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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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18 docs

FAQs on Lecture 5 - Cell membrane: Properties and Selective Permeability - Cell Biology- Botany

1. What are the properties of the cell membrane?
Ans. The cell membrane has several important properties. It is selectively permeable, meaning it allows certain substances to pass through while restricting others. It is also flexible and can change its shape, allowing the cell to move and maintain its structure. Additionally, the cell membrane is composed of a lipid bilayer, which provides stability and acts as a barrier to hydrophilic molecules.
2. How does the cell membrane maintain its selective permeability?
Ans. The cell membrane maintains its selective permeability through various mechanisms. One important mechanism is the presence of transport proteins that facilitate the movement of specific molecules across the membrane. These transport proteins can be either channel proteins, which form pores for passive diffusion, or carrier proteins, which undergo conformational changes to transport molecules. Additionally, the lipid bilayer itself restricts the movement of certain molecules based on their size, charge, and hydrophobicity.
3. What is the significance of the cell membrane's selective permeability?
Ans. The selective permeability of the cell membrane is crucial for the survival and proper functioning of cells. It allows cells to regulate the movement of substances in and out of the cell, ensuring a controlled internal environment. This selective permeability is necessary for maintaining appropriate concentrations of ions and molecules, regulating cell volume, and facilitating various cellular processes such as nutrient uptake, waste removal, and cell signaling.
4. How does the cell membrane contribute to cell communication?
Ans. The cell membrane plays a vital role in cell communication. It contains receptor proteins that bind to specific signaling molecules, such as hormones or neurotransmitters. When a signaling molecule binds to its receptor on the cell membrane, it initiates a series of intracellular events that can lead to changes in cell behavior or gene expression. This allows cells to respond to external signals and coordinate their activities with other cells, ultimately regulating processes like growth, differentiation, and immune response.
5. Can the cell membrane be damaged or disrupted?
Ans. Yes, the cell membrane can be damaged or disrupted under certain conditions. Physical factors such as extreme temperatures, mechanical forces, or osmotic imbalances can cause the membrane to rupture or lose its integrity. Chemical factors such as certain toxins or detergents can also disrupt the lipid bilayer or damage the proteins embedded within the membrane. When the cell membrane is damaged, it can lead to loss of cell function, leakage of cellular contents, and ultimately cell death. However, cells have mechanisms to repair or replace damaged membranes to maintain their integrity and functionality.
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