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
Read More