M&R Session 2 - Membrane Permeability Flashcards
Give two examples of substances unable to pass through the plasma membrane unaided.
Glucose
Sucrose
Ions (H+, Na+, Cl-)
What are the roles of transport processes?
Maintenance of ionic composition
Maintenance of intracellular pH
Regulation of cell volume
Concentration of metabolic fuels/building blocks
Extrusion of waste
Generation of ion gradients necessary for nerves/muscles
Define (simple) passive diffusion.
Movement of substances down their concentration gradient through a channel.
Define facilitated diffusion.
Passive movement of substances down their concentration gradient with the help of a protein carrier.
Define active transport.
Movement of substances up their concentration gradient, driven by the hydrolysis of ATP.
What is the difference between primary and secondary active transport?
Primary - ATP is hydrolysed at site of membrane protein.
Secondary - ATP is hydrolysed to creat a gradient which then goes on to passively transport a different molecule out while itself re entering the cell via an antiport
What is a uniport and give an example.
A channel through which one particle is transported in one direction
eg. PMCA
What is a symport and give an example.
A channel through which two different particles are transported in one direction
e.g. Glucose-Na+ Symport (also a secondary active transporter)
What is an antiport and give an example.
A channel through which two different particles are transported in opposite directions.
e.g. Na+/K+/ATPase
What is the difference between intracellular and extracellular concentrations of Ca2+ ?
Intracellular - 50-100nM
Extracellular - ~2mM
(~20,000 fold difference)
What are the four channels involved in resting Calcium ion concentration?
Plasma Membrane Ca2+ ATPase (PMCA) - High affinity, low capacity
Sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) - High affinity, low capacity
Na+/Ca2+ Exchange (NCX) - Low affinity, high capacity (secondary active transport)
Mitochondrial Ca2+ uniports - operate at high [Ca2+]i, act as buffer.
Why does a high intracellular calcium pose a threat to the cell? Hint: what compound does it form?
High [Ca2+]i is toxic to cells.
Would form Calcium Phosphate.
Why is the NCX electrogenic?
3Na+ substitute for one Ca2+ (3+ for 2+)
In depolarisation (extracellular membrane more -ve) Na+ effluxes out of cell. This reverses in polarisation.
What happens with Na-K-ATPase and NCX in ischaemia?
In ischaemia, ATP is depleted
Na-K-ATPase stops pumping
Na accumulates intracellularly
NCX reverses to efflux Na and influx Ca.
Give two examples of molecules able to pass through the plasma membrane unaided.
O2 CO2 N2 Benzene H20 Urea Glycerol Alcohol
At what point do plasma membrane transporters involved in cellular pH regulation activate?
When cellular buffering capacity is exceeded.
Name an acid extruder and its mechanism for increasing pH.
Na/H Exchanger (NHE)
One Na+ in, one H+ out.
(Inhibited by Amiloride)
Name Base extruder and give its mechanism of action.
Anion exchanger
One Cl- in, one HCO3 out.
What is the most important transporter in maintenance of pH and why?
Na/K/ATPase
Creates Na+ gradient to provide energy for other pH regulatory transporters.
What is the advantage of using more than one transporter when regulating cell volume?
Allows movement of osmotically-active molecules while remaining electrostatically neutral.
Give an example of how a cell may move ions to resist swelling.
Efflux of Na+ and K+
H20 follows.
Give an example of how a cell resists cell shrinking.
Influx of K+, influx of Cl-
H20 follows
Give an example of a diuretic and describe its mechanism of action.
Loop Diuretics
- Block NKCC2 channels in Thick Ascending Limb
- Sodium remains in tubule
- Water remains in tubule and is excreted.
Thiazides
- Block NCCT channels in Distal Convoluted Tubule
- Sodium remains in tubule
- Water remains in tubule
Amiloride
- Blocks ENaC (epithelial Sodium channels) in collecting ducts
- Na remains in tubule
- Water remains in tubule
