MEMBRANES. Discovering the Composition of Membranes 1875 Charles Overton 1875 Charles Overton Deduced that the membrane is composed of lipids. Deduced.

MEMBRANESMEMBRANES

Discovering the CompositionDiscovering the Composition of of MembranesMembranes

1875 Charles Overton1875 Charles Overton Deduced that the membrane is composed of lipids.Deduced that the membrane is composed of lipids. 20 years later it was found that the membranes also 20 years later it was found that the membranes also

contained proteins.contained proteins. 1917 Irving Langmuir1917 Irving Langmuir

Produced artificial membranes by adding phospholipids Produced artificial membranes by adding phospholipids dissolved in benzene to water.dissolved in benzene to water.

He found when the benzene evaporated it left a film on the He found when the benzene evaporated it left a film on the water.water.

1925 Gorter and Grendel1925 Gorter and Grendel Realized that the phospholipid layer was a boundary that Realized that the phospholipid layer was a boundary that

could separate two aqueous environments.could separate two aqueous environments. Isolated phospholipids from blood cells and found that Isolated phospholipids from blood cells and found that

there was enough to cover the surface area of a cell twice.there was enough to cover the surface area of a cell twice.

Davison and DanielliDavison and Danielli 1937 Hugh Davidson and James Danielli 1937 Hugh Davidson and James Danielli

Proposed the “Protein Sandwich Model”Proposed the “Protein Sandwich Model” Proteins reside laterally on top the phospholipid bilayer.Proteins reside laterally on top the phospholipid bilayer.

Electron micrograph supported this view because it Electron micrograph supported this view because it showed a triple layer with two dark stained bands.showed a triple layer with two dark stained bands.

This model was accepted for 30 years.This model was accepted for 30 years. Problems with the “Protein Sandwich Model”Problems with the “Protein Sandwich Model”

Proteins are not very soluble in water.Proteins are not very soluble in water. Not all membranes look the same under the microscope.Not all membranes look the same under the microscope.

Percent and type of proteins vary from membrane to Percent and type of proteins vary from membrane to membrane.membrane.

Different types of phospholipids.Different types of phospholipids. Amphipathic nature of proteins do not permit proteins to lay Amphipathic nature of proteins do not permit proteins to lay

laterally across the hydrophilic heads of the phospholipids.laterally across the hydrophilic heads of the phospholipids.

Singer and NicholsonSinger and Nicholson

Suggested that the membrane proteins Suggested that the membrane proteins are positioned vertically within the are positioned vertically within the membrane instead of laterally.membrane instead of laterally. This solves the amphipathic issue.This solves the amphipathic issue.

Suggested the “Fluid Mosaic Model”Suggested the “Fluid Mosaic Model” Proteins interdispersed among a fluid Proteins interdispersed among a fluid

phospholipid bilayer.phospholipid bilayer. Freeze fracture confirmed this theory.Freeze fracture confirmed this theory.

By freezing the cells and splitting them open By freezing the cells and splitting them open revealed pits and bumps that represent the way in revealed pits and bumps that represent the way in which the proteins are orientated in the membrane.which the proteins are orientated in the membrane.

TEM Verses SEM

The Fluidity of the The Fluidity of the MembraneMembrane

The membrane is held together mainly by The membrane is held together mainly by hydrophobic interactions.hydrophobic interactions.

Orientation of the membrane proteins.Orientation of the membrane proteins. Proteins drift laterally and do not flip due to Proteins drift laterally and do not flip due to

their amphipathic nature their amphipathic nature This lateral drift is primarily orchestrated by This lateral drift is primarily orchestrated by

cytoskeletal fibers within the cell.cytoskeletal fibers within the cell. This was proven by scientists when they fused a This was proven by scientists when they fused a

mouse cell to a human cell.mouse cell to a human cell. They found that the membrane proteins were They found that the membrane proteins were

intermingled over a period of time.intermingled over a period of time. Some proteins are completely immobile and Some proteins are completely immobile and

are firmly attached to cytoskeletal fibers are firmly attached to cytoskeletal fibers within the cell.within the cell.

Factors Affecting Fluidity of Factors Affecting Fluidity of a Membranea Membrane

Type of phospholipidType of phospholipid The cells of animals that live in colder The cells of animals that live in colder

climates tend to posses membranes climates tend to posses membranes composed of lipids containing composed of lipids containing unsaturated fatty acids.unsaturated fatty acids.

CholesterolCholesterol Wedged between phospholipids to Wedged between phospholipids to

prevent solidification at low prevent solidification at low temperatures.temperatures.

Membrane Structure

Mouse and Human Cell Fusion

Membrane ProteinsMembrane Proteins TypesTypes

IntegralIntegral Span the membraneSpan the membrane

PeripheralPeripheral Found on the cytoplasmic face of the membrane and are Found on the cytoplasmic face of the membrane and are

anchored to cytosketeltal fibers.anchored to cytosketeltal fibers. Can provide structure.Can provide structure.

IntegrinsIntegrins Attached to fibers of the ECM (extra cellular matrix) and Attached to fibers of the ECM (extra cellular matrix) and

provide additional structure to the plasma membrane.provide additional structure to the plasma membrane. New membrane is produced by the endoplasmic New membrane is produced by the endoplasmic

reticulum.reticulum. The inner face of the ER will become the outer face of The inner face of the ER will become the outer face of

the plasma membrane.the plasma membrane. Composition of a membrane depends on its function.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 Carbohydrates and the MembraneMembrane

Important in cell to cell recognitionImportant in cell to cell recognition Sorting cells into tissues during embryonic Sorting cells into tissues during embryonic

development.development. Rejection of foreign cellsRejection of foreign cells

Immune systemImmune system Some are glycolipidsSome are glycolipids

Carbohydrate attached to a lipid.Carbohydrate attached to a lipid. Most are glycoproteinsMost are glycoproteins

Carbohydrate attached to a protein.Carbohydrate attached to a protein. Blood types are determined by glycoproteins (A,B, Blood types are determined by glycoproteins (A,B,

AB,O)AB,O)

Functions of Membrane Proteins