5. Ionic basis of the membrane potential Flashcards
How does glucose move across membranes? 4
- glucose is polar but uncharged
- movement across the membrane is influenced by con. gradient but not membrane potential
- glucose is too big to move straight through the lipid bilayer
- requires a mechanism to move by diffusion
Describe ion gradients across cells. 6
- unequal distribution of ions across membrane
- ca2+ has the highest conc. gradient of any ion
- there are a lot more negatively charged ions outside the cell but inside there are lots of proteins with fixed nagative charges that don’t leave the cell
- ancestral organisms used energy with phosphate. calcium phosphate is insoluble so too must ca2+ in cell would ppt out all phosphate
- there are no ion channels permeable to mg2+. it has a huge charge density - small with 2 positive charges, doesnt give up hydration charge
- cell interior is -70mv, negative with respect to outside
Why is ion movement of K+ and cl- unpredictable? 2
- They have chemical gradients that oppose electrical gradients
- energy may favour or disfavour movement
What energy/work is needed due to electrical gradient? 6
- if the potential is 1V, it takes just 1J of work to move 1 coulomb of charge
- there are F coulombs of charge in 1 mole of univalent ions so to move 1 mole of z-valent ions through a membrane potential of Vm volts takes:
Work (J) = z.F.Vm - coulomb is a measure of charge
- F=faraday constant
- vm = voltage
- the equation gives us the energy needed to move an amount of ions in joules
What energy/work is needed to due to conc. gradients? 4
1. to move 1 mole of substance from a conc, Ci to Co, Work (J) = R.T.ln(Ci/Co) 2. R = universal gas constant, 8.314 3. T = temp in K 4. If Ci = Co, no work b.c ln1=0
What is the total work done when moving across conc. gradient? 4
- work (J) = (Z.F.Vm)+ [R.T.ln(Ci/Co)]
- if work is > 0, energy is needed to move an ion across the membrane, needs active transport
- if work is
What is the nernst equation?
If W=0, (Z.F.Vm)+[R.T.ln(Ci/Co)] = 0 Z.F.Vm = -RTln(ci/co) ln(1/a)=-ln(a) -ln(ci/co)=ln(co/ci) Z.F.Vm = RTln (co/ci) Vm= (RT/ZF)ln(co/ci)
Z can be -1, 1 or 0
can covert to body temperature to log10 and refer to body temperature but ONLY if at body temperature so
Vm = (61.5/Z) log (co/ci)
Calculate the equilibrium potential, Ek, when Z=1, Ko = 5mM and Ki= 100mM at body temperature.
Ek = (61.5/Z)log10(ko/ki)
Ek = 61.5log(5/100)
= -80mV
When resting potential is the same as the value defined by the nernst equation, ions are at equilibrium and no tendency for them to move
Can be positive or negative
Summarise the nernst equation. 3
- nernst equation gives the equilibrium potential for a particular ion
- this is the voltage at which the membrane potential balances the conc. gradient.
- at the reversal potential there is no net movement of the ion
Why do a few ions flow at resting membrane potential of -70mV? 1
- for K+, Ek is -80mV
What is the relationship between Na+ and resting membrane potential? 6
- with negative RMP, na+ will enter cell down both electrical and conc gradients
- na+ would only be at equilibrium at na+ equilibrium potential
- Ena = 61.5log(150/50)
= 61.5mV - slight permeability to na+ means RMP is more positive than Ek
- Permeability to K is much higher so RMP is much closer to Ek than Ena
- If permeability to na increases, RMP will become more positive, so neither ion is really at equilibrium
What is the importance of the sodium-potassium pump? 3
- finite permeability to na+ and K+ means the resting potential is not as equilibrium
- it is matched net fluxes of Na+ and K+
- These are restored by active transport via sodium-potassium pump, which uses atp
What are capacitors? 6
- a device for storing energy via separation of electrical charge
- charges stored on two plates separated by an insulator
- membranes act as capacitors
- hardly any ion movement is required to charge the capacitor and set up the membrane potential
- therefore, there is almost no change in ion conc., 0.006% of K+ ions leave
- This has an important consequences
