MEMBRANES

Discovering the Composition of Membranes 1875 Charles Overton 1875 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 contained proteins. 20 years later it was found that the membranes also contained proteins Irving Langmuir 1917 Irving Langmuir Produced artificial membranes by adding phospholipids dissolved in benzene to water. Produced artificial membranes by adding phospholipids dissolved in benzene to water. He found when the benzene evaporated it left a film on the water. He found when the benzene evaporated it left a film on the water Gorter and Grendel 1925 Gorter and Grendel Realized that the phospholipid layer was a boundary that could separate two aqueous environments. 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. Isolated phospholipids from blood cells and found that there was enough to cover the surface area of a cell twice.

Davison and Danielli Davison 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 showed a triple layer with two dark stained bands. Electron micrograph supported this view because it 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 membrane. Percent and type of proteins vary from membrane to membrane. Different types of phospholipids. Different types of phospholipids. Amphipathic nature of proteins do not permit proteins to lay laterally across the hydrophilic heads of the phospholipids. Amphipathic nature of proteins do not permit proteins to lay laterally across the hydrophilic heads of the phospholipids.

Singer and Nicholson Suggested that the membrane proteins are positioned vertically within the membrane instead of laterally. Suggested that the membrane proteins are positioned vertically within the 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 phospholipid bilayer. Proteins interdispersed among a fluid phospholipid bilayer. Freeze fracture confirmed this theory. Freeze fracture confirmed this theory. 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. 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 The membrane is held together mainly by hydrophobic interactions. The membrane is held together mainly by hydrophobic interactions. Orientation of the membrane proteins. Orientation of the membrane proteins. Proteins drift laterally and do not flip due to their amphipathic nature 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 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. This was proven by scientists when they fused a mouse cell to a human cell. They found that the membrane proteins were intermingled over a period of time. 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. Some proteins are completely immobile and are firmly attached to cytoskeletal fibers within the cell.

Factors Affecting Fluidity of a Membrane Type of phospholipid Type of phospholipid The cells of animals that live in colder climates tend to posses membranes composed of lipids containing unsaturated fatty acids. The cells of animals that live in colder climates tend to posses membranes composed of lipids containing unsaturated fatty acids. Cholesterol Cholesterol Wedged between phospholipids to prevent solidification at low temperatures. Wedged between phospholipids to prevent solidification at low temperatures.

Membrane Structure

Mouse and Human Cell Fusion

Membrane Proteins Types Types Integral Integral Span the membrane Span the membrane Peripheral Peripheral Found on the cytoplasmic face of the membrane and are anchored to cytosketeltal fibers. Found on the cytoplasmic face of the membrane and are anchored to cytosketeltal fibers. Can provide structure. Can provide structure. Integrins Integrins Attached to fibers of the ECM (extra cellular matrix) and provide additional structure to the plasma 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. New membrane is produced by the endoplasmic reticulum. The inner face of the ER will become the outer face of the plasma membrane. The inner face of the ER will become the outer face of 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 InstallationConstruction of the Cell Membrane - Learning Ac - Flash Player Installation

Membrane Structure and Function

Carbohydrates and the Membrane Important in cell to cell recognition Important in cell to cell recognition Sorting cells into tissues during embryonic development. Sorting cells into tissues during embryonic development. Rejection of foreign cells Rejection of foreign cells Immune system Immune system Some are glycolipids Some are glycolipids Carbohydrate attached to a lipid. Carbohydrate attached to a lipid. Most are glycoproteins Most are glycoproteins Carbohydrate attached to a protein. Carbohydrate attached to a protein. Blood types are determined by glycoproteins (A,B, AB,O) Blood types are determined by glycoproteins (A,B, AB,O)

Functions of Membrane Proteins