1. P L A S M A M E M B R A N
E :
S T R U C T U R E & M O D E
L S
Lecture 3
Dr. Sohail Raza
2. INTRODUCTIO
N
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Plasma membrane encloses every type of cell,
both prokaryotic and eukaryotic cells.
It physically separate the cytoplasm from the
surrounding cellular environment.
It is a ultrathin(8-10nm), elastic, living,
dynamic and selectively permeable barrier.
It control the entry and exit of molecules/ions
and generates difference in ion concentration
between the interior and exterior of the cell.
Plasma
membrane
3. HISTOR
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The term ‘cell membrane’ was coined by C. Nageli and C. Cramer in
1855.
In 1895, Charles Overton suggested that plasma membrane is
composed of lipids.
In 1900-1920, further studies on plasma membrane state that lipid must
be a phospholipid.
In 1925, E. Gorter and G. Grendel proposed that plasma membrane is
composed of phospholipid bilayer.
5. Lipi
d:-
A.
B.
C.
D.
Four major classes of lipids are-
Phospholipid (most abundant)
Sphingolipid
Glycolipid
Sterols (e.g. cholesterol)
All of them are amphipathic molecules possessing both hydrophobic
hydrophilic domains.
7. Protei
n:-
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According to their position in the plasma membrane, the proteins fall
two main categories:-
Integral or intrinsic proteins- these span the membrane.
Peripheral or extrinsic proteins- these are on one side or the other of
membrane.
9. Carbohydrates:-
They are present as short, unbranched or branched chains of sugar
attached either to exterior ectoproteins (forming glycoprotein), or to the
polar ends of phospholipids at the external surface of plasma membrane
(forming glycolipids).
10. MODELS OF PLASMA
MEMBRANE
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Sandwich model:-
It is proposed by Danielli and Dayson in 1935.
In this model a lipid bilayer was coated on its either side with proteins.
Mutual attraction between the hydrocarbon chains of the lipids and
electrostatic force between the protein and the head of lipid
were thought to maintain the stability of the membrane.
They predicted the lipid bilayer to be about 6.0 nm in thickness, and
each of the protein layer of about 1.0 nm thickness, giving a total
thickness of about 8.0nm.
11. Unit membrane
model:-
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In 1960, Robertson proposed the unit membrane model.
This hypothesis state that all cellular membrane have an identical
trilaminar structure(dark-light-dark).
12. Fluid mosaic
model:-
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In 1972, S.J. Singer and G.L. Nicolson suggested the widely accepted
fluid mosaic model.
According to this model, plasma membrane contain a biomolecular
layer, both surface of which are interrupted by protein molecules.
Proteins occur in the form of globular molecules and they are dotted
about here and there in a mosaic pattern.
13.
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Experiments on its viscosity suggest that it is of a fluid consistency
rather like the oil, and that there is a considerable sideway movement
of the lipid and protein molecules within it.
On account of its fluidity and the mosaic arrangement of protein
molecules.
This model of membrane structure is known as the fluid mosaic model.
Lateral movement
Flip-flop
movement
15. MEMBRANE
PROTEINS
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Cell – cell recognition protein:-
Carbohydrate chain of glycolipid and glycoprotein
serve as the fingerprint of the cell and identify the
type of cell(self vs foreign).
Glycoprotein on extra cellular surface serve as ID
tags which recognize individual, different species,
different type of cell.
16. Integri
n:-
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The integrins provide strength to the cell
membrane.
These integrins attached to cytoskeleton and the
extracellular matrix to maintain the cell shape
stabilize proteins.
18. Tight junction –
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It seal cells together into the sheets to provide impermeable barrier to
the cell membrane.
It prevent transfer of water and water-soluble substances through the
cell membrane.
E.g. intestinal epithelium
19. Gap junction –
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They provide direct connections between the cells.
They are some sort of channels or pores through the membranes of
cells and across the intercellular space.
E.g. epithelium cells
20. Enzymatic membrane
protein:-
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These proteins attached to integral membrane
proteins, or penetrate the peripheral regions of the
lipid bilayer.
The regulatory protein subunits of many ion
channels and transmembrane receptors, may be as
peripheral membrane proteins.
E.g. G-proteins (guanosine nucleotide-binding
proteins) and certain protein kinases.
21. Signal transduction (receptor)
proteins:-
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These receptor proteins bind hormones and other
substances on the outside of the cell membrane, that
binding process triggers a change inside the cell, called
as signal transduction.
Example: The binding of insulin to insulin receptors
causes the cell to put glucose transport proteins into
membrane.
22. Transport proteins:-
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a)
b)
The proteins are responsible for passive and active
transport.
Passive Transport Proteins-
Allow water soluble substances (small polar molecules and ions) to pass
through the membrane without any energy cost.
Active Transport Proteins-
The cell expends energy to transport water soluble substances against
their concentration gradient.
