Transcript Slide 1

CONCEPT OF NERST
POTENTIAL AND SODIUM
POTASSIUM PUMP
BY: DR.ANEEQA SHAHID
ECF and ICF
ECF and ICF
Molecular Gradients
inside
outside
(in mM)
(in mM)
Na+
K+
Mg2+
Ca2+
H+
HCO3ClSO42PO3-
14
140
0.5
10-4
(pH 7.2)
10
5-15
2
75
protein
40
142
4
1-2
1-2
(pH 7.4)
28
110
1
4
5
Types of Transport
• Passive and Active
– Passive
• Simple Diffusion
• Facilitated Diffusion
– Active
• Primary
– Co-Transport
– Counter Transport
• Secondary
• The lipid barrier and transport proteins
– Transport proteins
• Carreir
• Channel
Transport
• Passive Transport (Diffusion) (Kinetic energy
only)
– Random molecular movement of substances
molecule by molecule, either through
intermolecular spaces in membrane or in
combination with a carrier protein
• Active Transport (Kinetic + Additional Energy
for movement)
– Movement of ions or other substances
across the membrane in combination with a
carrier protein that can cause the substance
to move against an energy gradient (as
from low to higher concentration)
Basic Mechanisms of Transport
Molecular Gradients
inside
outside
(in mM)
(in mM)
Na+
K+
Mg2+
Ca2+
H+
HCO3ClSO42PO3-
14
140
0.5
10-4
(pH 7.2)
10
5-15
2
75
protein
40
142
4
1-2
1-2
(pH 7.4)
28
110
1
4
5
Simple Diffusion
inside
+
K
Na+
outside
K+
+
Na
Membrane Potential (Vm):
- charge difference across the membrane -
inside
+
K
Na+
outside
K+
+
Na
…how can passive
diffusion of potassium
and sodium lead to
development of
negative membrane
potential?
Basic Physics of Membrane Potentials
• Membrane Potential
Caused by Diffusion
– Diffusion Potential
• When equilibrium
established
– Equilibrium
potential
• Assuming freely
permeable
membrane for one
ion at a time
Simplest Case Scenario:
inside
outside
If a membrane were permeable
to only K+ then…
K+ would diffuse down its
concentration gradient until the
electrical potential across the
membrane countered diffusion.
+
K
The electrical potential that
counters net diffusion of K+ is
called the K+ equilibrium potential
(EK).
K+
K conductance
Simplest Case Scenario:
If a membrane were permeable
to only Na+ then…
Na+ would diffuse down its
concentration gradient until potential
across the membrane countered
diffusion.
The electrical potential that counters
net diffusion of Na+ is called the Na+
equilibrium potential (ENa).
inside
Na+
outside
+
Na
Na Conductance
Nernst Equation
• Relation of diffusion potential to the concentration
difference…… resulting in Nernst (equilibrium)
potential
• For any univalent ion at body temperature of 37° C
• EMF (mV)= +/-61log (Conc.inside/Conc.outside)
• Calculate for K+ and Na+
– K= -61log(140/4)
– Na= -61log(14/142)
– Sign is –ve for +ve ion and vice versa
The Potassium Nernst Potential
…also called the equilibrium potential
EK =
Ki
61 log
Ko
Example: If Ko = 5 mM and Ki = 140 mM
EK = -61 log(140/4)
EK = -61 log(35)
EK = -94 mV
So, if the membrane were permeable only to K+,
Vm would be -94 mV
The Sodium Nernst Potential
EK =
Nai
61 log
Nao
Example: If Nao = 142 mM and Nai = 14 mM
EK = -61 log(14/142)
EK = -61 log(0.1)
EK = +61 mV
So, if the membrane were permeable only to Na+,
Vm would be +61 mV
NOW
CONCENTRATE
further
Role of multiple ions
Multiple ions and diffusion potential
• 3 factors
– Polarity of each ion
– Membrane permeability of the ions
– Concentrations of respective ions on both
sides: (i= inside), (o= outside)
The Goldman-Hodgkin-Katz Equation
(also called the Goldman Field Equation)
Calculates Vm when more than one ion is involved.
+
+
+
+
p' [ K ] p' Na [ Na ]o p'Cl [Cl ]i
Vm = 61. log K + o
+
p'K [ K ]i + p'Na [ Na ]i + p'Cl [Cl ]o
or
+
+
+
+
p
'
[
K
]
p
'
[
Na
]
p
'
[
Cl
]o
K
i
Na
i
Cl
.
Vm = -61 log
+
+
p'K [ K ]o + p'Na [ Na ]o + p'Cl [Cl ]i
NOTE:
P’ = permeability
Multiple Channels
Active Transport
inside
outside
+
K
Na+
+
Na
ATP
K+
3 Na+
2 K+
ADP
Remember: sodium is
pumped out of the cell,
potassium is pumped in...
PRIMARY ACTIVE TRANSPORT
Sodium-potassium pump
It pumps sodium ions out of the cell and
potassium ions into the cell.
The pump contains a carrier protein which
is a complex of two separate globular
proteins, a larger one called the α-subunit
(mol. wt.= 100,000) and a smaller one
called the β-subunit (mol. wt.=55,000)
The smaller subunit anchors the protein
complex lipid membrane, while the larger
one has three important features:
i) It has three receptor sites for binding Na+
on the inside of the cell
ii) It has two receptor sites for K+ on the
outside of the cell.
iii) Inside of the protein near the binding site
of Na+ has ATPase activity.
Na+ K+ PUMP
When two potassium ions bind on the
outside of the carrier protein and three
sodium ions bind to the interior, the
ATPase function of the carrier protein is
activated. This cleaves one molecule of
ATP, with the liberation of high energy
phosphate bond. This energy brings a
chemical and conformational change in
the carrier protein molecule, extruding Na+
to outside and K+ to inside of the cell.
Functions of Na+/K+ pump
i) It establishes a Na+/K+ concentration
difference across the cell membrane
and makes a –ve electrical voltage
inside the cell.
ii) It regulates the cell volume by controlling
the concentration of solutes. Thus,
prevent the swelling or shrinking of the
cell.
iii) The energy stored during the transport of
Na+/K+ is used for secondary active
transport.
THANK YOU