Epithelial Transport Flashcards
What is important about the ENDOthelial cells of the lungs?
They do not contain membrane transporters=>
- O2 and CO2 diffuse across between air and blood
There are no O2 or CO2 membrane transporters
What does the apical surface of an epithelial cell face?
AKA: mucosal, lumenal
“special fluid”
- food in gut
- saliva in parotid duct
- urine in kidney
Contains specialize transporters that endow epithelium with its special properties
What does the basolateral surface of the epithelium face?
AKA: serosal, peritubular
interstitial fluid
Generic transport properties like plasma membranes of non-epithelial cells
What are tight junctions?
Cells in the epithelia sheet that are glued to one another
When are tight junctions tight, and when are they leaky?
Tight junctions are tight in “fine tuning” or finishing work
tight junctions are loose when the epithelia is involved in massive transport of substances
What are the 2 possible routes across epithelia?
- Cross 2 membranes -> enter epithelial cell on one side and leave on the other
- Cross no membraned -> pass between cells in pericellular shunt pathway
Where in epithelial cells is the Na/K pump located?
In the basolateral membrane
What is the benefit of using the Na/K pump to keep intracellular [Na] low?
provides energy to host of secondary transporters
What is the membrane potential on the basolateral side of an epithelial cell?
-70mV
What is the membrane potential on the apical side of an epithelial cell?
+10mV
How are NaCl and water absorbed/ transported by epithelial cells?
Na ions leak into apical membrane (down electrochemical gradient) -> pumped out basolateral side by Na/K pump
=> net transport of + charge so Cl- follows (electrical force)
=> Net transport of NaCl = osmotic gradient so water is drawn along as well
What are the rules for understanding transmembrane potentials?
- all membrane potentials are written as potential of the INSIDE with respect to the OUTSIDE
- the transepithelial potential is written as the potential of the apical solution with respect to the basolateral
- The cell is iosopotential - all voltage drops are at membranes
What are 2 important differences between leaky and tight epithelium?
Leaky allow for the passage of a larger amount of material
leaky “short out” transepithelial voltage => lower transepithelial voltage can be expected
What is the normal position of the Cl- channel at rest?
Closed
What happens when this channel is open?
When Cl Channel open it secretes electolytes-> driven by Cl- leaking out (away from high ICF [ ]-> electronegativity draws Na + with it (often intracellular shunt)-> osmotic gradient draws Water with them ->
=> net secretion of NaCl solution
When is the Cl- channel activated?
during digestion
by pathogens:
Cholera
On which side of epithelial cells is the Cl channel located?
apical
How are glucose and amino acids absorbed by epithelial cells and transported into the blood?
pumped across apical membrane -> passively move out of cell into interstitial fluid
-sodium dependent secondary active transport
not the same as facilitated diffusion of glucose in muscle cells
Why//how is Na available to power the secondary active transport of amino acids and sugars?
Na leaks across the leaky epithelium of the GI tract into the lumen of the gut
How many moles of metabolic wast solute are produced each day?
15mol
How is the majority of the waste excreted (most volatile 14.5mol)?
Carbon metabolism
- exhaled with CO2
How are the remainder of the non-volatile wastes excreted?
by the kidney as urea
- includes 50mmol of H+
How does the kidney remove waste?
“I KNOW WHAT I LIKE”
- forms ultrafiltrate of water, sugars, salts, plasma and all other beneficial solutes + all the non-volatile wastes products
- As filtrate passed through convoluted tubules of kidney -> renal tubules reabsorb what they want to keep
=> expensive in terms of ATP!
How much non-volatile waste is excreted daily by the GI tract?
30mmol
- mostly RBCs
To what point can human urine be concentrated?
1200mosM
