1. The document discusses facilitated diffusion, which is the transport of substances across a membrane with the aid of carrier proteins. It involves substances moving uphill against a concentration gradient, with the carrier protein having a fixed affinity for the substance and ATP being used to flip the orientation or change the affinity of the binding site. 2. It then covers the resting membrane potential, how cells create charge separation across the membrane by establishing ion concentration gradients and allowing diffusion through leak channels, and how this results in a membrane potential. Key ions involved are sodium, potassium, and macromolecular anions. 3. It defines graded potentials as local changes in membrane potential caused by transient opening of non-voltage gated ion channels that

1. 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 the
substance 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.

2. Lecture 4: Resting PotentialLecture 4: Resting Potential
Reading: Ch 3, section: membrane potential
Ch 4, section: electrical signals

3. 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.

4. Charge separation across a membrane
Vm = 0 mV
Vm does not equal 0

5. Charge separation across a cellular membrane

6. 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.

7. Kayak surfing

8. 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.

9. 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 charge
out of the cell per cycle

10. 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

11. ECF
ICF
Leak channels
Permit ions to flow down
concentration gradients
Na/K ATPase
Establishes and maintains
concentration gradients

12. Diffusion of K+
ions through the membrane

13. Diffusion of Na+
ions through the membrane

14. 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

15. How does the cell’s resting potential relate to EK and ENa?
EK = -90 mV; Ena= + 60 mV; Vm = -70 mV

16. Smartsite/Resources/DeBello/Animations/rest_potential_ct.swf

17. Neurons
(soma)
See also: Smartsite/Resources/DeBello/Animations/neuron_ct.swf

18. 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)

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

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

21. 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.)

22. 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 Morphology
3. Graded Potentials
- can be depolarizing or hyperpolarizing
- can vary in size
- decay with distance
- rely on non-voltage gated ion channels