—KK++ is high inside cells, Na is high inside cells, Na++ is high outside because of the is high outside because of the Na+/K+ ATPase (the sodium pump).Na+/K+ ATPase (the sodium pump).

—Energy is stored in the Energy is stored in the electrochemical gradientelectrochemical gradient::the chemical and electrical forces across the membrane the chemical and electrical forces across the membrane that arise from the asymmetric distribution of charges and that arise from the asymmetric distribution of charges and ion concentrationsion concentrations

—The intracellular pH is slightly lower than outside.—Cells take great pains to keep cytoplasmic [Ca++] very low.

12-1

Changes in membrane potentialmembrane potential are used by neurons for electrical

signalling

The actual number of ions that move is small:1/100,000th of the concentration can change the membrane potential, Vm by 100mV in a typical

cell,so ion concentrations are not measurably affected

during electrical signalling in neurons.

For ions moving down concentration gradient, ∆G<0

Chemical forces:The free energy change for one mole of ions

moving across a membrane: ∆Gconc = –RT ln Co/Ci

Electrical forces:The free energy change for charged ion

movement:∆Gvolt = zFV

At equilibrium, the chemical and electrical forces balance:

∆Gvolt + ∆Gconc = 0

(z=1)

substituting:

10mM K+ Cl–100mM K+Cl–

VeqK+ = 58mV•Log10Co/Ci = –58mV

by convention Vm = Vin – Vout

ion-selective K+ channel

K+ K+

+

++

+

–––

–

chemical forcechemical force

electrical forceelectrical forceCl–

VeqK+ = 58mV•Log10Co/Ci

10mM K+ClCl––100mM K+ClCl––

100mM Na+ClCl––

10mM Na+ClCl––

ion-selective K+ channel

ion-selective Na+ channel

VeqK+ = 58mV•Log10Co/Ci = –58mV

by itself VeqNa+ = 58mV•Log10Co/Ci = 58mV

Both channels together: Na+ in, K+ out;at steady state inward and outward currents match.What will the resting membrane potential (Vm) be?

Veq = 58mV•Log10Co/Ci

Since the concentration gradients do not change, the membrane potential can be set anywhere between –58 and +58mV simply by changing the ratio between sodium and potassium conductances, i.e., by opening and closing by opening and closing channelschannels.

conformationmay change to open and closegated channel

10mM K++ClCl––100mM K+ClCl––

100mM Na++ClCl––10mM Na+ClCl––

The resting potential depends on how fast ions flow through each channel,the relative conductance.

Chemiosmotic coupling• Energy is stored in the electrochemical gradientelectrochemical gradient:

the chemical and electrical forces across the membrane that arise the chemical and electrical forces across the membrane that arise from the asymmetric distribution of charges and ion concentrationsfrom the asymmetric distribution of charges and ion concentrations

• Mitochondria use energy from electrons (e–) to pump protons (H+) across a membrane and then use the electrochemical gradient to make ATP.

The The resting potentialresting potential for most cells is negative (–20 to –200mV) for most cells is negative (–20 to –200mV) because real cells are permeable to both Kbecause real cells are permeable to both K++ and Cl and Cl––, but have , but have low Na+ permeability. Klow Na+ permeability. K++ would flow out (conc. gradient) and would flow out (conc. gradient) and ClCl– – in (due to Vin (due to Vmm and intracellular anions), and H and intracellular anions), and H22O would flow O would flow in.in.

However, the driving force for Na+ in is high, driving Vm 0. Counteracting this is the electrogenic Na+/ K+ ATPase.

Proton-motive forceProton-motive forceElectrical forcesElectrical forces:

The free energy change for charged ion movement:∆Gvolt = zFVm

are large compared toare large compared toChemical forcesChemical forces:

The free energy change for one mole of ions moving across a membrane: ∆Gconc = RT ln Cmatrix/Ccytosol

The Nernst EquationThe Nernst Equation

Electrical forcesElectrical forces

Chemical forcesChemical forces

∆∆pHpH = pH= pHmatrixmatrix – pH – pHcytosolcytosol

pmf = Vpmf = Vmm + 2.3 + 2.3RT RT ∆pH∆pH/F/F

Proton-motive forceis used to drive ATP synthesis

ATP synthaseATP synthasecan go backwardsand hydrolyze ATP

inner membraneinner membrane

Oxidative phosphorylationATP synthase - ATP synthase -

an amazing machine!an amazing machine!100 ATPs per second100 ATPs per second

1 ATP for 3 H1 ATP for 3 H++

ATP synthase

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