3. passive transport and generation of membrane potentials

spontaneous processes in nature are driven by (blank- 3 words)

potential energy gradients

• process stops when potential energy decreased ΔE

ΔE = 0 means

equilibrium

driving force of diffusion?

concentration (another form of energy) or chemical potential gradient

high to low

membranes should be permeable to solute or solution?

both! different membrane permeabilities and desired end results

T or F: rate of diffusion decreases logarithmically

true! ΔEₓ = log (Cₓ^A - Cₓ^B) approach 1 bc log1=0

counter ions

ionic compounds dissolve in water dissociate into their negative and positive ions; still retain strong attractive forces to one another

• ex: if K+ is permeable and crosses membrane (cytosol → extracellular fluid) due to their concentration gradient, they are still close to M- bc of their charges (electrical forces are strong)

law of electroneutrality

ionic solutions must have equal numbers of (+) and (-) charges

what can make membrane polar and what does it gain?

a membrane with increasingly positive charges on the outside and negative on the inside makes the membrane polar (K= and M- example) and it gains electrical potential

• one each side the similar charges repel each other so additional ion accumulation is hard

  • eventually K+ is stopped from going to the outside

what are the two gradients that can occur across a membrane with Cl^- and K⁺?

• concentration gradient

• voltage gradient aka membrane potential (makes K⁺ wanna go back in)

*eventually K⁺ cannot follow concentration gradient due to repelling forces extracellularly; reach equilibrium

Nerst equation in relevant terms

equilibrium aka membrane potential is achieved when concentration gradient (K⁺ move outside cell) = electrical gradient (K⁺ move into cell)

T or F: the sign of E is indicative of the charge inside the cell membrane

true

E = (-) vs E = (+) means:

• (-): inside cell overall negative

• (+): inside cell positive

explain membrane potential

electrical charge on the membrane, measured in millivolts, “E”, has polarity and magnitude, measure with voltmeter with electrode in and out of cell

E indicative of membrane interior

nerst equation in cells

E = 60 log (C ion out/ C ion in)

for a membrane whose E_K= -80mV…

… this means membrane is negative on the inside and for a cell with K+ as the only permeable ion, K+ would have a higher intracellular concentration than extracellular (K is on the inside diffusing out)

when does Nerst equation work?

only one permeable ion; to determine membrane changes at equilibrium if concentrations of permeable ion in and out of cell are known

the four important ions in neurophysiology?

• K+ E_K= -93mV

• Na+ E_Na= +65 mV

• Ca2+ E_Ca= +120-150 mV

• Cl- E_Cl= -89mV

more permeable ion will

leave some ions on the membrane (accumulation)

more than one ion is permeable is indicated by what E?

E_m value would be between the permeability of each ion (there is a max and min)

• could be closer to one of the individual potentials; the one that is closer to E_m has a smaller driving force than the individual ion E that is farther away from E_m

T or F: if the permeability of any ion increases, the Em will move towards the equilibrium potential of that ion

true!

what is the typical E_m for a resting neuron?

-80 mV and it is called resting potential

what is happening in a resting neuron?

• permeable to Na and K

• Em = -90 to -70 mV meaning it has higher permeability for K ions (more K leaving and positive charge on outside and negative ions inside)

  • some Na and K are leaking in so diffusing down concentration gradient

• Na/K pumps:

  • active transport

  • uses cellular energy to move ions against their concentration gradients

  • returns diffusing ions to the original sides maintaining the concentration gradients of the two ions (make K that escaped out back in and Na that escaped in back out)

NOT AT EQUILIBRIUM; ongoing steady state system

Na/K pumps move K (blank) and Na (blank)

2 K in and 3 Na out

membrane potential E_m is influenced only by

permeable ions

ions move (passively or actively) down electrochemical gradients

passively

the influence of any ion on E_m is proportional to that ion’s …

… relative permeability

T or F: a large proportion of all available ions move to set up/change E_m

FALSE very small proportion

T or F: ionic concentration gradients do not change significantly with E_m changes

true

if E_Na = -70 mV

• what ion is permeable

• concentrated more inside cell or outside?

• which direction is the ion diffusing?

• what ion is permeable: K

• concentrated more inside cell or outside? inside

• which direction is the ion diffusing? outside