GIT physiology 4 Flashcards
paracellular absorption
movement of solute is passive and the permeability is inverse to resistance and is dependant on the structure of tight junctions
transcellular absorption
movement of solute across two cell membranes (apical and basal) so will often have contrasting concentration gradients
transcellular absorption is driven cy
2 degree active transport
drives passive transport by setting up an electrochemical gradient
paracellular through tight junctions
TJs occlude paracellular passage of water and solutes
‘tightness’ varies and determined whether/how much paracellular route is used
colon TJs
tight
small intestine TJs
leaky
carbohydrates that are absorbed
only monosaccharides
monosaccharides
fructose
galactose
glucose
starch is made up of
glucose
broken down by saliva and pancreatic juices into subunits maltoses
maltoses are brown down by
maltase and a-dextinase
maltoses brown down into
glucose
lactose broken down into
galactose
lactose broken down by
lactase
lactose broken down in
the intestine
sucrose broken down to
fructose
sucrose broken down by
sucrase
sucrase
break sucrose down into fructose
lactase
breaks lactose down into galactose
maltase and a-dextrinase
breaks maltose down into glucose
GLUT5
transports fructose
depends on driving gradient of fructose
SGLT1
sodium dependant transport of glucose/galactose
able to scavenge
GLUT2
allows glucose and galactose to diffuse out if there is enough in the cell
disaccharides
broken down into monosaccharides
lactase deficiency
causes lactose to go straight through the digestive system without being absorbed unchanged
consequence of not absorbing lactose
osmotically active
creates osmotic drag and water will follow
bacteria anaerobically metabolise lactose creating gas
gas and water overpresent in stool
protein digestion
pepsin digests some proteins
pancreatic proteases convert most luminal orcein to oligopeptides and amino acids
peptidases liberate free amino acids
apical AA transporters
present on apical membrane
- distinct transporters for differently charged AAs
- Na co-transport
- scavenger transporter
basal AA transporters
Na-independent
pepT1
peptide transporter
requires a proton gradient
3 classes of lipids
- triglycerides
- phospholipids
- cholesterol
triglycerides
glycerol with 3 fatty acids
storage and transformation form of energy
fast deposits for insulation and protection
phospholipids
lecithins, cephalins, sphingomyelins
glycerol and 2 fatty acids
phosphorous group
cholesterol
synthesised from acetyl-CoA structural role in membranes precursor for - bile salts - steroid hormones - vitamin D
digestion of triglycerides
lingual lipase and gastric lipase (less important)
mainly pancreatic lipase
gives 2 fatty acids and monoglyceride
fat absorption
can diffuse through bilayer
also cavoelae-mediated uptake and carrier-mediated transport eg. fatty acid translocase
packed into lipoprotein complexes that carry lipids from small intestine through circulation
- chylomicrons
- exocytosed into lacteals (SI lymphatic vessels), then secreted into bloodstream
water is mostly absorbed in
small intestine
water is absorbed by
osmotic gradient
electrolytes and minerals
active or passive (requires electrochemical gradient)
used to set up the osmotic gradient required to move water across membranes
Chloride is absorbed via
passive diffusion and Na, H, Cl, HCO3 exchange
chloride is secreted via
Cl- channels
-CTFR
-CFTR
cystic fibrosis transmembrane conductive regulator
regulated by cell volume or signalling
CFTR gating
CL- diffuses out down its concentration gradient
water follows
CTFR is regulated by
cAMP
when cAMP goes up chloride goes out
bioavailability of minerals depends on
pH
redox state of the metal
dietary complexes with enhance and diminish solubility
dietary complexes which enhance solubility
organic acids - ascorbate (fruit juice), lactate (fermented milk), pyruvate/cysteine/histidine
dietary complexes which diminish absorption
phytates (in cereals), tannins (in tea), oxalates (rhubarb, greens), carbonates (Perth)
daily calcium absorption
1g of Ca ingested
mostly most bioavailable
absorbed transcellularly or paracellularly
leaky tight junctions allow calcium to pass through
TRPV6 calcium pump
calbindin binds systolic Ca2+ - prevents pree Ca2+ from blocking TRPV6
TRPM6
transports magnesium
dietary iron is eaten in 2 forms
haem iron
non-haem iron
haem iron
iron in meat, blood, muscle
bound to haem
more readily absorbed
taken up as Fe-porphyrin complex by haem transporter HT
non-haem iron
iron in vegetables
either ferrous or ferric
ferric iron is completely insoluble
divalent metal transporter (DMT)
ferric iron
completely insoluble and must be reduced first
DMT
divalent metal transporter
can only pick up ferrous iron
ferric iron must first be reduced but this is not an efficient process and most will be lost
iron inside the cell
very reactive
needs to be bound to ferritin for storage
basal iron transporter
IREG1 - ferroportin
iron the the blood
bound to transferrin for transport
Haem-Fe transporter
more efficient
enzyme used to break it apart
if cells with iron die
iron is lost from the system
hepcidin regulates plasma iron levels
when there is too much iron in the system
hepcidin is secreted
hepcidin switches off all ferroportin channels (IREG1)
iron enters cells but is all stored as ferritin which is toxic to the cell so the cell dies and the iron is lost to the system
hepcidin
regulates plasma iron levels
secreted from the liver
when ferroportin is switched off by hepcidin
iron is still take up by the cell and stored as ferritin
too much ferritin is toxic to the cell
cell dies and iron is lost
