Membrane Permeability -- 2.1 & 2.2 Flashcards
How do small polar uncharged molecules pass though the bilayer?
down their concentration gradient by diffusion.
How do hydrophobi molecules move across the bilayer?
Hydrophobic molecules (O2 and CO2) diffuse across easily.
How do large, uncharged polar molecules pass through the bilayer?
Large, uncharged polar molecules and ions must pass through by transport proteins, again down a concentration gradient.
The smaller the membrane coefficient …
… the larger the net rate of transport
Define secondary transport
transport of a substance is linked to the concentration gradient and transport of another. Usually uses sodium gradient.
What are symports?
Give an example of a symport
two molecules move from the same side to another e.g. glucose cotransporter in small intestine
What are antiports and give an example
molecules move to opposite sides e.g. Na/Ca exchange.
What is the purpose of the Na/K/ATPase?
It is necessary for setting up the membrane potential but does not produce the membrane potential
– sets up the Na conc gradient that drive the majority of particle transport
Describe some characteristics of Na/K/ATPase
antiport, 3Na for 2K, uses ATP hydrolysis, primary AT = ATP hydrolysis, inhibited by digoxin and ouabain (cardiac glycosides)
Describe PMCA characteristics and what its purpose is
antiport, high affinity, low capacity for Ca, Ca in H out, primary AT = ATP hydrolysis, maintains low intracellular calcium
Describe SERCA characteristics and what its purpose is
antiport, high affinity low capacity for Ca, increase Ca in SR/ER in exchange for H, primary AT = ATP hydrolysis, maintains stores of Ca in SR/ER to prevent cell death
What cellular response does NCX have in ischaemia?
Low ATP levels due to low O2.
Na pump inhibited, NCX reverses to lower Ns a conc, high Ca is toxic and activates potent enzymes causing cell death
Describe the characteristics of NCX
secondary AT, 3Na in 1Ca out, current follows Na so not dependent on ATP hydrolysis, involved in ischaemia
Describe the characteristics of NBC
Na and HCO3 into cell and Cl out, follows Na gradient
Describe the characteristics of NHE
uses Na gradient to move H out of the cell, increasing the intracellular pH
How is cell volume controlled?
Efflux or influx of ions. Water will follow.
What happens in response to cell swelling?
- cells extrude ions and water follows
- - usually efflux K, Na, Cl or solute molecules
What happens in response to cell shrinkage?
- cells influx ions and water follows
- - influx K, Na, Cl
How is bicarbonate resorbed in the kidneys?
Starts as NaHCO3 in lumen of kidney.
Splits into Na+ and HCO3-
Na+ taken up by NHE (exchanges H+)
HCO3- + H+ –> H2CO3 (carbonic anhydrase)
H2CO3 –> H20 + CO2 which diffuse across
H20 + CO2 –> H2CO3 (carbonic anhydrase)
H2CO3 –> H+ + HCO3-
HCO3- moved into capillary by anion exchanger (exchange Cl-)
Na into capillary by Na pump (exchange K)
How much bicarbonate is reabsorbed by the kidneys and why is it resorbed?
All of it in order to retain a base for the buffers
Where does Na resorption take place in the kidneys?
Thick ascending limb, distal convoluted tubule and cortical collecting duct.
What is the mechanism for Na resorption in the thick ascending limb?
Na moves out of kidney lumen via NKCC2 (Na,K,2Cl)
K is filtered back into lumen via ROMK
KClCT moves K & Cl into capillaries
ClC-Kb moves Cl into capillaries
Na/K/ATPase transports 3Na into capillaries and 2K in exchange
What inhibits resorption at the thick ascending limb and what does it treat?
Loop diuretics – inhibit NKCC2
Limits Na resorption, more Na stays in lomerular filtrate reducing blood volume which treats hypertension
What is the main cotransported used for Na resorption in the distal convoluted tubule and what is its inhibitor drug?
NCC2 is inhibited by thiazides
What is the main cotransported used for Na resorption in the cortical collecting duct and what is its inhibitor drug?
ENaC and Aquaporins
Inhibited by Spironolactone
