MEMBRANES
Discovering the Composition of Membranes
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1875 Charles Overton
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1917 Irving Langmuir
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Deduced that the membrane is composed of lipids.
20 years later it was found that the membranes also contained proteins.
Produced artificial membranes by adding phospholipids dissolved in
benzene to water.
He found when the benzene evaporated it left a film on the water.
1925 Gorter and Grendel
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Realized that the phospholipid layer was a boundary that could separate
two aqueous environments.
Isolated phospholipids from blood cells and found that there was enough
to cover the surface area of a cell twice.
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Davison and Danielli
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1937 Hugh Davidson and James Danielli
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Proposed the “Protein Sandwich Model”
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Proteins reside laterally on top the phospholipid bilayer.
Electron micrograph supported this view because it showed a triple
layer with two dark stained bands.
This model was accepted for 30 years.
Problems with the “Protein Sandwich Model”
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Proteins are not very soluble in water.
Not all membranes look the same under the microscope.
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Percent and type of proteins vary from membrane to membrane.
Different types of phospholipids.
Amphipathic nature of proteins do not permit proteins to lay laterally
across the hydrophilic heads of the phospholipids.
Singer and Nicholson
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Suggested that the membrane proteins are positioned
vertically within the membrane instead of laterally.
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This solves the amphipathic issue.
Suggested the “Fluid Mosaic Model”
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Proteins interdispersed among a fluid phospholipid bilayer.
Freeze fracture confirmed this theory.
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By freezing the cells and splitting them open revealed pits and bumps
that represent the way in which the proteins are orientated in the
membrane.
TEM Verses SEM
The Fluidity of the Membrane
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The membrane is held together mainly by hydrophobic
interactions.
Orientation of the membrane proteins.
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Proteins drift laterally and do not flip due to their
amphipathic nature
This lateral drift is primarily orchestrated by cytoskeletal
fibers within the cell.
This was proven by scientists when they fused a mouse cell to
a human cell.
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They found that the membrane proteins were intermingled over a
period of time.
Some proteins are completely immobile and are firmly
attached to cytoskeletal fibers within the cell.
Factors Affecting Fluidity of a
Membrane
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Type of phospholipid
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The cells of animals that live in colder climates tend
to posses membranes composed of lipids
containing unsaturated fatty acids.
Cholesterol
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Wedged between phospholipids to prevent
solidification at low temperatures.
Membrane Structure
Mouse and Human Cell Fusion
Membrane Proteins
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Types
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Integral
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Peripheral
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Found on the cytoplasmic face of the membrane and are anchored to
cytosketeltal fibers.
Can provide structure.
Integrins
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Span the membrane
Attached to fibers of the ECM (extra cellular matrix) and provide
additional structure to the plasma membrane.
New membrane is produced by the endoplasmic reticulum.
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The inner face of the ER will become the outer face of the plasma
membrane.
Composition of a membrane depends on its function.
• Construction of the Cell Membrane Learning Ac - Flash Player Installation
• Membrane Structure and Function
Carbohydrates and the Membrane
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Important in cell to cell recognition
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Sorting cells into tissues during embryonic development.
Rejection of foreign cells
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Some are glycolipids
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Immune system
Carbohydrate attached to a lipid.
Most are glycoproteins
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Carbohydrate attached to a protein.
Blood types are determined by glycoproteins (A,B, AB,O)
Functions of Membrane Proteins