cell membrane structure

1. “COMPOSITION AND STRUCTURE OF BIOLOGICAL
MEMBRANE”

2. SYNOPSIS
1) INTRODUCTION OF BIOLOGICAL MEMBRANE
2) INTRODUCTION AND HISTORY OF PLASMA MEMBRANE
3) ISOLATION AND ANALYSIS
4) CHEMICAL COMPOSITION
a) LIPIDS
b) PROTEINS
c) CARBOHYDRATES
5) STRUCTURE OF PLASMA MEMBRANE- MODELS
a) LIPID BILAYER MODEL
b) SANDWICH MODEL
c) UNIT MEMBRANE MODEL
d) FLUID MOSAIC MODEL
6) ROLE OF LIPID MOLECULES IN MAINTAINING FLUIDITY
7) MEMBRANE ASYMMETRY
8) FUNCTION OF PLASMA MEMBRANE
9) CONCLUSION
10) REFERENCE

3. INTRODUCTION OF BIOLOGICAL MEMBRANE
 Biological membrane or biomembrane is an enclosing of
separate amphipathic layer that acts as barrier within or around a
cell.
 It is almost invariably a lipid bilayer, composed of a double
layer of lipid molecules(usually phospholipids) and proteins
that may constitute close to 50% membrane content.

4. INTRODUCTION
Plasma membrane encloses every type of
cell, both prokaryotic and eukaryotic cells.
It physically separates the cytoplasm from
the surrounding cellular environment .
Plasma membrane is ultra thin, elastic,
living, dynamic and selective transport
barrier
It is a fluid-mosaic assembly of molecules
of lipids (phospholipids and cholesterol),
proteins and carbohydrates.

5. It controls the entry of nutrients and exit
of waste products, and generates
differences in ion concentration between
the interior and exterior of the cell.
HISTORY
Plasma membrane is also called as Cell
membrane, Cytoplasmic membrane, or
Plasma lemma.
The term cell membrane was coined
by C. Cramer in 1855 and the term
Plasma lemma has been given by J. Q.
Plowe in 1931

6. ISOLATION AND ANALYSIS
 The mammalian erythrocytes and myelin sheath of
nerve fibres, however have provided the bulk of
information regarding the structure and properties of
plasma membrane.
 E.Gorter and F.Grendel (1925) have been selected
human erythrocytes for following advantages-
 a. These cells are easy to obtain.
 b. Extremely simple.
 c. Contains no other organelles or membrane
 d. Plasma membrane is relatively tough and
does not readily fragment.
 Plasma membrane are most easily isolated from
erythrocytes subjected to haemolysis.

7. CHEMICAL COMPOSITION
– Chemically plasma membrane contains lipids, proteins, and
carbohydrates in different ratio, shown in the table.
42 54 4
Human
erythrocyte
40 52 8
MEMBRANE LIPID PROTEIN CARBOHYDRATE
Mouse liver(P.M) 52 44 4
Amoeba(P.M) 42 54 4
Human
erythrocyte(P.M)
40 52 8
Bacteria(P.M) 64 30 6
Mitochondrial
inner membrane
24 72 0

8. 1.LIPIDS
 Four major classes of lipids are commonly present in the
plasma membrane:
 Phospholipids (most abundant)
 Sphingolipids
 Glycolipids and
 Sterols (e.g., cholesterol)
 All of them are amphipathic molecules, possessing both
hydrophilic and hydrophobic domains.
1).PHOSPHOLIPIDS
 It contains a phosphate group, glycerol, choline in the
hydrophilic head, while its hydrophobic tail contains fatty
acid chain.

9. fig: structure of phospholipid

10. STURCTURE OF PHOSPHOLIPID
fig: stucture of phospholipid

11. These are of 2 types-a)Neutral phospholipids
b)Acidic phospholipids
Neutral phospholipids- They are neutral in nature
and are closely packed in bilayer lipid layer along
with cholesterol.e.g. sphingomyelin, lecithin etc.
Acidic phospholipids-They are negatively charged
and are associated with proteins by lipid-protein
interraction.e.g. sulpholipid, Phosphatidyl glycerol
etc.

12. These are class of lipids and are derived from the
aliphatic amino alcohol.
It consist of sphingosine linked to a fatty acid by
its amino group.
This molecule is a ceramide
fig: structure of sphingolipid
2)Sphingolipid

13. 3).GLYCOLIPIDS
It Contain one or more monosaccharide residues and are
based on ceramide or sphingosine
A ceramide is a sphingosine to which a fatty acid is linked
by an amide bond
fig: structure of glycolipid

14. 4.STEROLS
Sterols are steroid alcohols which contain a
cyclopentanoperhydrophenantrene.
Cholesterol is the most abundant in animal tissue, while
phytosterol are the major sterol components of plant cell
membrane.
It is smaller than other lipids of the membrane and less
amphipathic
fig: structure of sterols

15. 2.PROTEINS
2).PROTEINS
 The plasma membrane proteins fall in 2 main
categories:
 1). Intrinsic or integral proteins
 2). Extrinsic or peripheral proteins
1).INTRINSIC PROTEIN
It is also known as Integral protein.
It is a protein moleclules that is permanently
attached or firmly anchored in the plasma membrane
via its hydrophobic domains interacting with the
membrane phospholipid.
These can be easily separated by non polar solvent

16. STRUCTURE OF PROTEINS
r
2.EXTRINSIC PROTEINS
 They are also called as Peripheral protein.
 They are bound to the surface membrane by electrostatic and
hydrogen bond interaction.
 They form outer and inner layers on the lipoid layer of plasma
membrane.
 These can be separated by addition of salt, are soluble in
aqueous solution and are usually free of lipids.
fig: structure of proteins

17. On the basis of their functions, proteins of can also
be classified into 3 main types: structural proteins,
enzymes and transport protein(carriers).
 Structural proteins are extremely lipophilic and form
the main bulk i.e. back bone of the plasma
membrane.
 Enzymes of plasma membrane are either
ectoenzymes or endoenzymes and are of about 30
types.
 Transport proteins transport specific substances
across plasma membrane.

18. 3). CARBOHYDRATES
These are present only in the plasma
membrane.
They are present as short, unbranched or
branched chains of sugars(oligosacchrides)
attached either to exterior ectoproteins
(forming glycoproteins) or to the polar ends of
phospholipids at the external surface of
plasma membrane(forming glycolipids).

19. Fig: structure of carbohydrate

20.  The carbohydrates of glycolipids of the red
blood cell plasma membrane determine whether
a person’s blood type is A,B,O, or AB.
 A person having A blood group has N-
acetylgalactosamine enzyme at the end of
chain, B blood group has galactose enzyme
to the chain terminus, AB type posses both
type of enzyme, while O type lacks these
enzymes.

21. Ref : fig-4.11 Gerald karp (Blood group antigens)

22. STRUCTURE OF PLASMA MEMBRANE1. LIPID BILAYER MODEL
This model was proposed by Gorter and Grendel in
1925
Ref: fig-4.3 Gerald karp

23. 2.SANDWICH MODEL
This model was proposed by Danielli and Davson in 1935.
They suggested the presence of proteins in the plasma
membrane, in addition to the lipid.
The lipid bilayer was coated by on its either side with hydrated
proteins (globular proteins).
Ref: fig-5.2 Verma and Agarwal

24. 3. UNIT MEMBRANE
MODELIt was proposed by Robertson in 1960.
He stated that all cellular membranes have a identical trilaminar
structure( or dark-light-dark or railway track).
Ref: fig -5.4 Verma and Agarwal

25. 4. FLUID MOSAIC MODEL
It was proposed by S.J.Singer and G.L.Nicolson in 1972.
It was widely accepted of all model.
Ref: fig-4.4 Gerald karp

26. EXPERIMENTAL EVIDENCE IN SUPPORT OF FLUD MOSAIC
MODEL OF PLASMA MEMBRA
A. Evidence in support of mosaic arrangement of proteins- Freeze
structure of plasma membrane by Branton(1968) revealed the
presence of bumps and depressions(7 to 8nm in diameter)
distributed randomly.
These were later shown to be transmembrane integral protein
particals.
NE

27. B. Evidence in support of fluid property of lipid bilayer- It was
demonstrated by a classical experiment of D.Frye and
M.Edidin(1970).
They fused 2 different type of cultured cells having different
antigens.
Cell fusion is achieved by the use of some fusogen such as an
inactivated parainfluenza virus, called sendai virus.
This virus facilitates fusion of the plasma membrane and cytoplasm of
both cells to produce a hybrid cell or heterokaryon.
If the 2 cells are labelled with fluorescein(green) and rhodamine(red)
than the parts can be recognised, at the onset of fusion.
However, intermixing occurs as the antigens are dispersed and the 2
colour become less detectable.
After 40min(37ºC) the intermixing of 2 colours is complete and the 2
antigens can no longer be distinguish.

29. ROLE OF LIPID MOLECULES IN MAINTAINING FLUID
PROPERTY OF MEMBRANE
A. TYPES OF MOVEMENT OF LIPID MOLECULES: i)Flip-flop movement - It
occurs once a month for any individual lipid molecules. for e.g. smoothE.R.
ii) Lateral diffusion.
iii)Rotate- occur near the centre of the bilayer

30. B. ROLE OF UNSATURATED FATS IN INCREASING MEMBRANE
FLUIDITY:
 Double bonds in unsaturated hydrocarbon chains tend to increase
the fluidity of a phospholipid bilayer by making it more difficult to
pack together.
Thus to maintain fluidity of the membrane cells of organisms living
at low temperature have high proportions of unsaturated fatty acids
in their membranes then do cells at higher temperature.
C. ROLE OF CHOLESTEROL IN MAINTAINING FLUIDITY OF
MEMBRANES:
Eukaryotes plasma membranes are found to contain a larger
amount of cholesterol, up to one molecules for every phospholipid
molecule.
Cholesterol inhibits phase transition by preventing hydrocarbon
chain from coming together and crystallizing.

31. Cholesterol molecules orient themselves in the liquid bilayer in such a
way that their hydroxyl groups remain close to polar head groups of
the phospholipids, there rigid plate like steroid rings interact with and
partly immobilize those regions of hydrocarbon chains that are
closest to the polar head groups, leaving the rest of the chains flexible
fig: cholesterol moleclules

32. MEMBRANE ASYMMETRY
The lipids of the plasma membranes are distributed in a highly
asymmetric pattern.
Lipid digesting enzyme cannot penetrate the plasma
membranes and consequently are only able to digest liquids that
recite in the outer leaflet of the bilayer.
It follows that the liquid bilayer can be thought of as composed
of 2 or more less stable independent monolayer's having
different physical and chemical properties

33. The asymmetric distribution of phospholipid and cholesterol in the plasma membrane
of human erythrocytes.(SM-sphingomyelin,PC-phosphatidylcholine
PS-phosphatidylserine,PE-phosphatidylethanolamine,PI-phosphatidylinositol
CL-cholesterol)

34. FUNCTIONS
Compartmentalization- Because of it, specialized
activities can proceed with a minimum of outside
interference can be regulated independently of one
another.
Providing a selectively permeable barrier-
Membranes prevent the unrestricted exchange of
molecules from one side to the other.
Transporting solutes- Transport substances from one
side of membrane to another from low concentration to a
high concentration.
Energy transduction- Membranes involved in transfer
of one type of energy to another type. For e.g.
photosynthesis.

35. Responding to external signals- Plasma membrane
plays a important role in responding to external stimuli, known
as signal transduction. Membrane possess receptors that
combine with specific molecules having complementary
structure. Interaction of receptor with external ligand cause
the membrane to generate signals that stimulates or inhibits
internal activities.
Intercellular interaction- The plasma membrane mediates
the interaction ns between a cell and its neighbours to
recognize and signal one another, to adhere when
appropriate, to exchange materials and information.

36. REFFERENCE
 A TEXT BOOK OF CELL BIOLOGY- BY
P.S.VERMA AND AGARWAL.
CELL BIOLOGY BY- GERALD KARP.
CELL BIOLOGY BY- C.B.POWAR.
CELL BIOLOGY BY- COOPER
W.W.W.GOOGLE.COM (IMAGES, AND
INFORMATION)
W.W.W.WEKIPEDIA (IMAGES)
W.W.W.BING.COM (IMAGES)

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