Absorption Processes of the Digestive Tract Flashcards
Whats absorption?
Food biomolecules broken down into their component parts
(digestion) can be absorbed across the epithelial mucosa
and into the body proper.
where does absorption occur?
stomach
small intestine
largwe intestine
describe absorption in the stomach
no food stuffs, but lipid-soluble NSAIDS and alcohol can pass through the epithelia and
into the blood stream
describr absorption in the small intestine
the site of most absorption,
water and all digested nutrients derived from
food absorbed here.
describe absorption in the large intrstine
absorbs water and salt
how many litres per day are absorbed?
9 litres per day
where do all secretions come from?
All secretions come from plasma therefore secretion closely
parallels absorption!
- 95 % of fluid is absorbed back ….
… the plasma
Only bilirubin is lost as
a waste product
slide 8
To absorb food molecules efficiently the small intestine has
3 main adaptations
To absorb food molecules efficiently the small intestine has which are
1) A large surface area
2) A thin wall (adapted epithelial cells with transporters)
3) Good blood supply
the different parts of the the small intestine
duodenum
jejunum
ileum
The small intestine is very
lon (6m long)
describe the small intestines large surface area
- the small intestine is very long - 6m long
- circular folds - inner mucosa surface is folded
- have villi
- microvilli
what is the small intestine covered with
epithelial cells on villi are covered with
villi and microvilli greatly increase
each villus hass an extremely thin
wall
- only one cell thick
- ## intestinal absorptive cells
simple columnar epithelial cells found in
the small intestines and colon
thin walls allow for
rapid absorption
each villus is uspplied with
blood vessels to receive the absorbed foods
whats abosorbed into blood capilleries
Glucose / amino acids / vitamins / minerals are absorbed into blood capillaries
Products of fat digestion absorbed into
central lacteal
sodium absorption can be both…
… passive and active
describe aodium absorption through cells
- transcellular uptake
- active process creasted by a basal membrane Na+ pump that pumps sodium out of the cells
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amino acids absorption is us
secondary active trasport -
AA absorption is accomplished by
Na+ dependant symport
siyrces if protein
ingested food
endogenous sources
what are endogenous sources of protein?
- Digestive enzymes secreted into lumen-
- Proteins within cells shed from the villi into the lumen
- Plasma proteins that leak from capillaries in
proteins are absorbed primarily as
amino acids and as a few small peptide fragments
transport proteins are selective for
different amino acidds
whats teritary active transport
proton (H+) gradient used to transport peptides against
their concentration gradient.
Proton gradient maintained by
Na+ moving into the cell down its concentration gradient and
pumping H+ out against its concentration gradient.
how is carbohydrate absorptiojn accomplished?
Secondary active transport – accomplished by Na+ - dependent symport
Carbohydrate arrives at the small intestine as
small disaccharides (maltose, lactose, sucrose)
- Carbohydrate arrives at the small intestine as small disaccharides (maltose, lactose, sucrose)
what digests these disaccharides?
The integral enzymes (disaccharidases) of the microvilli digest these disaccharides into
their component monosaccharides (glucose, galactose, fructose)
Glucose and galactose are absorbed by
secondary active transport / co-transport
Na+ and glucose or galactose are carried across
the membrane simultaneously (symport) by the SGLT transport protein.
Na+ and glucose or galactose are carried across the membrane simultaneously (symport) by the SGLT transport protein..
Is energy used here?
No energy is used here, we use the Na+ concentration gradient, which is created actively by
the Na+ / K+ pump which uses ATP hydrolysis.
Fructose enters and leaves the cell only
passively by facilitated diffusion (GLUT5 / GLUT2)
once concentrated in the cell, the monosaccharides
leave the cell down their concentration
gradients and into the villus by facilitated diffusion (GLUT2).
cholesterol and lecithin play important rolesin
fat absorption
in a micelle, the bile salts and lecithin aggregate in
small clusters with the fat-soluble parts in the middle to form a hydrophobic core and a hydrophilic surface.
Micelles are thus water soluble, but they…
… dissolve water insoluble substances in their hydrophobic cores
In addition cholesterol (water insoluble) dissolves in
the micelles core
portions of a phopholipid ?
Phospholipid: has both a lipid-soluble and water soluble component
Free Fatty Acids (FFA) can diffuse through
the plasma membrane
Monoglycerides and FFA can be
transported and enter the cells where they move to the endoplasmic reticulum
in th eER they are
re-synthesised into triglycerides
triglycerides coalesce into
droplets are are coated with a layer of lipoprotein
what are the different lipids?
hdl
ldl
idl
vldl
chylomicrons
what do chylomicrons pass through
the secretory pathway of the cell.
what are chylomicrons transported in
speciliased veciles to the golgi apparatus for further processing
what further processing do chylomicrons undergo in the golgi apparatus
protein coating - and are eventually secreted into the interstitial fluid via exocytosis
after chylomicroms are excreted from golgi into interstitial fluid, what then?
here the pass into the central lacteal of the
villus
chylomicrons do not
enter the blood capillaries
why dont chylomicrons enter the blood cappilleries?
Chylomicrons do not enter the blood capillaries as the capillaries possess a
basement membrane that prevents chylomicrons from entering. The lacteal does
not have such a membrane
the amount of caclium absorbed in
regulated
calcium enters the cells down ints
concentration gradient through special channels (TRPV5/6) where it is bound by calbindin
calcium absorbtion is
passive transport
calbindin mediates the
transport of calcium across the enterocytes from the apical side to the basolateral site
calcium exits the cell by two active processes:
- Primary active transport Ca 2+– ATPase pump called PMCA1b
- Secondary active transport: Na+ – Ca2+ anti-porter called NCX1
vitamin D is
active calcitriol
what does vitamin D regulate?
regulates the amount of calcium by increasing the expression of the genes that encode the above mentioned proteins for transport of Ca2+ into / out of the cells and also by increasing production of calbindin.
calbindin action is a
rate limited process
why is calbindin action a rate limited process?
The presence of calbindin increases the amount ofcalcium crossing the cell but does not raise
the free concentration
iron absorption essential for
haemoglobin production
how much iron ingested inuk diet
5-20 mg per day ingested
how much iron absorbed per day
only 0.5 to 1 mg per day men
1-1.5 mg per day for women
haem iron is a
animal protein
non haem iron (aka elemental iron) are found in
plant based foods (less efficienct absorbtion than haem iron)
iron is absorbed into
the small intestine
iron is actively sborbed into
epithelial cells as either haem or elemental iron (non-haem)
the two types of iron
haem or non haem (elemental)
haem iron is absorbed more efficiently via the
haem carrier protein 1 - active transport)
Dietary iron is
oxidised Fe3+ form, but the divalent Fe2+ is more easily absorbed. Dietary Fe3+ reduced to Fe2+ by membrane-bound Dcytb protein.
- Transported by divalent metal transporter (DMT1).
Fe within haem is converted to
Fe2+ by haem oxygenase and forms an Fe2+ iron pool wihth other Fe2+ from other sources
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