Origin of membrane potentials Flashcards
Revision
In what way are the plasma membranes of all cells polarised?
The plasma membranes of all cells are polarised electrically.
What is the definition of membrane potential?
Membrane potential (Em (m is a subunit)): separation of opposite charges across the membrane.
What are the units of membrane potential?
mV (1/1000 volt)
What is the process for membrane potential?
Membrane has no potential when the charges on either side are equal.
Membrane has potential when the charges on either side are unequal.
The membrane separates the charges and on either side is the remainder of fluid electrically fluid. Separates charges responsible for potential.
Magnitude of potential: membrane B has more potential than membrane A and less potential than membrane C.
Separated charges form a layer along the plasma membrane.
Is the membrane itself charged?
NO
What does Em (m is subscript) refer to?
Em refers to the difference in charge between the thin layers of ECF and ICF located next to the inside and outside of the membrane, respectively.
What is EM due to?
differences in the concentration and permeability of key ions.
What cells have membrane potential?
All cells have membrane potential.
What do excitable cells (nerve and muscle) have the ability to do?
They have the ability to produce rapid, transient changes in their membrane potential when excited (action potentials).
What is the resting membrane potential?
Resting membrane potential is the constant in non-excitable cells, and in excitable cells at rest.
What is responsible for the electrical properties of the membrane?
Unequal distribution and their selective movement through the plasma membrane are responsible for the electrical properties of the membrane.
What ion concentration is always greater extracellularly than intracellularly?
The sodium ion concentration is always greater extracellularly than intracellularly.
Although the values will vary between cell type (and species), what are the estimated concentrations of sodium and potassium ions inside and outside the cell?
The sodium ion concentration (millimoles/litre; mM) extracellularly is 150 and intracellularly is 15 with a relative permeability of 1.
The potassium ion concentration (mllimoles/litre; mM) extracellularly is 5 and intracellularly is 150 giving a relative permeability of 100 (for a skeletal cell).
What are the concentration gradients for K+ and Na+?
The concentration gradient
- for K+ is outward
- for Na+ is inward
As K+ and Na+ are cations, the electrical gradient for both will always be toward what?
As K+ and Na+ are cations, the electrical gradient for both will always be towards the negatively charged side of the membrane.
What is the difference is permeability of the membrane at resting potential for K+ and Na+?
At resting potential the membrane is 100X more permeable to K+ then Na+.
What is the plasma membrane impermeable to?
The plasma membrane is impermeable to the large negatively charged anionic intracellular proteins (A-).
What is the effecto of the movement of K+ alone on Em?
The concentration gradient for K+ tends to move this ion out of the cell.
The outside of the cell becomes more positive as K+ ions move to the outside down their concentration gradient.
The membrane is impermeable to the large intracellular protein anion (A-). The inside of the cell becomes more negative as K+ ions move out, leaving behind A-.
The resulting electrical gradient tends to move K+ into the cell.
No further net movement of K+ occurs when the inward electrical gradient exactly counterbalances the outward concentration gradient. The membrane potential at this equilibrium point is the equilibrium potential for K+ (EK+ (K+ is subscript__ at -90mV.
what are the 2 opposing forces acting on K+?
- The concentration gradient (tending to move K+ out of the cell).
- The electrical gradient (tending to move K+ into the cell).
What happens when the 2 opposing forces acting on K+ both exactly balance each other?
When both exactly balance each other (i.e. equilibrium) no further net movement of K+ would occur.
What is the potential that would exist at this equilibrium when the 2 opposing forces acting on K+ both exactly balance each other?
The equilibrium potential for K+ (Ek (k is subscript)).
The membrane potential at Ek is -90mV.
- Important: the sign (+ or -) is the polarity of the excess charge on the inside of the membrane.
The equilibrium potential for any given ion can be calculated using the Nernst equation. What is this equation?
Eion(ion is subscript) = (RT/zF)ln((ion)o(o is subscript)/(ion)i(i is subscript))
For a monovalent ion at 37degrees the Nernst equation can be simplified to what?
Eion = 61log10((ion)o/(ion)i)
What is the equilibrium potential for Na+?
The concentration gradient for Na+ tends to move this ion into the cell.
The inside of the cell becomes more positive as Na+ ions move to the inside down their concentration gradient.
The outside becomes more negative as Na+ ions move in, leaving behind in the ECF unbalanced negatively charged ions, mostly CI-.
The resulting electrical gradient tend to move Na+ out of the cell.
No further net movement of Na+ occurs when the outward electrical gradient exactly counterbalances the inward concentration gradient. The membrane potential at this equilibrium point is the equilibrium point is the equilibrium potential for Na+ (ENa+(Na+ is subscript) at +60mV.
How does a cells permeability for a given ion, affect the drive of the membrane potential?
The greater the permeability for a given ion, the greater the tendency for that ion to drive membrane potential towards the ion’s own equilibrium potential..
Pk is 100x PNa, and
Ek = -90mV cf. ENa = +61mV.
