Prepared by Dr Thirunahari Ugandhar

1. PLASMA MEMBRANES STRUCTURE
AND FUNCTIONS
Presented by
Dr. Thirunahari Ugandhar
Head & Assistant Professor of Botany

2. What are membranes?
Membranes cover the surface of every cell, and
also surround most organelles within cells. They
have a number of
functions, such as:
 keeping all cellular components
inside the cell
 allowing selected molecules to move in and out of the cell
 isolating organelles from the rest of the cytoplasm, allowing cellular
processes to occur separately.
 a site for biochemical reactions
 allowing a cell to change shape.

3. The Plasma Membrane
A plasma membrane is
common to all cells.It forms
their outer limit. It forms a
boundary for dissolved
substances-allows exchange.
Allows cells to maintain
themselves
Bacteria, fungi, and plant cells
have a cell wall, but it is a
structurally distinct feature and
lies outside the
plasma membrane.
This colored Bacillus
megaterium cell clearly shows
the plasma membrane, which
lies inside the distinct structure
of the cell wall.
Plasma
membrane
Cell wall

4. Cells and Membranes
The membrane surrounding a cell, called
the plasma membrane, forms the
boundary that separates the living cell
from its non-living surroundings.
Although the plasma membrane
(arrowed) is only about 8 nm (0.01
micrometere) thick, it:
selectively controls the movement of
materials into and out of the cell
(selectively permeable)
is responsible for cell-cell
recognition (e.g. when cells
aggregate into tissues
is a dynamic structure, with distinct
inside and outside faces.
Animal cell
Plant cell

5. Membrane Structure
The currently accepted model for the structure of
the plasma membrane (and cellular membranes
generally) is the fluid mosaic model.
In this model there is a double layer of
phospholipids (fats),
which are arranged with their hydrophobic tails
facing inwards.(repel water)
The hydrophilic head (phosphate) is attracted to
water-both inside and outside cell-cell is in a watery
environment
The double layer of lipids is quite fluid, with
proteins floating within it.
Glycoproteins, glycolipids, and cholesterol are also
an integral part of the membrane structure.
Double layer of
phospholipids (lipid
bilayer)
} Hydrophilic end
Hydrophobic end

6. Membrane Structure
Some proteins, called
peripheral proteins,
are stuck to the surface
of the membrane.
Glycolipids act as
surface receptors and
stabilize the membrane.
Common in brain cells
and nerves.(CHO and
lipid)
Some proteins completely penetrate
the phospholipid layer. These proteins
may control the movement of specific
molecules into and out of the cell.
Glycoproteins play an
important role in cellular
recognition and immune
responses. They help stabilize
the membrane
structure(adhesion between
cells)
Carbohydrates-
found on outer layer-
linked to protruding-
protein(glycoprotein(

7. Membrane Structure
Cholesterol in the membrane
disturbs the close packing of
the phospholipids and keeps
the membrane more fluid.
Provides rigidity and water
resistance. Membranes would
break down without it. Plants
have phytosterol.
Some substances, particularly
ions and carbohydrates, are
transported across the
membrane via the proteins.
Some substances,
including water, are
transported directly through
the phospholipid bilayer.
But mostly impermeable
to water soluble(polar)
molecules-most
movement via proteins.
Glycolipids also have a role
in helping cells to aggregate
in the formation of tissues.
Glycoproteins-cell recognition-
between
cells,antibodies,hormones and
viruses

8. Fluid –Mosaic model
• Fluid- individual phospholipids and some
proteins can move sideways(laterally) in each
layer-therefore FLUID
• Mosaic-range of different proteins resting on
the surface or through the phospholipid layer
gives it a mosaic appearance

9. Located:Plasma Membrane
Surrounds the cell
forming a boundary
between the cell
contents
and the extracellular
environment.
Structure:
Semi-fluid
phospholipid bilayer
in which proteins are
embedded. Some of
the proteins fully span
the membrane.
Function:
Forms the boundary
between the cell and
the extracellular
environment.
Phospholipid
bilayer
The plasma membranes of two adjacent
cells joined with desmosomes
Protein

10. PLASMA MEMBRANE

12. THREE CLASSES OF MEMBRANE LIPIDS

16. • Plasma Membrane
• A lipid/protein/carbohydrate complex, providing a
barrier and containing transport and signaling
systems.

17. Fluid Mosaic Model of the Plasma
Membrane
Carbohydrate
chain
Glycoprotein
Intrinsic
Protein
Phospholipids
Non-polar hydrophobic
fatty acid

18. Membranes: timeline of discovery

19. 2nd cell membrane
1st cell
membrane
intracellular space (blue)
This was taken to be the
phospholipid bilayer (light)
surrounded by two layers of
protein (dark).
1 light layer = phospholipid
bilayer
2 dark layers: protein
Evidence for the Davson–Danielli model
When clear electron micrographs of membranes became available, they appeared
to show support for Davson–Danielli’s model, showing a three-layered structure.

20. Later, it was discovered that the light layer represented the phospholipid tails and
2nd cell membrane
1st cell
membrane
intracellular space (blue)
1 light layer = phospholipid
tails
2 dark layers:
phospholipid heads
Evidence for the Davson–Danielli
the dark layers represented the m
phos
o
phd
olip
e
idlheads.

21. By the end of the 1960s, new evidence cast doubts on the viability of the Davson–
Danielli model.
 The amount and type of membrane
proteins vary greatly between different
cells.
 It was unclear how the proteins in the
model would permit the membrane to
change shape without bonds being
broken.
 Membrane proteins are largely hydrophobic and therefore should not be found
where the model positioned them: in the aqueous cytoplasm and extracellular
environment.
Problems with the Davson–
Danielli model

22. Evidence from freeze-fracturing
E-face: looking up
at outer layer of
membrane
This revealed a smooth
surface with small
bumps sticking out.
These were later
identified as proteins.
In 1966, biologist Daniel Branton used freeze-fracturing to split cell membranes
between the two lipid layers, revealing a 3D view of the surface texture.
P-face: looking
down on inner layer
of membrane

23. The fluid mosaic model
This model suggested that proteins are found within, not outside, the
phospholipid bilayer.
The freeze-fracture images of cell membranes were further evidence against the
Davson–Danielli model.
E-face
They led to the
development of the fluid
mosaic model,
proposed by Jonathan
Singer and Garth
Nicholson in 1972.
P-face protein

24. Side view
Surface view
Biochemical Composition of the Plasma
Membrane

25. Side view
Biochemical Composition of the Plasma
Membrane
The main components are protein and phospholipid:
Protein
Phospholipid

26. Membrane Structure and
Function

27. Membrane Function
• Membranes organize the chemical activities
of cells.
• The outer plasma membrane
– forms a boundary between a living cell and its
surroundings
– Exhibits selective permeability
• Controls traffic of molecules in and out

28. Membrane Function
• Internal membranes provide structural order
for metabolism
• Form the cell's organelles
• Compartmentalize chemical reactions

29. Fluid Mosaic Model of the PM
• A membrane is a mosaic
– Proteins and other molecules are embedded in a
framework of phospholipids
• A membrane is fluid
– Most protein and phospholipid molecules can
move laterally

30. Membrane Structure
Phospholipids are
the major structural
component of
membranes.

31. Membrane Structure
All membranes are phospholipid bilayers with
embedded proteins.
Label the:
Hydrophilic
heads
Hydrophobic
tails
Phospholipid Bilayer

32. • Embedded in the bilayer are proteins
–Most of the membrane’s functions are
accomplished by the embedded proteins.
• Integral proteins span the membrane
• Peripheral proteins are on one side or the other of the
membrane

33. Plasma Membrane Components
• Glycoproteins and glycolipids are
proteins/lipids with short chain
carbohydrates attached on the
extracellular side of the membrane.

34. Fig.5-1a
Cholesterol
Glycoprotein
Glycolipid
Carbohydrate of
glycoprotein
Phospholipid
Microfilaments
of cytoskeleton
Integrin

35. Types of Membrane Proteins
1. Cell-cell recognition proteins
2. Integrins
3. Intercellular junction proteins
4. Enzymes
5. Signal transduction proteins
• Aka - Receptor proteins
1. Transport proteins
– Passive and active

36. • Cell-cell recognition proteins - identify type of
cell and identify a cell as “self” versus foreign
– Most are glycoproteins
• Carbohydrate chains vary between species, individuals,
and even between cell types in a given individual.
• Glycolipids also play a role in cell recognition

37. • Integrins are a type of integral protein
– The cytoskeleton attaches to integrins on the
cytoplasmic side of the membrane
– Integrins strengthen the membrane
• Intercellular junction proteins - help like cells
stick together to form tissues

38. • Many membrane proteins are enzymes
– This is especially important on the membranes of
organelles.

39. • Signal transduction (receptor) proteins bind
hormones and other substances on the
outside of the cell.
– Binding triggers a change inside the cell.
• Called signal transduction
• Example: The binding of insulin to insulin receptors
causes the cell to put glucose transport proteins into
the membrane.

40. Fig.5-1c
Activated
molecule
Messenger molecule
Receptor

41. Transport Proteins
• 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

42. Transport of Substances Across the
Plasma Membrane (PM)
1. Passive Transport
–
–
–
(Simple) Diffusion (5.3)
Facilitated diffusion (5.6)
Osmosis (5.4, 5.5)
1. Active Transport (5.8)
2. Bulk Flow (5.9)
–
–
Endocytosis
Exocytosis

43. Passive Transport
• In passive transport substances cross the
membrane by diffusion
– Diffusion - net movement of substances from an
area of high concentration to low concentration
• no energy required

45. Blood group antigens

48. INTEGRAL MEMBRANE PROTEINS
Integral membrane proteins typically contains one or more transmembrane helices

49. Peripheral proteins
Peripheral proteins are non covalently bonded to the polar head groups of the lipid bilayer

50. Lipid anchored proteins
Lipid –anchored proteins are covalently bonded to a lipid group

52. Various classes of proteins are associated with the lipid bilayer

53. Freeze fracture : A technique for investigating the cell membrane
structure

54. Freeze- fracture technique

56. Integral proteins resides in the plasma membrane

57. Glycoporin a integral protein with a single transmembrane domain

58. Various amino acid residues within trans membrane helices

59. Transition temparature

60. LIPID RAFTS – GPI ANCHORED PROTEINS

61. The possible movements of phospholipids in a membrane

62. PATTERN OF MOVEMENT OF INTEGRAL PROTEINS

64. Plasma membrane functions

65. RBCS

66. Plasma membrane of the human erythrocyte

67. The dynamic properties of the plasma membrane

68. 1
GUL
MUNEER
S/O ABDUL
RAZZAQUE
BIOCHEMIST
RY
BS PART II
UNIVERSITY OF
SINDH
2K10/BCH/2
3
PROUD TO BE
BIOCHEMIST
WE EXPLORE THE
SECRETS OF LIFE

69. PLASMA MEMBRANE
By
GUL MUNEER
2

70. 3

71. 4

72. What is plasma membrane?
 Outermost layer (animal)
 Thickness is 5-8 nm
 Selectively permeable
 Serve as outer boundary
Allows some substances to cross more easily than
others
Made of Phospholipids, proteins & conjugated
molecules
 Separate and protect cell from external environment
 Provide connecting system b/w cell & its environment
 Also called cell membrane
5

73. How could you define plasma
membrane?
An outermost envelope surrounding the cell
that separates and protects the cell from the
external environment
connecting system B/w
and provides a
the cell and its
environment is called plasma membrane.
6

74. 7

75. Structure of the Plasma
Membrane
The plasma
membrane is
composed of two
layers of
phospholipids
back-to-back.
Phospholipids are lipids with a
phosphate attached to them.
8

76. Phospholipids
 Fatty acid tails
 hydrophobic
 Phosphate group head
 hydrophilic
 Arranged as a bilayer
Fatty acid
Phosphate
Phospholipids, glycolipids,
cholesterol are amphipathic
lipids containing
hydrophobic and
hydrophillic ends
9

77. Phospholipid bilayer
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
10

78. Sandwitch model
OR
Danielli- Davson Model
Proposed by Davson and Danielle in 1935
“Cell membrane is lipid bilayer sandwitched
B/w two monomolecular protein layers”
11

79. Fluid mosaic model
In 1972, S.J. Singer & G. Nicolson
proposed Fluid mosaic model.
12

80. What is Fluid mosaic model?
“Cell membrane is lipid bilayer in
which proteins are partially
embeded like Floating iceburgs in
sea”
The fluid mosaic model
describes the plasma membrane as a
flexible boundary of a cell. The
phospholipids move within the
membrane.
13

81. OVERVIEW
Lipid molecules are present in a fluid
state capable of rotating and moving.
The proteins occur as a „mosaic‟of
discontinuous particles that penetrate
deeply into and even through the lipid
sheet.
Globular proteins are irregularly
embedded in the lipid bilayer.
14

82. Membrane proteins
Membrane proteins are categorized into two
groups:
1- Extrinsic (peripheral) membrane proteins
2- Intrinsic (integral) membrane proteins
15

83. Extrinsic membrane protein
Proteins loosely associated with
membrane surface
Located entirely outside of the lipid
bilayer
Either on the extracellular or
cytoplasmic surface
Also called Peripheral membrane
proteins:
 Example:
 Cytochrome C of Mitochondria
 Cell surface identity marker (antigens)
16

84. Intrinsic membrane proteins
Directly incorporated within the lipid
bilayer
 Tightly bound to lipid bilayer
Provides channel for the water-soluble
substances
Also called Integral membrane
proteins
 Example:
 Transmembrane protein
 Transport proteins
 Channels, permeases (pumps)
17

85. Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Extracellular fluid
Cytoplasm
Glycolipid
Filaments of
cytoskeleton
Peripheral
protein
Glycoprotein
Phospholipids
Cholesterol
Transmembrane
proteins
18

86. Proteins domains anchor molecule
 Within membrane
 nonpolar amino acids
hydrophobic
anchors protein
into membrane
 On outer surfaces of
membrane
 polar amino acids
hydrophilic
extend into
extracellular fluid &
into cytosol
Polar areas
of protein
Nonpolar areas of protein
19

87. NH2
H+
COOH
Cytoplasm
Retinal
chromophore
Nonpolar
(hydrophobic)
-helices in the
cell membrane H+
proton pump channel
in photosynthetic bacteria
water channel
in bacteria
Porin monomer
-pleated sheets
Bacterial
outer
membrane
function through
conformational change =
shape change
Examples

88. Classes of amino acids
What do these amino acids have in common?
nonpolar & hydrophobic
21

89. Classes of amino acids
What do these amino acids have in common?
polar & hydrophilic 22

90. Association of membrane proteins with the lipid bilayer
Transmembrane Proteins
1. A single α-helix
2. Multiple α-helices
3. Rolled up β-sheet
23

91. 4.α- helix (hydrophobic face)
embedded in lipid bilayerthe.
24

92. Cytoplasmic side 
5. Protein covalently
attaches lipid chain
– fatty acid chain or
prenyl group
(cytoplasmic side)
6. Protein attaches
phosphatidylinositol
via
an oligosaccharide
linker
7, 8. Noncovalent
interaction between
proteins

93. Plasma Membrane :
Membrane Proteins
Functional classification
Transmembrane
Proteins

94. Plasma Membrane : Membrane Proteins
Functional classification
Transmembrane Proteins
27

95. Plasma Membrane :
Membrane Proteins
Functional classification
Peripheral Proteins
(only one side of the
membrane)
28

96. Chemical composition
Composed of Lipids, Proteins and
Carbohydrates.
Actual components differs from
tissue to tissue.
 Lipids of cell membrane are
 Phospholipids
 Glycolipids
 Sterol
 Cholesterol
29

97. Why carbohydrates are not inserted
into the biological membrane?
The carbohydrates are not inserted into
the membrane -- they are too
hydrophilic for that. They are attached
to embedded proteins -- glycoproteins.
30

98. Membrane carbohydrates
 Play a key role in cell-cell recognition
 ability of a cell to distinguish one cell from
another
Antigens (MHC)
 important in organ &
tissue development
 basis for rejection of
foreign cells by
immune system
31

99. Four major phospholipids found in mammalian plasma
membrane
There are many „minor‟phospholipids exists, too.

100. Cholesterol
Unique to plasma membrane
Stabilize membrane
33

101. Cholesterol
Unique to plasma
membrane
Stabilize membrane
Cholesterol
34

102. 35

103. 36

104. Movement
across the
Cell
Membrane
37

105. Many Functions of Membrane Proteins
Outside
Plasma
membrane
Inside
Transporter Cell surface
receptor
Enzyme
activity
Cell surface
identity marker
Attachment to the
cytoskeleton
Cell adhesion
38

106. Transport
proteins allow
needed substances or
waste materials to
move through the
plasma membrane.
39

107. Aquaporins
 Water moves rapidly into & out of cells
 evidence that there were water channels
1991 | 2003
Peter Agre
John Hopkins
Roderick MacKinnon
Rockefeller 40

108. Diffusion
 2nd Law of Thermodynamics
governs biological systems
 universe tends towards disorder (entropy)
 Diffusion
 movement fromhighlowconcentration 41

109. Diffusion
 Move from HIGH to LOW concentration
 “passive transport”
 no energy needed
diffusion osmosis
movement of water
42

110. Diffusion through phospholipid bilayer
 What molecules can get through directly?
 fats & other lipids
inside cell
outside cell
lipid
salt
H2O
sugar aa
NH3
 What molecules can
NOT get through
directly?
 polar molecules
 H2O
 ions
 salts, ammonia
 large molecules
 starches, proteins
43

111. Channels through cell membrane
 Membrane becomes semi-permeable
with protein channels
 specific channels allow specific material
across cell membrane
inside cell
outside cell
sugar
aa
H2O
salt
NH3 44

112. Osmosis: Diffusion of Water
The diffusion of water across a selectively
permeable membrane is called osmosis.
Regulating the water flow through the plasma
membrane is an important factor in
maintaining homeostasis within a cell.
45

113. freshwater balanced saltwater
Managing water balance
 Cell survival depends on balancing
water uptake & loss
46

114. Passive Transport
When a cell uses no energy to move particles
across a membrane passive transport occurs.
Concentration
gradient
Plasma
membrane
47

115. Passive Transport by proteins
Plasma
membrane
Passive transport of materials across the
membrane using transport proteins is called
facilitated diffusion.
Channel
proteins
Concentration
gradient
48

116. Facilitated Diffusion
 Diffusion through protein channels
 channels move specific moleculesacross
cell membrane
 no energyneeded
open channel = fast transport
facilitated = with help
high
low
“The Bouncer”49

117. Passive transport by proteins
The movement is with the concentration
gradient, and requires no energy input from
the cell.
Concentratio
n gradient
Plasma
membrane
Step 1 Step 2
Carrier proteins
50

118. Active Transport
Plasma
membrane
Concentration
gradient
Movement of materials through a membrane
against a concentration gradient is called
active transport and requires energy from the
cell.
Carrier
proteins
Cellular
energy
Step 1 Step 2
51

119. Active Transport
 Cells may need to move molecules against
concentration gradient
 shape change transports solute from
one side of membrane to other
 protein “pump”
 “costs” energy = ATP
conformational change
ATP
low
high
“The Doorman52”

120. symport
antiport
Active transport
Many models & mechanisms
ATP ATP
53

121. Getting through cell membrane
 Passive Transport
 Simple diffusion
 diffusion of nonpolar, hydrophobic molecules
 lipids
 high  low concentration gradient
 Facilitated transport
 diffusion of polar, hydrophilic molecules
 through a protein channel
 high  low concentration gradient
 Active transport
 diffusion against concentration gradient
 low  high
 uses a protein pump
 requires ATP
ATP
54

122. Transport summary
simple
diffusion
facilitated
diffusion
active
transport
ATP
55

123. How about large molecules?
 Moving large molecules into & out of cell
 through vesicles & vacuoles
 Endocytosis ( 2 Types)
1. phagocytosis = “cellular eating”
2. pinocytosis = “cellular drinking”
 exocytosis
exocytosis 56

124. ENDOCYTOSIS
- Cell membrane surrounds the substances by
infolding in the form of vacuole or forms a vesicle.
Two Types of Endocytosis
Phagocytosis – engulf solid particles
Pinocytosis – engulf liquid material
57

125. Transport of Large Particles
Endocytosis Exocytosis
Endocytosis is a process by which a cell
surrounds and takes in material from its
environment.
Nucleus
Wastes
Digestion
58

126. Transport of Large Particles
Endocytosis Exocytosis
Exocytosis is the expulsion or secretion of
materials from a cell.
Nucleus
Wastes
Digestion
59

127. THE END
60

128. THANK YOU
61