Page 1
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 0
Lesson: Cell membrane: Properties and Selective Permeability
Lesson Developer: Rina Majumdar
College/Department: Maitreyi College, University of Delhi
Page 2
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 0
Lesson: Cell membrane: Properties and Selective Permeability
Lesson Developer: Rina Majumdar
College/Department: Maitreyi College, University of Delhi
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 1
Table of Contents
? Introduction-Unique Properties of Biological
Membranes
? Fluidity of the membranes
o Role of Cholesterol on Membrane Fluidity
o Importance of membrane fluidity
? Membrane Asymmetry
? Phase Transition of the membrane
? Selective Permeability of the membrane
? Passive Transport
o Facilitated diffusion
o Probable mechanisms for FD
? Active Transport
o The Na
+
/ K
+
exchange pump
o Other important Ion Transport systems
? Channel protein
? Carrier protein.
? Bulk Transport
o Endocytosis
? Pinocytosis
? Receptor-mediated endocytosis
? Phagocytosis
o Exocytosis
? Summary
? Glossary
? Exercises
? References
Page 3
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 0
Lesson: Cell membrane: Properties and Selective Permeability
Lesson Developer: Rina Majumdar
College/Department: Maitreyi College, University of Delhi
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 1
Table of Contents
? Introduction-Unique Properties of Biological
Membranes
? Fluidity of the membranes
o Role of Cholesterol on Membrane Fluidity
o Importance of membrane fluidity
? Membrane Asymmetry
? Phase Transition of the membrane
? Selective Permeability of the membrane
? Passive Transport
o Facilitated diffusion
o Probable mechanisms for FD
? Active Transport
o The Na
+
/ K
+
exchange pump
o Other important Ion Transport systems
? Channel protein
? Carrier protein.
? Bulk Transport
o Endocytosis
? Pinocytosis
? Receptor-mediated endocytosis
? Phagocytosis
o Exocytosis
? Summary
? Glossary
? Exercises
? References
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 2
Introduction-Unique Properties of Biological Membranes
Video: A breif overview of plasma membrane
Source: http://www.youtube.com/watch?v=moPJkCbKjBs
All living organisms are characterised by a highly complex cellular system, which have to
perform a wide range of biochemical processes for its existence. Cell is the ultimate unit
of structure and function of all living systems. The membrane that delimits each cell
plays a vital role in the cellular function. The biological living membranes of all cells
share certain physical properties, which makes them adapt to different environmental
conditions and perform a wide range of different functions. The cell membrane is a highly
dynamic entity and the protein-lipid interactions within a membrane keep changing
according to their functions. Some of the properties, which maintain the dynamism of
the membrane are discussed below.
Fluidity of the membranes
Membranes are highly flexible and the various molecules within these,are not tightly
packed, rather held together by hydrophobic interactions, which are much weaker than
the covalent bonds. Both the lipid and the protein components of the membranes are in
a constant state of dynamism. The molecular weight of the lipids is much smaller than
the proteins hence the lipids can travel much faster than the proteins. A phospholipid
molecule can possibly make three types of movements within a bilayer .
Page 4
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 0
Lesson: Cell membrane: Properties and Selective Permeability
Lesson Developer: Rina Majumdar
College/Department: Maitreyi College, University of Delhi
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 1
Table of Contents
? Introduction-Unique Properties of Biological
Membranes
? Fluidity of the membranes
o Role of Cholesterol on Membrane Fluidity
o Importance of membrane fluidity
? Membrane Asymmetry
? Phase Transition of the membrane
? Selective Permeability of the membrane
? Passive Transport
o Facilitated diffusion
o Probable mechanisms for FD
? Active Transport
o The Na
+
/ K
+
exchange pump
o Other important Ion Transport systems
? Channel protein
? Carrier protein.
? Bulk Transport
o Endocytosis
? Pinocytosis
? Receptor-mediated endocytosis
? Phagocytosis
o Exocytosis
? Summary
? Glossary
? Exercises
? References
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 2
Introduction-Unique Properties of Biological Membranes
Video: A breif overview of plasma membrane
Source: http://www.youtube.com/watch?v=moPJkCbKjBs
All living organisms are characterised by a highly complex cellular system, which have to
perform a wide range of biochemical processes for its existence. Cell is the ultimate unit
of structure and function of all living systems. The membrane that delimits each cell
plays a vital role in the cellular function. The biological living membranes of all cells
share certain physical properties, which makes them adapt to different environmental
conditions and perform a wide range of different functions. The cell membrane is a highly
dynamic entity and the protein-lipid interactions within a membrane keep changing
according to their functions. Some of the properties, which maintain the dynamism of
the membrane are discussed below.
Fluidity of the membranes
Membranes are highly flexible and the various molecules within these,are not tightly
packed, rather held together by hydrophobic interactions, which are much weaker than
the covalent bonds. Both the lipid and the protein components of the membranes are in
a constant state of dynamism. The molecular weight of the lipids is much smaller than
the proteins hence the lipids can travel much faster than the proteins. A phospholipid
molecule can possibly make three types of movements within a bilayer .
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 3
Figure: Fluidity of lipid molecules in the membrane
Source: Author
a) Phospholipids can diffuse laterally within the same leaflet with a considerable
ease, around 10
-9
times per second.
b) A very rare phospholipid movement is to flip-flop transversely across the
membrane which takes place around 10
5
times per second. The hydrophilic head
group of the lipids have to cross the hydrophobic (centre) barrier of the
membrane, which is thermodynamically unfavourable. Enzymes called flippases
activate the movement of lipids from one leaflet to the other.
c) Phospholipid molecules can rotate rapidly without changing their position within a
bilayer, which is termed as flexion motion , around 10
-9
per second. The kink in
one of the unsaturated fatty acid tail of phospholipid molecules plays a dramatic
role in preventing the compact packaging of the lipids within the membrane
monolayers.
The protein molecules of the membrane also do not maintain a fixed position but may
slowly diffuse laterally. D. Frye and M. Edidin by hybrid cells (heterokaryons)
experiment demonstrated the lateral diffusion of the protein molecules. They prepared
hybrid cell from the fusion of human and mouse cells, using fluorescentlabelled antibody
and followed the distribution of the plasma membrane protein in the hetero-karyon (two
different nucleus within the hybrid cell) in different time intervals. Initially the protein on
the two „halves? of the fused cells were different , characteristic of each cell, but in less
than an hour, the protein of both the cell membranes were evenly distributed through
simple diffusion. No ATP (metabolic energy) was required since the metabolic inhibitor
did not prevent the movement.
Page 5
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 0
Lesson: Cell membrane: Properties and Selective Permeability
Lesson Developer: Rina Majumdar
College/Department: Maitreyi College, University of Delhi
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 1
Table of Contents
? Introduction-Unique Properties of Biological
Membranes
? Fluidity of the membranes
o Role of Cholesterol on Membrane Fluidity
o Importance of membrane fluidity
? Membrane Asymmetry
? Phase Transition of the membrane
? Selective Permeability of the membrane
? Passive Transport
o Facilitated diffusion
o Probable mechanisms for FD
? Active Transport
o The Na
+
/ K
+
exchange pump
o Other important Ion Transport systems
? Channel protein
? Carrier protein.
? Bulk Transport
o Endocytosis
? Pinocytosis
? Receptor-mediated endocytosis
? Phagocytosis
o Exocytosis
? Summary
? Glossary
? Exercises
? References
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 2
Introduction-Unique Properties of Biological Membranes
Video: A breif overview of plasma membrane
Source: http://www.youtube.com/watch?v=moPJkCbKjBs
All living organisms are characterised by a highly complex cellular system, which have to
perform a wide range of biochemical processes for its existence. Cell is the ultimate unit
of structure and function of all living systems. The membrane that delimits each cell
plays a vital role in the cellular function. The biological living membranes of all cells
share certain physical properties, which makes them adapt to different environmental
conditions and perform a wide range of different functions. The cell membrane is a highly
dynamic entity and the protein-lipid interactions within a membrane keep changing
according to their functions. Some of the properties, which maintain the dynamism of
the membrane are discussed below.
Fluidity of the membranes
Membranes are highly flexible and the various molecules within these,are not tightly
packed, rather held together by hydrophobic interactions, which are much weaker than
the covalent bonds. Both the lipid and the protein components of the membranes are in
a constant state of dynamism. The molecular weight of the lipids is much smaller than
the proteins hence the lipids can travel much faster than the proteins. A phospholipid
molecule can possibly make three types of movements within a bilayer .
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 3
Figure: Fluidity of lipid molecules in the membrane
Source: Author
a) Phospholipids can diffuse laterally within the same leaflet with a considerable
ease, around 10
-9
times per second.
b) A very rare phospholipid movement is to flip-flop transversely across the
membrane which takes place around 10
5
times per second. The hydrophilic head
group of the lipids have to cross the hydrophobic (centre) barrier of the
membrane, which is thermodynamically unfavourable. Enzymes called flippases
activate the movement of lipids from one leaflet to the other.
c) Phospholipid molecules can rotate rapidly without changing their position within a
bilayer, which is termed as flexion motion , around 10
-9
per second. The kink in
one of the unsaturated fatty acid tail of phospholipid molecules plays a dramatic
role in preventing the compact packaging of the lipids within the membrane
monolayers.
The protein molecules of the membrane also do not maintain a fixed position but may
slowly diffuse laterally. D. Frye and M. Edidin by hybrid cells (heterokaryons)
experiment demonstrated the lateral diffusion of the protein molecules. They prepared
hybrid cell from the fusion of human and mouse cells, using fluorescentlabelled antibody
and followed the distribution of the plasma membrane protein in the hetero-karyon (two
different nucleus within the hybrid cell) in different time intervals. Initially the protein on
the two „halves? of the fused cells were different , characteristic of each cell, but in less
than an hour, the protein of both the cell membranes were evenly distributed through
simple diffusion. No ATP (metabolic energy) was required since the metabolic inhibitor
did not prevent the movement.
Cell Membrane : Properties and Selective Permeability
Institute of Life Long Learning, University of Delhi 4
Figure : Membrane mobility demonstrated by cell fusion experiment
Source:
Evidence is there to show that mobility of some integral proteins is restricted through a
system of cytoskeleton, that radiate through the cytoplasm.
http://bealbio.wikispaces.com/Cell+and+Cell+Membrane
Role of Cholesterol on Membrane Fluidity
In animals, cholesterol is present in both the monolayers, which constitute almost 50 per
cent of the total membrane lipids on a molecular basis. One cholesterol molecule is
present per 2-3 molecules of phospholipids. The small polar hydroxyl group of the highly
hydrophobic cholesterol lies close to the polar head group of a neighbouring phospholipid
molecule where it forms a hydrogen bond with the oxygen of the phospholipid. The
highly hydrophobic rigid steroid rings and the hydrocarbon chain of the cholesterol lie
close to the saturated region of the hydrocarbon tails of the fatty acid, which lies next to
the polar head group of the phospholipid molecules. This prevents even the saturated
hydrocarbon chains of the fatty acid to come closer, maintaining a loose arrangement.
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