Chapter 3
The Plasma Membrane and
Membrane Potential
VII edit. Pag 54-58, Pag 75-83
VI edit. Pag 53-58, Pag 73-83
V edit. Pag 57-62, Pag 87-97
Cell Structure
Cells are the basic unit of structure and
function in living organisms
© Brooks/Cole - Thomson Learning
Main cellular components:
1) Plasma membrane
2) Nucleus
3) Cytoplasm
a) Endoplasmic
reticulum
b) Golgi complex
c) Mitochondria
d) Cytosol
e) …
What is the Plasma Membrane?
-fluid lipid bilayer consisting of lipids and
proteins
-separates intracellular content from
external environment
-regulates the movement of nutrients
and substances in or out of the cell
(differential permeability)
What is composition of the Plasma
Membrane?
1) Phospholipids
2) Cholesterol
3) Proteins
4) Carbohydrates
Phospholipid Bilayer
© Brooks/Cole - Thomson Learning
Cholesterol adds fluidity to the lipid
bilayer
Plasma Membrane
(Fluid Mosaic Model)
© Brooks/Cole - Thomson Learning
Proteins in the Plasma Membrane
1) Integral proteins: ion channels, receptors,
transporters or carriers
2) Peripheral proteins: enzymes
Ion Channels
1)
2)
3)
4)
Leak channels
Voltage-activated ion channels
Ligand-gated ion channels
G-protein coupled receptors
Functions of the Plasma Membrane
1) Control the entry and exit of substances
into or out the cell
(transport & selective permeability)
2) Intercellular communication and signaling
3) Maintain a resting membrane potential
(RMP)
What is the Membrane Potential?
Separation of opposite charges (ions)
across the cell membrane:
membrane is polarized
What does the
Resting Membrane Potential
DO?
It allows nerve/muscle cells to work:
generate and transmit electrical signals
The intracellular and extracellular space of all
cells contain charged particles or ions: if
the number of negative and positive
charges is equal on both sides then the
membrane is electrically neutral
The intracellular and extracellular space of all
cells contain charged particles or ions: if
the number of negative and positive
charges is different on both sides then the
membrane is polarized
How is the Membrane Potential
generated?
Charged particles can not cross the cell
membrane easily -Why?
Ion channels allow some ions to
cross the cell membrane in a
selective manner
Notice: Some transporters or carriers also allow the selective
movement of ions across the cell membrane
When the membrane becomes permeable
to some ions but not others
a separation of charges occurs
Unequal distribution of ions across
the cell membrane generate the
Resting Membrane Potential (RMP)
Normal distribution of ions in a
neuron at rest
Ion
Extracellular
concentration
(mM / L)
K+
5
150
50-75
Na+
150
15
1
A-
0
65
0
Cl-
~100
~7
~10
Intracellular
Relative
concentration Permeability
(mM / L)
How is the Resting Membrane
Potential generated?
1) Unequal distribution of charged
particles or ions across the membrane
(electrochemical gradient)
2) Selective permeability to different ions
3) Active transport
I. How is the resting membrane potential
generated?
High concentration of negatively charged
ions inside the cell (generated by protein
groups) that can not cross the plasma
membrane
OUT
IN
II. How is the resting membrane potential
generated?
High concentration of positively charged
potassium (K+) ions inside the cell
that can cross the membrane easily
(by diffusion)
through leak potassium channels
OUT
IN
OUT
IN
Why K+ ions do
not escape
from the
cytoplasm and
do not collapse
the unequal
distribution of
K+ if they can
cross the
membrane
easily?
Potassium equilibrium potential
Nernst equation and K+ equilibrium
potential (at room temperature)
EK= 61 log Co/Ci ~ -90 mV
Co: extracellular K+ concentration
Ci: intracellular K+ concentration
Potassium equilibrium potential
OUT
IN
III. How is the resting membrane potential
generated?
High concentration of positively charged
sodium ions outside the cell
that cannot cross the membrane easily
WHY?
There are few leak sodium channels in the
membrane
Ion
Extracellular
concentration
(mM / L)
K+
5
Na+
150
15
1
A-
0
65
0
Cl-
~100
~7
~10
Intracellular
Relative
concentration Permeability
(mM / L)
150
50-75
OUT
IN
Sodium equilibrium potential
Nernst equation and the Na+ equilibrium
potential (at room temperature)
ENa= 61 log Co/Ci ~ +60 mV
Co: extracellular Na+ concentration
Ci: intracellular Na+ concentration
Resting Membrane Potential
In nerve cells the RMP is -70 mV
The greater the permeability of an ion,
the greater the contribution of the ion to
the resting membrane potential
Ion
Extracellular
concentration
(mM / L)
K+
5
Na+
150
15
1
A-
0
65
0
Cl-
~100
~7
~10
Intracellular
Relative
concentration Permeability
(mM / L)
150
50-75
Resting Membrane Potential
If the RMP is determined by the K+
permeability and the EK=-90 mV, why is the
RMP=-70 mV and not -90 mV?
OUT
IN
Do negatively charged proteins
have an equilibrium potential? Why?
OUT
IN
Depolarization and hyperpolarization
of the resting membrane potential
OUT
IN
+60
ENa
0
-70
-90
EK
© Brooks/Cole - Thomson Learning
IV. Role of active transport in the generation
and maintenance of the resting membrane
potential
Sodium-Potassium pump (Na+/K+ ATPase)
OUT
IN
Sodium-Potassium Pump
http://www.brookscole.com/chemistry_d/templates/student_resources/shared_r
esources/animations/ion_pump/ionpump.html
Sodium-Potassium Pump (Na+/K+ ATPase)
-electrogenic active transporter (?)
-pumps out 3 Na+ for every 2 K+ pumped in
-pump activity is regulated by negative
feedback
-generate ~20% of the RMP
What will happen if you inhibit
the Na+/K+ pump?
OUT
IN
Digitalis-like compounds like digitoxin and
ouabain can block the Na+/K+ pump and cause
membrane depolarization of neuronal and
heart tissue: used to stimulate heart beats
after cardiac failure
purple foxglove, digitalis purpurea.
How do chloride ions Cl- move
across the cell membrane?
Ion
Extracellular
concentration
(mM / L)
K+
5
Na+
150
15
1
A-
0
65
0
Cl-
~100
~7
~10
Intracellular
Relative
concentration Permeability
(mM / L)
150
50-75
Why is there more Cl- outside
than inside the cell?
What is the chloride equilibrium potential?
Nernst equation and the Cl- equilibrium
potential (at room temperature)
ECl= -61 log Co/Ci ~ -70 mV
Co: extracellular Cl- concentration
Ci: intracellular Cl- concentration
Will Cl- move in or out of the cell at rest?
The membrane potential determines the
movement of Cl- ions in or out of the cell
(passive distribution)
OUT
IN
In what direction Cl- ions move if…
-membrane potential = -90 mV?
-membrane potential = -50 mV?
OUT
IN
Ion
Extracellular
conc
(mM / L)
Intracellular
conc
(mM / L)
Relative
Permeability
Equilibrium
Potential
(mV)
K+
Na+
5
150
150
15
50-75
1
-90
+60
ACl-
0
~100
65
~7
0
~10
none
-70
http://www.lifesci.ucsb.edu/~mcdougal/neurobehavior/modules_homework/lect2.dcr
http://www.sumanasinc.com/webcontent/anisamples/neurobiology/signaling.html
What is the contribution of all ions to the
resting membrane potential?
Goldman equation (at room temperature):
EV= 61 log [(PNa+ Co+ PK+Co + PCl-Ci) /
(PNa+Ci+ PK+Ci + PCl-Co)]
EV ~ -70 mV
Where: P= Permeability for Na+, K+, and Cl- ions
Co: extracellular ion concentration
Ci: intracellular ion concentration
Function of the Resting Membrane
Potential?
1) Generation of electrical signals (action
potentials and synaptic potentials) in excitable
cells: neurons and muscle cells
2) Secretion of hormones and other substances
from endocrine/glandular tissue
3) Maintenance of a constant milieu for cell
function
Conclusions:
1) The resting membrane potential (RMP) is
determined by: an unequal distribution of
charged particles across the membrane (K+, Na+,
A-), differential permeability of the membrane to
ions, and the work of the Na+/K+ pump
2) The RMP is close to the K+ equilibrium potential
because K+ ions are more permeable at rest
(through leak K+ channels)
3) Cl- ions are distributed passively across the
membrane, Cl- ions made the bulk of negatively
charged particles in the extracellular space