The document discusses cell membranes and transport across membranes. It notes that plasma membranes are composed of lipids, proteins, and carbohydrates arranged in a fluid mosaic structure. Transport across membranes can occur through passive diffusion, facilitated diffusion, or active transport. Active transport uses carrier proteins and ATP hydrolysis to move substances against their concentration gradient. A key example is the sodium-potassium pump, which transports 3 sodium ions out of the cell in exchange for 2 potassium ions into the cell, contributing to the cell's membrane potential.

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.