become senescent → naturally slough off → broken down and reabsorbed
what does continuous escalator allow
effects of gut-borne toxins/drugs to run course, lesions will be short-lived and repaired quickly
what impairs stem cell activity and hence cause GI dysfunction
radiation therapy/exposure
where does first stage of carbohydrate digestion occur
mouth via salivary a-amylase, which is destroyed in stomach
where does sugar digestion resume and via what
duodenum via pancreatic amylase (requires Cl- and alkaline conditions) and brush border enzymes of enterocytes
what are complex carbohydrates
long polymers in basic chains or branched chains
what are simple carbohydrates
disaccharides and monosaccharides
goal of digestion
break the long polymers into simple monomers that can be absorbed by the gut wall
3 different disaccharides produced by pancreatic amylase and enzymes used
sucrose, maltose, lactose (sucrase, maltase, lactase)
what is maltose converted into
2 glucose monomers
what is sucrose converted into
1 glucose, 1 fructose
what is lactose converted into
1 glucose, 1 galactose
what transporter are glucose and galactose absorbed through
SGLT-1 transporter (sodium glucose linked transporter 1) in apical surface by secondary active transport (as Na+ allowed into cell, energy released as flows down concentration gradient used to pump glucose or galactose into cell in exchange for Na+)
what transporter does fructose undergo facilitated diffusion through
GLUT-5 (glucose transporter 5)
what channel do all 3 monosaccharides diffuse through in baseolateral membrane
GLUT-2 channels
what ensures sodium gradient
Na+/K+ ATPase pump
where are ingested proteins broken down in and by what
stomach, by pepsin into smaller peptides
why does pepsin stop working effectively into duodenum
relies on acidic environment to function
where does digestion of proteins continue from stomach
duodenal lumen
what breaks down smaller peptides in duodenum and what do they become
protease-rich pancreatic juice, into tripeptides and dipeptides
examples of pancreatic enzymes for protein breakdown
trypsin, chymotrypsin, carboxypeptidase
where does final stage of digestion of proteins occur and where are the enzymes secreted from
brush border by enzymes secreted from epithelial cells
what are the typical enzymes
tripeptidases and dipeptidases
specific enzymes to break down tripeptides and dipeptides
endopeptidase, dipeptidase, aminopolypeptidase, carboxypeptidase
what transporters does single amino acid absoprtion into enerocytes occur through
secondary active transporters such as AA/Na+ symporter
what transporters does dipeptide and tripeptide absoprtion into enerocytes occur through
secondary active transporters such as AA/H+ symporters
what breaks down dipeptides and tripeptides into amino acids
cytoplasmic peptidase enzymes in enterocytes
how do amino acids move across basolateral transporter
facilitated diffusion
first stage of fat digestion
in mouth where lingual lipase hydrolyses triglycerides
what continues fat digestion in stomach
ingested lingual lipase, secreted gastric lipase
what does gastric lipase do
cleaves single fatty acid chains from free triglycerides
what increases fat surface area
mechanical churning of the stomach (slightly emulsifies)
what state are fats in when leaving the stomach
largely intact
what does bile do to fats in the duodenum
provides chemical emulsification with huge increases in SA by creating small fat droplets
what is secreted simultaneously with bile
pancreatic juices
function of pancreatic juices
luminal digestion: cleave 2 fatty acid chains from triglycerides to form monoglycerides and free fatty acids (by pancreatic colipase which is activated later - prevents bile salts from displacing lipase from fat droplet)
outcome of luminal digestion products
reach brush border where combine with bile salts to form micelles which cross aqueous unstirred layer (still fat so must form mixed micelles to get to wall of gut in aqueous solution to increase rate of absorption); absorption of micelle is much quicker than emulsion
fate of lipolytic products after passing through brush border
diffuse through apical membrane in jejunum while bile salts remain in lumen (travel through small intestine and are reabsorbed in terminal ileum)
fate of free fatty acids and monoglycerides in cell
resynthesised into triglycerides
primary mechanism of triglyceride synthesis
monoglyceride acylation pathway (fatty acids bind to apical membrane, FA binding proteins facilitate transfer of fatty acids from apical membrane to smooth ER, where fatty acids esterified into di/tri glycerides)
secondary mechanism of triglyceride synthesis
phosphatidic pathway (triglycerides synthesised from CoA fatty acid and a-glycerophosphate)
outcome of resynthesised triglycerides in enterocytes as emulsion
packaged with proteins, phospholipids, cholesterols into chylomicron lipoprotein (80-90% triglycerides, 8-9% phospholipids, 2% cholesterol, 2% protein, trace carbohydrates)
fate of chylomicron
sent to Golgi, where secreted across basement membrane by exocytosis and enter lymphatics via villi’s lacteal, as too large to enter capillaries
reason for rapid turnover of cells
first line of defence vs GI pathogens (may be directly affected by toxic substances), effects of agents which interfere with cell function, metabolic rate diminished, lesions will be short-lived, if escalator-like transit of enterocytes interrupted (e.g. radiation), severe intestinal dysfunction
effect of cholera on enterocytes, affecting gut and response
releases enterotoxin (affects opening of Cl- channel in enterocytes); more open so lose water as can’t retain; treatment is rehydration with salts also as diarrhoea clears bacteria
4 stages of fat digestion
secretion of bile and lipases, emulsification, enzymatic hydrolysis of ester linkages, solubilization of lipolytic products in bile salt micelles
pancreatic cholesterol esterase function
hydrolyses cholesterol ester to free cholesterol and fatty acid
structure of bile salt
flat, amphipathic, hydrophobic face (nucleus and methyl) dissolves fat, hydrophilic face (hydroxyl and carboxyl) dissolves in water
