Lecture 5 - Cell membrane: Properties and Selective Permeability | Cell Biology- Botany PDF Download

 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.