sodium and potassium pump

1. M.Prasad Naidu
MSc Medical Biochemistry,
Ph.D.Research Scholar

2. MEMBRANES
Plasma membrane is an envelop surrounding the cell.
Separates & protects the cell from the external hostile
environment.
Besides being a protective barrier, PM provides a
connecting system b/n the cell & its environment.

3. Chemical composition
Membranes are composed of lipids, proteins &
carbohydrates.
Among the lipids, amphipathic lipids
( containing both hydrophilic & hydrophobic groups)
namely PL, GL and Cholesterol are found in the
animal membranes.

4. Structure of membranes
Lipid bilayer model-- Davson&Danielle(35)
Fluid mosaic model-Singer&Nicolson(1972
FM model is more recent and acceptable
Thickness---5-8nm
A membrane is essentially composed of a lipid bilayer.
The hydrophobic (non-polar) regions face each other at
the core of the bilayer while the hydrophilic (polar)
regions fact outward
Globular proteins are irregularly embedded in the lipid
bilayer.

5. Membrane proteins
Membrane proteins are categorized into two groups.
1. Extrinsic (peripheral)membrane proteins
Eg: Cyt.c of mitochondria
2. Intrinsic (integral)membrane proteins.
Eg: hormone receptors, Cyt.P450
The membrane is asymmetric due to the irregular
distributions of proteins.

6. Transport across membranes
The biological membranes are relatively
impermeable.
The membrane therefore forms a barrier for the free
passage of compounds across it.
3 distinct mechanisms have been identified for the
transport of solutes (metabolites) through the
membrane.
1. Passive diffusion
2. Facilitated diffusion
3. Active Transport

7. Passive diffusion
A simple process which depends on the
concentration gradient of a particular substance
across the membrane.
Passage of water & gases through the membrane
occurs by passive diffusion.
The process does not require energy.

8. Facilitated diffusion
This is somewhat comparable with PD, since the
solute moves along the concentration gradient
(higher to lower) and no E is needed.
FD occurs through the mediation of carrier or
transport proteins.
Specific carrier proteins for the tpt of glu, gal, leu, phe
etc have been isolated and characterized.

9. Factors affecting rate of
diffusion
1. Permeability of the cell membrane
2. Temparature
3. Conc.gr / electrical gr of the substance across the
cell membrane
4. Solubility of the substance
5. Thickness of the cell membrane
6. Size of the molecules
7. Size of the ions.
8. Charge of the ions

10. Active transport
AT occurs against a conc. gradient & this is dependent on
the supply of metabolic E (ATP)
This is also a carrier mediated process like FD.
Movement of substances against the chemical or electrical
or electrochemical gradient is called AT.
It is like swimming in the opp direction of water flow in a
river( also called uphill transport)
The E required is liberated mostly by break down of high E
compounds like ATP.

11. Active transport
AT occurs with the help of carrier proteins as in the
case of FD. (mech .is diff)
Each CP can carry only one substance or more than
one across the membrane.
Those CPs transporting only 1 substance are called
uniports/uniport pumps.
Those CPs carrying more than one substance are
called symports/ antiports

12. Mechanism of active transport
When a substance to be transported across the cell
membrane comes near the cell, it combines with the CP
of the membrane and a Sub-Pro Complex is formed.
Now this SP complex moves towards the inner surface
of the membrane.
Now the substance is released from the CPs
The same CP moves back to outer surface of the
membrane to transport another molecule of the
substance.

13. Substances transported by AT
Both ionic form and non-ionic forms
Ionic forms:- Na+
, K+
, Ca2+
, H+
, Cl-
& I-
Non-ionic forms:- Glu, AAs, Urea

14. Types of AT
1. Primary AT:- In this type of AT, the E is liberated
directly from the break down of ATP.
By this method Na+
, K+
, Ca2+
, H+
, Cl-
are transported across
the membrane
2. Secondary AT:- When Na+
is transported by a CP,
another substance is also transported by the same protein
simultaneously, either in the same direction or in the opp
direction.
This type of tpt of a sbustance with Na+
by means of a CP is
called the secondary AT.
i)CP tpts 2 diff molecules in the same direction (symport)
Ii)CP tpts 2 diff molecules in opp.direction (antiport)

15. Primary active transport of Na+
& K+
(Na+
-
K+
Pump)
Na+
& K+
are transported across the membrane by means of
common mechanism called Na+
-K+
Pump.
This tpts Na+
from inside the cell to outside and K+
from
outside into the cell.
This pump is present in all parts of the body.
Cells have a high intracellular K+
conc and a low Na+
conc.
This is essentially needed for the survival of the cells.
High cellular K+
is needed for the optimal glycolysis & for
protein biosynthesis.
Further Na+
& K+
gradients across the membranes are
needed for the transmission of nerve impulse.

16. CP of Na+
-K+
Pump
The CP involved in Na+
-K+
Pump has got 6 sites
3 receptor sites for Na+
. These are on the inner (towards
the cytoplasm) surface of the protein molecule
2 receptor sites for K+
. These are on the outer (towards
extra cellular fluid) surface of the protein molecule
1 site for the enzyme ATPase (mol.wt250000) which is
near the sites for Na.
ATPase consists of 2α & 2β subunits and represented as
(αβ)2.

17. Mechanism of action of
Na+
-K+
Pump
3 Na ions from the cell get attached to the receptor sites of
Na+
on the inner surface of the CP.
2 K ions outside the cell bind to the receptor sites of K+
located on the outer surface of the CP
The binding of Na+
& K+
to the CP immediately activates
the ATPase.
ATPase causes the break down of ATP to ADP with the
release of one high E phosphate.

18. Mechanism of action of
Na+
-K+
Pump
Now the E liberated causes some sort of conformational
change in the CP.
Bcos of this, the outer surface of the CP with K+
, now faces
the inner side of the cell.
And the inner surface of the CP with Na+
faces the outer
side of the cell.
Now dissociation and release of the ions takes place so
that the Na+
are released out side the cell and the K+
are
released inside the cell.
The exact mechanisms involved in the dissociation and
release of the ions are not yet known.

19. Electrogenic activity of Na+
-K+
Pump
Na+
-K+
Pump moves 3 Na+
outside the cell and 2 K+
inside
the cell.
Thus, when the pump works once, there is a net loss of
one +vely charged ion from the cell.
The continuous activity of these Na+
-K+
Pump causes
reduction in the no.of +vely charged ions inside the cell.
Leading to the development of –ve potential inside the
cell.
This is called electrogenic activity of Na+
-K+
Pump.