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
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
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
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.
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.
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
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
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
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
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
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.
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 fromhighlowconcentration 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
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
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”
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
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