During facilitated diffusion

a) substances are moved uphill – from the side of the membrane with lower concentration to the side with higher.

b) the binding site of the carrier has fixed affinity for thesubstance to be transported.

c) ATP is used to flip the orientation of the carrier.

d) ATP is used to change the affinity of the binding site.

Lecture 4: Resting PotentialLecture 4: Resting Potential

Reading: Ch 3, section: membrane potentialCh 4, section: electrical signals

Resting Membrane Potential - voltage difference across the plasma membrane, in millivolts, when the cell is at rest (i.e. no perturbing influences). Often abbreviated as Vm.

Charge separation across a membrane

Vm = 0 mV

Vm does not equal 0

Charge separation across a cellular membrane

How does the cell create charge separation?

1. Establishes and maintains concentration gradients for key ions (Na+, K+, A-).

2. Ions diffuse through the membrane down their concentration gradients.

3. Diffusion through the membrane results in charge separation, creating a membrane potential (electrical gradient).

4. Net diffusion continues until the force exerted by the electrical gradient exactly balances the force exerted by the concentration gradient.

Kayak surfing

How does the cell create charge separation?

1. Establishes and maintains concentration gradients for key ions (Na+, K+, A-).

2. Ions diffuse through the membrane down their concentration gradients.

3. Diffusion through the membrane results in charge separation, creating a membrane potential (electrical gradient).

4. Net diffusion continues until the force exerted by the electrical gradient exactly balances the force exerted by the concentration gradient.

Concentration gradient - Na+/K+ ATPase establishes the unequal distribution of Na+ and K+ ions inside and outside of the cell

ECF

ICF

Na+/K+ ATPase: pumps 3 Na+ out of the cell for every 2 K+ pumped into the cell.

Net movement of 1 positive chargeout of the cell per cycle

Concentration gradients

Macromolecular anions (A-) - high intracellular concentration of nucleic acids and proteins which carry a net negative ionic charge.

Na+ - low intracellular concentration due to the Na+/K+ ATPase

K+ - high intracellular concentration due to the Na+/K+ ATPase

Concentration (millimoles/liter)

Ion extracellular intracellular relative permeability

Na+ 150 15 1

K+ 5 150 50-75

A- 0 65 0

See also: Smartsite/Resources/DeBello/Animations/ion_concentration_ct.swf

ECF

ICF

Leak channels

Permit ions to flow down concentration gradients

Na/K ATPase

Establishes and maintains concentration gradients

Diffusion of K+ ions through the membrane

Diffusion of Na+ ions through the membrane

Nernst equation - equation describing the equilibrium potential for a particular ion (i)

Ei = RT/zF ln [i]o/[i]i = 61/z log [i]o/[i]i

where R is the gas constant, T is the temperature in degrees Kelvin, z is the valence of the ionic species, and F is the Faraday constant.

Thus, the equilibrium potential for K+ is:

EK = 61 log (5/150) = - 90 mV

the equilibrium potential for Na+ is:

ENa = 61 log (150/15) = + 60 mV

How does the cell’s resting potential relate to EK and ENa?

EK = -90 mV; Ena= + 60 mV; Vm = -70 mV

Smartsite/Resources/DeBello/Animations/rest_potential_ct.swf

Neurons

(soma)

See also: Smartsite/Resources/DeBello/Animations/neuron_ct.swf

Graded Potentials - local changes in membrane potential that decay over short distance

They result from a transient injection of current, usually the consequence of synaptic transmission and the opening of non-voltage gated ion channels (not the same as leak channels that give rise to Vm)

Graded Potentials - local changes in membrane potential that decay over short distance.

Graded Potentials – the size of the graded potential often correlates with the size of the stimulus.

Depolarization - decrease in membrane polarization to more positive values than rest. Hyperpolarization - increase in membrane polarization to more negative values than rest.

Graded Potentials (cont.)

SUMMARY - Resting Potential and Graded Potentials

1. Resting Membrane Potential- Na/K pump establish and maintain concentration gradients for Na and K- diffusion thru leak channels creates charge separation- equilibrium potential proportional to concentration gradient- resting permeability of membrane for K > Na- Vm largely determined by EK

2. Neuronal Morphology3. Graded Potentials

- can be depolarizing or hyperpolarizing- can vary in size- decay with distance- rely on non-voltage gated ion channels