M&R - Membrane Permeability Flashcards
What is passive transport dependent upon?
Permeability and concentration gradient
What is the relationship between the rate of passive transport and concentration gradient?
Increases linearly
How can you confer permeability to a lipid bilayer?
Insert proteins that are specific
E.g. Cl- normally exceedingly low permeability but is 10 to 7 fold higher through erythrocyte membrane because an anion transporter (band 3) is present.
What are the roles of transport processes? (6)
Maintain ionic composition
Maintain intracellular pH
Regulate cell volume
Concentration of metabolic fuels and building blocks
Extrusion of waste products of metabolism and toxic substances
Generation of ion gradients necessary for electrical excitability of nerve and muscle
What are the models of membrane transport?
Ping pong transport - protein changes conformation, binds to substrate, releases substrate on other side
Facilitated diffusion via ion channels (ligand gated or voltage gated) - protein which opens channel, selective for ion, conformational change, channel opens, substrate can run tbrought
Give two examples of ligand gated ion channels?
Nicotinic acetylcholine receptor
ATP sensitive K+ channel
Give an example of a voltage gated ion channel.
Na+ channel - Voltage sensors open when change in membrane potential detected
What type of process is facilitated diffusion?
Saturable transport process - each carrier can only interact with one of a few ions at any moment and there are only a finite number of transporters present in the membrane
What is active or passive transport dependent on? (2)
Concentration ratio
Membrane potential
Define active and passive transport
Passive transport - transport of ion/molecule can occur spontaneously
Active - Transport of molecules against an unfavourable concentration/electrical gradient. Requires energy (directly or indirectly) from ATP hydrolysis to be transported.
What are the values of free ion distribution across the cell membrane?
Na+. 145/12 mM
Cl- 123/4.2 mM
Ca2+. 1.5mM/ 10-7M
K+ 4/155 mM
Name two primary active transporter (2)
PMCA - Plasma membrane Ca2+ ATPase
SERCA - Sarcoplasmic recticulum Ca2+ ATPase
Define Uniport, Symport, Antiport
Uniport - one solute molecule transported from one side of membrane to another in a single direction
Symport - Transport of a solute molecule depends on the simultaneous transfer of a second solute in same direction. Two molecules in one direction.
Antiport - transport of two molecules in different directions
Name two secondary active transporters
NCX Na+Ca+ exchanger - removes most calcium
Mitochondrial Ca2+ uniports - removes residual calcium
What is co-transport?
More than one type of ion or molecule may be transported on a membrane transporter per reaction cycle
Describe the Na+ pump (Na+/K+ ATPase)
Found in all cells
Cruical to maintenance of gradients for Na+ and K+ across plasma membrane
Requires ATP
3 Na+ are removed from the cell
2 K+ are brought into the cell
What is Ouabin (cardiac glycoside) used for?
Congestive heart failure
Inhibits Na+ pump
Reduces Na+ gradients and therefore increases membrane potential, action firing and force of contraction
What are the two subunit proteins of the sodium pump?
Alpha subunit - does everything
Beta subunit - directs protein to plasma membrane
What types of molecules can permeate a lipid bilayer?
Hydrophobic molecules
- 02, CO2, N2, Benzene
Small uncharged polar molecules - H20, Urea, Glycerol
What is the Na+ Ca2+ exchanger?
High capacity, low affinity secondary active transport system
Removes Ca2+ from the cell by using the energy from the influx of Na+ down its electrochemical gradient and membrane potential
1 Ca2+ removed
3 Na+ in
Depolarised membranes - exchanger reverses and contributes to Ca2+ influx during cardiac action potential
Describe the Na+ H+ Exchanger (NHE).
Secondary active transport exchanger
Uses inward electrochemical gradient for Na+ to expel H+, leading to cell alkalinisation
1 Na+ in
1 H+ out
Can regulate pH and cell volume.
Important in cell growth
Describe the Na+ glucose co-transport.
Where are these co-transports found?
Entry of Na+ provides the energy for glucose to move against concentration gradient
Small intestine and kidney
Relate the transports to a medical condition
Cystic fibrosis - defective CFTR gene.
Lots of proteins working together to get Cl- into cell
Cl- not being transported out of cell
Knock on effect to other transporters
Diarrhoea
Increased function of transporters = Lots of Cl- being transported out of cell.
Water follows Cl-
Leads to diarrhoea
What is the role of the Na+ pump? (2)
Form Na+ and K+ gradients - necessary for electrical excitability
Drives secondary active transport
- control pH
- regulate cell volume
- absorption of Na+ in epithelia
- nutrient uptake
What is the problem of having too much calcium in cells?
High intracellular calcium is toxic
Cells can signal by small changes in Ca2+
What transporters control cell pH?
Acid extruders
Na+/H+ exchanger (NHE)
Na+ dependent Cl-/HCO3- exchanger (Sodium bicarbonate cotransporter)
Base Extruders
Cl-/HCO3- Anion exchanger
Which bicarbonate transporters are involved in cell volume regulation?
Na+ bicarbonate-chloride cotransporter - alkalinises cell
Anion exchanger - acidifies cell
What transporter would be used for alkali extrusion?
Anion exchanger - band 3
What do you want to do to cell during cell swelling and cell shrinking?
What ions would be involved?
Cell swelling - extrude ions
Cell shrinking - influx ions
Na+
K+
Cl-
What organ reabsorbed all bicarbonate that is filtered into the proximal tubule? and why?
Kidney - to retain base for pH buffers
What are the three areas Na+ undergoes reuptake in the kidney?
Thick ascending limb
Distal convoluted tubule
Cortical collecting duct
How is Na+ re-uptaken in the thick ascending limb?
Loop diuretics inhibit the Na+/K+ transporter
This allows more Na+ than would normally be lost is lost as less is re- uptaken
More water follows
Reduced BP
What happens to Na+ reuptake in the distal convoluted tubule?
Amiloride inhibits ENaC so Na+ can not be re uptaken
Thiazides - inhibit NCCT
Less Na+ is taken up into the cell and more lost
What happens to Na+ uptake in the cortical collecting duct?
ADH acts on aquaporin an stimulates the production to increase water retention
Spironoalactone - mineralocorticoid receptor antagonist. Used to stop over expression of aldosterone
Aldosterone increases sodium and water retention
K+ equilibrated by ROMK channels
