STRUCTURE, MODELS AND FUNCTIONS

1. Plasma Membrane
Structure Models and functions
DR. DILIP V. HANDE
ASSOCIATE PROFESSOR, DEPT OF BOTANY
SHRI SHIVAJI SCIENCE COLLEGE,
AMRAVATI MS.

2. 1. The protoplasm of the cell is bounded
by an exceedingly thin elastic, semi-
permeable living limiting membrane
called the plasmalemma or Plasma
membrane or Cytoplasmic membrane or
cell membrane.
2. A plasma membrane forms the outer
limiting boundary of most animal cells.
In plant cell and bacterial cell the
plasma membrane lies between the cell
wall and cytoplasm.
3. Many botanists prefer the term
plasmalemma to plasma membrane. The
earlier biologists doubted its
appearance.
Plasma membrane

3. 1. Existence of plasma membrane was proved
when phenol-red dye was injected into the
cytoplasm.
2. In the experiment it was found that the dye
colored the whole of cytoplasm and did not pass
out of the cell.
3. Further when some colorless cells were placed
in the dye, only the cell wall was colored and not
the cytoplasm.
4. These observations suggested the existence of a
boundary layer between the outer non-living
cell wall and internal cytoplasm.
Plasmamembrane

4. 1. The membranes are composed mainly of proteins,
phospholipids and a small percentage (1-5%) of
oligosaccharides.
2. In general, plasma membranes contain about 60%
protein and 40% lipids by dry weight but there is wide
variation in the protein lipid ratio of different cell
membranes.
3. Oligosaccharides may be found attached to either
lipids (glycolipids) or proteins (glycoprotein).
4. Carbohydrates usually form 1-10% of the total dry
weight. .
Chemical Composition

5. .
1. Plasma membranes contain three different classes
of protein: structural proteins, enzymes and carrier
proteins.
2. The structural proteins form the mechanical
structure of the membrane.They have little
catalytic activity and are extremely hydrophilic in
nature.
3. The plasma membrane consists largely of
structural proteins.The structural proteins of
plasma membrane have been classified into two
main groups:
1. The peripheral or extrinsic proteins.
2. Intrinsic or integral proteins.
Proteins of the Plasma Membranes

6. 1.The peripheral proteins are
weakly associated with the
membrane surface and may
be separated by mild
treatments.
2.They are soluble in aqueous
solutions and are usually free
from phospholipids.
Proteins of the Plasma Membranes

7. Integral or Intrinsic proteins are associated
strongly with the membrane and represent
more than 70% of the structural proteins.
They are usually associated with the lipid
extending all the way through it and
require drastic procedures for isolation
such as treatment with hydrophobic bond
breaking agents like detergents, organic
solvents.
Proteins of the Plasma Membranes

8. Homeostasis – Maintaining a
Balance
▪ Cells must keep the proper
concentration of nutrients and
water and eliminate wastes.
▪ The plasma membrane is
selectively permeable – it will
allow some things to pass
through, while blocking other
things.

9. LAMELLAR MODELS
(I) Lipid bilayer Hypothesis
Overton (1895) suggested that the
cell membranes contain lipids. This
conclusion was based on the fact that fat
solvents dissolved the membrane easily
and fat soluble substances passed easily
through the cell membranes.

10. (II) Protein-lipid-protein Hypothesis (Sandwich
Models)
Danielli and Davson in 1935 studied
surface tension of cell membrane and on the
basis of their observation they pointed out the
existence of protein molecules adsorbed on the
surface of lipid droplets which reduce the
surface tension of droplets. This conclusion led
James Danielli and Hugh Davson in 1935 to
suggest bimolecular leaflet model of cell
membrane.

11. Unit Membrane Model.
1. This model was suggested by J. David Robertson.
In late 1950 Robertson summarized a large number of
ultra-structural data obtained by him .
2. Some other workers concluded that the plasma
membrane and the membranes of all cell organelles were
similar in structure.
3. Robertson's unit membrane model has been widely
accepted as it accounted for a number of properties of cell
membrane

13. Fluid Mosaic Model.
* The fluid mosaic model of cell membrane
was proposed in 1972 by S.J. Singer and
G.L. Nicolson.
* According to this model, the cell
membranes have been visualized as mosaics
of lipids and proteins.
* The lipids are thought to be arranged
primarily in a bilayer in which Peripheral
and integral proteins are embedded to
varying degrees.
* This suggests that membrane proteins are
not fixed within the lipid layer but are
free to move laterally like icebergs
floating in a sea of lipids.

14. 1.Singer and Nicolson considered
the lipoprotein association to
be hydrophobic and fluidity of
the membrane results due to
hydrophobion interaction.
2.It should be noted that
phospholipids and many intrinsic
proteins are amphipatic
molecules, i.e., both
hydrophilic and hydrophobic
groups occur within the sam
molecule.

15. Structure of the Plasma
Membrane
▪ Lipid bilayer – two sheets of lipids
(phospholipids).
 Found around the cell, the
nucleus, vacuoles, mitochondria,
and chloroplasts.
 Embedded with proteins and
strengthened with cholesterol
molecules.

16. What’s a Phospholipid?
▪ It’s a pair of fatty acid chains and a
phosphate group attached to a
glycerol backbone.
 Polar (water-soluble) heads face
out and the nonpolar fatty acids
hang inside.

17. Membrane Proteins
▪ 1. Determine what particles can
pass through the membrane.
▪ 2. Serve as enzymes (may speed
reactions).
▪ 3. Act as markers that are
recognized by chemicals and
molecules from the inside and the
outside of the cell (the immune
system).

20. Cellular Transport
▪ Diffusion – movement of particles from
an area of high concentration to an
area of low concentration.
 Caused by Brownian motion (movement
of particles because of the movement of
their atoms).
 Continues until an equilibrium is reached
(no gradient).
 Dynamic equilibrium – particles move
freely and are evenly distributed.

21. Osmosis
▪ Diffusion of water across a
selectively permeable membrane.
▪ Occurs until water is balanced on
both sides of the membrane.

22. Cell Concentrations
▪ Hypertonic solutions – more
dissolved solute.
▪ Hypotonic solutions – less dissolved
solute.
▪ Isotonic solutions – the same
dissolved solute.

23. Overcoming Osmosis
▪ Contractile vacuoles – expel excess
water from bacterial cells that live in
water.
▪ Turgor pressure – water pressure in
a plant cell. Loss of turgor pressure
causes wilting (plasmolysis).

24. Functions
Permeability:
▪ The plasma membrane is of great
physiological importance because of its
semi-permeable nature.
▪ It controls the cellular semi-permeability,
secretion and excretion.
▪ It regulates the cytoplasmic contents of a
cell and acts as a selective barrier between
the living cell contents and its external
environment.
▪ It permits only those substances to
pass through which are needed by the
cytoplasm and does not allow those
substances to enter which are not
required.
▪ .

25. Functions
Permeability:
▪ Plasma membrane is permeable to water.
The diffusion of ions through plasma
membrane depends upon the
concentration and electrical gradients.
▪ Plasma membrane is not only a passive
diffusion barrier but also carries
catalytically active regions.
▪ The machinery which utilizes energy for
carrying out osmotic work is also located
in or near plasma membrane.

26. Functions
▪ Cell reorganization:
▪ Certain cell recognize foreign cells like
bacteria and engulf them by phagocytosis,
the plasma membrane plays important role
in the recognition of foreign cells. The
site of cell recognition is present on the
structure of plasma membrane.
▪ Transport:
▪ The transport of different material from one cell
to other via plasma membrane takes place as it is
containing the carrier protein, this transport may
be active or passive.