Describes the structure of the plasma membrane, transport, modifications of plasma membrane

1. • The Plasma membrane
Dr. Gauri Haval
Abasaheb Garware College,
Pune

2. • Plasma membrane encloses the cell, defines
boundaries and maintains the essential
differences in the cytosol and external
environment.
• All biological membranes have a common
structure of thin film of lipid and protein
molecules.
• The cell membranes are fluid, dynamic
structures.

3. Danielli & Davson model:
• This model was prepared in 1935 called as
Sandwich model. It says membrane has two
lipid layers coated on its either side with
hydrated proteins therefore trilamellar
structure. Phospholipid molecules have their
non polar ends i.e. tails extending into the
centre of the membrane while hydrophilic
heads are in contact with surface proteins

4. • The Danielli and Davson model got support from electron
microscope studies. It showed that the plasma membrane
contains two dark layers (electron dense granular protein
layers) both separated by a light area. The total thickness
turned out to be 7.5nm.
• The stability of the structure is maintained by mutual
attraction between hydrocarbon chains of the lipid
molecule and electrostatic forces between the proteins
and the head of the lipid molecule. Danielli and Davson
have also predicted the lipid bilayer be about 6.0 nm in
thickness and each of protein about 1nm thick. The total
thickness of the plasma membrane comes to about
8.0nm

5. Unit Membrane
• Robertson (1960) proposed the unit
membrane hypothesis. He used the evidence
from electron microscope. This hypothesis
states that all cellular membranes have an
identical trilamellar structure- dark-light-dark.
However, the thickness of the unit membrane
has been found to be greater in plasma
membrane (10 nm) than in intracellular
membranes of E.R. and Golgi apparatus i.e. 5-
7nm.

6. Fluid Mosaic model:
• S.J. Singer and G.L. Nicholson
suggested this model which is
widely accepted.
• The plasma membrane
contains a bimolecular lipid
layer. Both the surfaces are
interrupted by protein
molecules. The proteins are
located here and there in a
mosaic pattern. Some proteins
are attached at the polar
surface of the lipid (Extrinsic
proteins) while other proteins
partially penetrate the bilayer
or span the membrane
entirely. These are called Trans
membrane proteins

7. • The extrinsic proteins or ectoproteins frequently
contain chains of sugar or oligosaccharides to
form glycoprotein. Some oligosaccharides are
also attached to lipids to form glycolipids. As
there is variety of chemicals required to form a
membrane it is called mosaic i.e. mosaic of
chemicals
• The fluid mosaic model suggests that plasma
membrane is not rigid. It has a fluid consistency.
Also there are movements of molecules in the
lipid and protein. This model represents the
fluidity of the membrane and mosaic of
chemicals hence it is called fluid mosaic model.

8. • The lipid bilayer is formed spontaneously by lipid molecules as they
are amphipathic. Lipid molecules are insoluble in water but
dissolve readily in organic solvent. The three major types of lipids
in plasma membrane are phospholipids (most abundant),
cholesterol and glycolipids. All the three are amphipathic. i.e. they
have a hydrophilic end and a hydrophobic end. In the structure
tails are hydrophobic and heads are hydrophilic. The length of the
tail varies. One tail is saturated and other one is unsaturated.
Differences in length of the tail and in saturation are important
because they influence the ability of Phospholipid molecule to
pack against one another and thereby affect the fluidity of the
membrane. Thus the fluidity of the plasma membrane depends on
its composition.

9. • Osmosis (Greek, osmos “to push”)
– Movement of water down its concentration
gradient
• Hydrostatic pressure
– Movement of water causes fluid mechanical
pressure
– Pressure gradient across a semi-permeable
membrane
Osmotic Properties of Cells

10. Erythrocyte Cell
Equilibrium
•No osmotic pressure
- cell is in an isotonic solution
- Water does not cross
membrane
•Increased [Osmotic] in cytoplasm
- cell is in an hypotonic solution
- Water enters cell, swelling
•Decreased [Osmotic] in cytoplasm
- cell is in an hypotonic solution
- Water leaves cell, shrinking

11. Cell Permeability
• Passive transport is carrier mediated
– Facilitated diffusion
– Solute molecule combines with a “carrier” or
transporter
– Electrochemical gradients determines the
direction
– Integral membrane proteins form channels

12. Crossing the Membrane
• Simple or passive diffusion
• Passive transport
– Channels or pores
• Facilitated transport
– Assisted by membrane-floating proteins
• Active transport pumps and carriers
– ATP is required
– Enzymes and reactions may be required

13. Cellular Transport
• Passive transport – no energy is
needed to move particles.
–Facilitated diffusion – embedded
proteins act as tunnels allowing
particles to “fall” through.

14. Modes of Transport

16. Channel Mediated Transport
• Proteins form aqueous pores allowing specific solutes to pass across the
membrane
• Allow much faster transport than carrier proteins

17. Coupled Transport
• Some solutes “go along for the ride” with a
carrier protien or an ionophore
Can also be a Channel
coupled transport

18. Cellular Transport
• Active transport – energy is
needed to move particles.
– Carrier proteins – embedded proteins
change shape to open and close
passages across the membrane.
– Endocytosis – taking something into
the cell.
– Exocytosis – expelling something
from the cell.

19. Active Transport
• Energy is required

20. •Against their
electrochemical
gradients
•For every 3 ATP, 3
Na+ out, 2 K+ in
Na+/K+ Pump
• Actively transport Na+ out of the cell and K+ into the cell

21. • Na+ exchange
(symport) is also
used in
epithelial cells in
the gut to drive
the absorption
of glucose from
the lumen, and
eventually into
the bloodstream
(by passive
transport)
Na+/K+ Pump

22. Endocytosis and Exocytosis
• Exocytosis
- membrane vesicle fuses with cell membrane,
releases enclosed material to extracellular
space.
• Endocytosis
- cell membrane invaginates, pinches in,
creates vesicle enclosing contents

23. Receptor Mediated Endocytosis

24. Membrane Receptors
• Converting chemical signal into
electrical one.
• These are fast acting receptors,
a typical ex. of nervous system.
Chemical signals in the form of
neurotransmitters are
transduced by ion channel ion
channel linked receptors directly
into an electrical signal in the
form of voltage difference
across the plasma membrane.
Ion Gated Channels

25. G-Protein membrane receptors
Importance of protein phosphorylation
(kinases, conformation change,
enzyme activation)

26. • This is the largest family
of cell surface receptors.
The binding of a signal to
this receptor results in
the switching ‘on’ of a G
protein on the internal
face of the membrane.
Once activated this G
protein will initiate a
process that will alter
cellular behaviour.
•

28. Second Messengers: cAMP

29. Modifications in Plasma
Membrane

30. microvilli finger-like projections of cell cytoplasm lined by cell membrane

31. cilia actively motile processes
structure:
9 pairs of microtubules
+ 1 central pair
associated proteins:
dynein
nexin
basal body of cilia:
9 triplets of microtubules

32. desmosome
transmembrane proteins: desmosomal cadherin,
desmocolin, desmoglein
proteins of plaque: plakoglobin, desmoplakin,
plakophilin, plektin
cytokeratin intermediate filaments anchored to plaque

33. hemidesmosome, a half of
desmosom
connecting net of intermediary filaments of
cell to the basal part of an extracellular
matrix
ultrastructural
dense plaque composed of intracellular
protein (desmoplakin, plektin, BP 230),
cytokeratin intermediary filaments are
anchored to that
binding to the extracellular matrix is
provided by the set of transmembrane link
proteins (α, a β, integriny)

34. Plasmodesmata
• Found in plants, but
are very similar to gap
junctions
• Extension of the
plasma membrane
• Desmotubule links
ER's of both cells and
transports
hydrophobic
molecules
• Neck region contains
proteins that regulate
permeability