Lecture 2 - Cell Membrane Structure | Cell Biology- Botany PDF Download

 Page 1


Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 
 
 
 
 
 
 
 
 
Subject : Cell Biology 
Lesson : Cell Membrane Structure 
Lesson Developer : Rina Majumdar 
College/Department : Maitreyi college, University of Delhi 
  
Page 2


Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 
 
 
 
 
 
 
 
 
Subject : Cell Biology 
Lesson : Cell Membrane Structure 
Lesson Developer : Rina Majumdar 
College/Department : Maitreyi college, University of Delhi 
  
Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 1 
 
 
Table of Contents       
 
Chapter: Cell membrane - I 
? Introduction  
o  Membrane Models (A Historical Overview) 
o Chemical Composition 
? Membrane lipids 
? Phospholipids 
? Sphingolipids 
? Cholesterol 
? Membrane proteins 
? Peripheral (Extrinsic) Protein 
? Integral (Intrinsic) Protein 
? Functions of membrane Proteins 
? Membrane Carbohydrate 
? Summary  
? Exercise/ Practice 
? Glossary 
? References/ Bibliography/ Further Reading 
 
 
 
 
Page 3


Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 
 
 
 
 
 
 
 
 
Subject : Cell Biology 
Lesson : Cell Membrane Structure 
Lesson Developer : Rina Majumdar 
College/Department : Maitreyi college, University of Delhi 
  
Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 1 
 
 
Table of Contents       
 
Chapter: Cell membrane - I 
? Introduction  
o  Membrane Models (A Historical Overview) 
o Chemical Composition 
? Membrane lipids 
? Phospholipids 
? Sphingolipids 
? Cholesterol 
? Membrane proteins 
? Peripheral (Extrinsic) Protein 
? Integral (Intrinsic) Protein 
? Functions of membrane Proteins 
? Membrane Carbohydrate 
? Summary  
? Exercise/ Practice 
? Glossary 
? References/ Bibliography/ Further Reading 
 
 
 
 
Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 2 
 
Introduction  
? Cell is the smallest living entity of all living organisms, and is delimited from the 
surrounding cellular environment by cell membrane. 
? Cell membrane is also known as plasma membrane or cytoplasmic membrane since 
it encloses the cellular content called cytoplasm (plasma was the original name for 
cellular content). 
? Membranes form the outermost boundaries of the animal cells whereas a rigid cell wall 
is present outside the membrane in plant cells.  
? The intracellular organelles in eukaryotic cells are also surrounded by membranes, hence 
the cell membranes also help in intracellular compartmentalization of the eukaryotic 
cells. 
? All membrane compartments have specific functions related to the unique molecular 
structure of the membrane. 
? The cell membrane regulates the transport of the substances into and out of the cell.  
The exchange of nutrients, ions, gases, water and other substances takes place through 
the membranes. 
? Plasma membrane provides differential or selective permeable barrier which allows 
only selective molecules to cross the membrane.  Unrestricted exchange of molecules is 
not possible through the membrane. 
? Cellular waste is also removed through the membrane hence helps in excretion of 
waste. 
? Intercellular interaction in a multicellular organism is possible through the cell 
membrane since each cell is connected to its adjacent cell. 
? In animals and plants, cell-to-cell communication is possible through the gap junctions 
and plasmodesmata respectively. 
? The outer surface of the membrane plays key role in signal transduction (both 
electrical and chemical).  This includes both the detection of specific signals on the outer 
surface of the cell and the response that results within the cell. 
 
Membrane Models (A Historical Overview) 
Several models have been proposed to explain the structure of membranes. The models are 
proposed based on experimental observations and results.  With the rapid progress in 
Page 4


Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 
 
 
 
 
 
 
 
 
Subject : Cell Biology 
Lesson : Cell Membrane Structure 
Lesson Developer : Rina Majumdar 
College/Department : Maitreyi college, University of Delhi 
  
Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 1 
 
 
Table of Contents       
 
Chapter: Cell membrane - I 
? Introduction  
o  Membrane Models (A Historical Overview) 
o Chemical Composition 
? Membrane lipids 
? Phospholipids 
? Sphingolipids 
? Cholesterol 
? Membrane proteins 
? Peripheral (Extrinsic) Protein 
? Integral (Intrinsic) Protein 
? Functions of membrane Proteins 
? Membrane Carbohydrate 
? Summary  
? Exercise/ Practice 
? Glossary 
? References/ Bibliography/ Further Reading 
 
 
 
 
Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 2 
 
Introduction  
? Cell is the smallest living entity of all living organisms, and is delimited from the 
surrounding cellular environment by cell membrane. 
? Cell membrane is also known as plasma membrane or cytoplasmic membrane since 
it encloses the cellular content called cytoplasm (plasma was the original name for 
cellular content). 
? Membranes form the outermost boundaries of the animal cells whereas a rigid cell wall 
is present outside the membrane in plant cells.  
? The intracellular organelles in eukaryotic cells are also surrounded by membranes, hence 
the cell membranes also help in intracellular compartmentalization of the eukaryotic 
cells. 
? All membrane compartments have specific functions related to the unique molecular 
structure of the membrane. 
? The cell membrane regulates the transport of the substances into and out of the cell.  
The exchange of nutrients, ions, gases, water and other substances takes place through 
the membranes. 
? Plasma membrane provides differential or selective permeable barrier which allows 
only selective molecules to cross the membrane.  Unrestricted exchange of molecules is 
not possible through the membrane. 
? Cellular waste is also removed through the membrane hence helps in excretion of 
waste. 
? Intercellular interaction in a multicellular organism is possible through the cell 
membrane since each cell is connected to its adjacent cell. 
? In animals and plants, cell-to-cell communication is possible through the gap junctions 
and plasmodesmata respectively. 
? The outer surface of the membrane plays key role in signal transduction (both 
electrical and chemical).  This includes both the detection of specific signals on the outer 
surface of the cell and the response that results within the cell. 
 
Membrane Models (A Historical Overview) 
Several models have been proposed to explain the structure of membranes. The models are 
proposed based on experimental observations and results.  With the rapid progress in 
Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 3 
scientific research and technology one model may replace another, but it does not imply 
that earlier models are worthless.  Every model has to face new challenges, which inspire 
experiments leading to the proposal of new model to explain the challenges faced by earlier 
models.  Therefore, all the historical models are of immense importance for understanding 
the precise structure and function of the membrane. 
 
? E. Overton (1899) did extensive studies on the penetration of more than 500 different 
chemical compounds in both plant and animal cells.  He established that the membranes 
of both plants and animals are ‘impregnated’ by lipid like material since, compounds 
soluble in organic solvents entered the cell more rapidly than water-soluble compounds. 
Overton later suggested that the two important lipids present in the membrane might 
be cholesterol and lecithins. 
 
 
 
Figure: Lipid nature of membrane 
 Source: Author 
 
? Irving Langmuir (1917) made the second important advancement by spreading the lipid 
molecules in a monolayer on water surface in a specially designed apparatus called 
Langmuir Trough.  This helped in measuring the area occupied by a single lipid 
molecule along with the surface area of the entire thin film of lipid.  He later received 
Nobel Prize in chemistry in 1932.  This important technique was widely used in the 
study of specific membrane lipids. 
 
 
 
Figure: Lipid monolayer 
Source :Author 
Page 5


Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 
 
 
 
 
 
 
 
 
Subject : Cell Biology 
Lesson : Cell Membrane Structure 
Lesson Developer : Rina Majumdar 
College/Department : Maitreyi college, University of Delhi 
  
Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 1 
 
 
Table of Contents       
 
Chapter: Cell membrane - I 
? Introduction  
o  Membrane Models (A Historical Overview) 
o Chemical Composition 
? Membrane lipids 
? Phospholipids 
? Sphingolipids 
? Cholesterol 
? Membrane proteins 
? Peripheral (Extrinsic) Protein 
? Integral (Intrinsic) Protein 
? Functions of membrane Proteins 
? Membrane Carbohydrate 
? Summary  
? Exercise/ Practice 
? Glossary 
? References/ Bibliography/ Further Reading 
 
 
 
 
Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 2 
 
Introduction  
? Cell is the smallest living entity of all living organisms, and is delimited from the 
surrounding cellular environment by cell membrane. 
? Cell membrane is also known as plasma membrane or cytoplasmic membrane since 
it encloses the cellular content called cytoplasm (plasma was the original name for 
cellular content). 
? Membranes form the outermost boundaries of the animal cells whereas a rigid cell wall 
is present outside the membrane in plant cells.  
? The intracellular organelles in eukaryotic cells are also surrounded by membranes, hence 
the cell membranes also help in intracellular compartmentalization of the eukaryotic 
cells. 
? All membrane compartments have specific functions related to the unique molecular 
structure of the membrane. 
? The cell membrane regulates the transport of the substances into and out of the cell.  
The exchange of nutrients, ions, gases, water and other substances takes place through 
the membranes. 
? Plasma membrane provides differential or selective permeable barrier which allows 
only selective molecules to cross the membrane.  Unrestricted exchange of molecules is 
not possible through the membrane. 
? Cellular waste is also removed through the membrane hence helps in excretion of 
waste. 
? Intercellular interaction in a multicellular organism is possible through the cell 
membrane since each cell is connected to its adjacent cell. 
? In animals and plants, cell-to-cell communication is possible through the gap junctions 
and plasmodesmata respectively. 
? The outer surface of the membrane plays key role in signal transduction (both 
electrical and chemical).  This includes both the detection of specific signals on the outer 
surface of the cell and the response that results within the cell. 
 
Membrane Models (A Historical Overview) 
Several models have been proposed to explain the structure of membranes. The models are 
proposed based on experimental observations and results.  With the rapid progress in 
Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 3 
scientific research and technology one model may replace another, but it does not imply 
that earlier models are worthless.  Every model has to face new challenges, which inspire 
experiments leading to the proposal of new model to explain the challenges faced by earlier 
models.  Therefore, all the historical models are of immense importance for understanding 
the precise structure and function of the membrane. 
 
? E. Overton (1899) did extensive studies on the penetration of more than 500 different 
chemical compounds in both plant and animal cells.  He established that the membranes 
of both plants and animals are ‘impregnated’ by lipid like material since, compounds 
soluble in organic solvents entered the cell more rapidly than water-soluble compounds. 
Overton later suggested that the two important lipids present in the membrane might 
be cholesterol and lecithins. 
 
 
 
Figure: Lipid nature of membrane 
 Source: Author 
 
? Irving Langmuir (1917) made the second important advancement by spreading the lipid 
molecules in a monolayer on water surface in a specially designed apparatus called 
Langmuir Trough.  This helped in measuring the area occupied by a single lipid 
molecule along with the surface area of the entire thin film of lipid.  He later received 
Nobel Prize in chemistry in 1932.  This important technique was widely used in the 
study of specific membrane lipids. 
 
 
 
Figure: Lipid monolayer 
Source :Author 
Cell Membrane -Structure 
Institute of Lifelong Learning, University of Delhi 4 
? E. Gorter and F. Grendel did extensive studies on the organization of membrane lipids in 
red blood cells (erythrocytes) from a variety of mammals including dogs, sheep, rabbit, 
goat, human and obtained consistent results.  Using Langmuir trough, the lipids from 
the erythrocytes, were allowed to form a tightly packed monolayer and the surface area 
of the lipid monolayer was calculated.  His experiment led to an interesting finding that 
the total surface area of the monolayer is almost twice that of the surface area of the 
individual erythrocyte plasma membrane.  Gorter and Grendel then proposed that the 
lipids of the cell membrane are organised in a bimolecular lipid sheet.  They also 
added that the membrane lipids have hydrophobic (non-polar) and a hydrophilic (polar) 
ends which result in such an arrangement where all cell membranes face aqueous 
environment on their either side resulting in the polar ends of the lipids facing the outer 
surface of the membranes keeping the non-polar ends of the lipids embedded in the 
centre.  This model named bimolecular lipid leaflet model got support even after the 
use of advance scientific techniques like X-ray diffraction, ESR, Freeze fracture etc.  
 
 
 
Figure: Lipid bilayer – the hydrophobic tails are positioned inwards while the hydrophilic 
heads point outwards 
Source: Author