Biological Membranes
The cell membrane creates a barrier between the outside and inside environment of the cell, is
involved in transport, signal transduction, and storage. Membranes are amphipathic molecules
which from lipid bilayers this is because the phosphate head is hydrophilic and fatty acid tails
hydrophobic. Amphipathic molecules have a hydrophobic and hydrophilic end. Lipids are
amphipathic. When it is placed into an aqueous environment, the polar region of the lipid tends
to interact with the water molecules while the non-polar region does not interact with water.
Micelles form when the hydrophobic fatty acid chains face inwards away from water forming a
hydrophobic centre, and the hydrophilic phosphate heads face outwards. This minimises the
polar-non-polar interactions and this stabilises the molecule. The hydrophobic effect drives this.
A liposome is an aqueous compartment surrounded by a phospholipid bilayer. It is formed by
mixing a lipid solution into an aqueous solution sonicating the solution. Sonication involves
bombarding the solution with sound waves. The energy carried by the sound waves disperses the
lipids allowing them to spontaneously aggregate into bilayer membranes. By mixing proteins
with detergent and then mixing them with phospholipids, liposomes can be built.
Detergents are amphipathic molecules which contain polar and hydrophobic groups. In aqueous
solutions, they form organised structures known as micelles.
Membrane Fluidity
Membrane fluidity is controlled by fatty acid composition and cholesterol content. The relative
movement of lipids determines the strength of the interactions in the membrane. As the
temperature is increased there is a sharp transition from the rigid state to the fluid state. Phase
transition between two states occurs at Tm (melting temperature). Tm depends on fatty acid
chain length, number of unsaturated fatty acids, and amount of cholesterol present.
Longer fatty acids can form more Van der Waals forces therefore the presence of them
decreases the fluidity and increases the melting temperature (Tm).
Saturated fatty acids create a well-structured arrangement of hydrocarbons with the straight
chain forming stronger intermolecular forces raising Tm.
Unsaturated fatty acids have kinks in the chain which interfere with the well-ordered
structure favouring membrane fluidity raising Tm.
Biological Membranes 1
, Cholesterol inserts itself between phospholipids. The OH group of cholesterol aligns with
the polar head groups of lipids. The rigid steroid ring partially immobilises the hydrocarbon
chain at the point closest to the head. At high concentrations, this prevents the hydrocarbon
chains from coming together. This blocks phase transitions. Cholesterol forms complexes
with glycosphingolipids which form lipid rafts making the membrane less fluid and more
resistant to phase changes raising Tm.
Proteins In Membranes
Integral (intrinsic) proteins - bind to the membrane extensively and span the width of the
bilayer. The portion of the protein within the core interacts with van der waals forces in the
hydrocarbon tails of the phospholipids.
Integral membrane proteins have one or more domains embedded in the lipid bilayer. They may
have single or multiple transmembrane (TM) domains.
Peripheral (extrinsic) proteins - interact with the membrane less extensively being found on
the surface. They typically attach on either side of the polar heads of the phospholipids
through typically hydrogen or ionic bonds. The weaker interactions mean they can more
readily dissociate from the membrane.
Membrane proteins are difficult to solubilise and detergents are used to solubilise them.
Can be used to analyse the interiors of phospholipid bilayers. Freeze
the cell in water immobilising the cell. Fracturing then occurs causing
Freeze Fracture Electron fractures in the ice. These fractures occur in points of weakness on the
Microscopy protein. This usually occurs down the centre of the phospholipid
bilayer where the hydrophobic core is. A thin layer of metal is then
added allowing the sample to be viewed under a TEM.
FRAP (Fluorescence Recovery Live cell visualisation detects movement of lipids and proteins in
After Photobleaching) living cells.
The protein is marked with a fluorescent antibody.
This can be viewed under a microscope.
The cell should then be bleached with a lazer so they are no longer
fluorescent. Photobleaching - bleach with light so sample can no
longer fluoresce.
Fluorescence recovery - measure how and when sample recovers
fluorescent properties. Photo bleached molecules replaced by non
photo bleached molecules over time bc of diffusion/movement of
biomolecules.
Biological Membranes 2
The cell membrane creates a barrier between the outside and inside environment of the cell, is
involved in transport, signal transduction, and storage. Membranes are amphipathic molecules
which from lipid bilayers this is because the phosphate head is hydrophilic and fatty acid tails
hydrophobic. Amphipathic molecules have a hydrophobic and hydrophilic end. Lipids are
amphipathic. When it is placed into an aqueous environment, the polar region of the lipid tends
to interact with the water molecules while the non-polar region does not interact with water.
Micelles form when the hydrophobic fatty acid chains face inwards away from water forming a
hydrophobic centre, and the hydrophilic phosphate heads face outwards. This minimises the
polar-non-polar interactions and this stabilises the molecule. The hydrophobic effect drives this.
A liposome is an aqueous compartment surrounded by a phospholipid bilayer. It is formed by
mixing a lipid solution into an aqueous solution sonicating the solution. Sonication involves
bombarding the solution with sound waves. The energy carried by the sound waves disperses the
lipids allowing them to spontaneously aggregate into bilayer membranes. By mixing proteins
with detergent and then mixing them with phospholipids, liposomes can be built.
Detergents are amphipathic molecules which contain polar and hydrophobic groups. In aqueous
solutions, they form organised structures known as micelles.
Membrane Fluidity
Membrane fluidity is controlled by fatty acid composition and cholesterol content. The relative
movement of lipids determines the strength of the interactions in the membrane. As the
temperature is increased there is a sharp transition from the rigid state to the fluid state. Phase
transition between two states occurs at Tm (melting temperature). Tm depends on fatty acid
chain length, number of unsaturated fatty acids, and amount of cholesterol present.
Longer fatty acids can form more Van der Waals forces therefore the presence of them
decreases the fluidity and increases the melting temperature (Tm).
Saturated fatty acids create a well-structured arrangement of hydrocarbons with the straight
chain forming stronger intermolecular forces raising Tm.
Unsaturated fatty acids have kinks in the chain which interfere with the well-ordered
structure favouring membrane fluidity raising Tm.
Biological Membranes 1
, Cholesterol inserts itself between phospholipids. The OH group of cholesterol aligns with
the polar head groups of lipids. The rigid steroid ring partially immobilises the hydrocarbon
chain at the point closest to the head. At high concentrations, this prevents the hydrocarbon
chains from coming together. This blocks phase transitions. Cholesterol forms complexes
with glycosphingolipids which form lipid rafts making the membrane less fluid and more
resistant to phase changes raising Tm.
Proteins In Membranes
Integral (intrinsic) proteins - bind to the membrane extensively and span the width of the
bilayer. The portion of the protein within the core interacts with van der waals forces in the
hydrocarbon tails of the phospholipids.
Integral membrane proteins have one or more domains embedded in the lipid bilayer. They may
have single or multiple transmembrane (TM) domains.
Peripheral (extrinsic) proteins - interact with the membrane less extensively being found on
the surface. They typically attach on either side of the polar heads of the phospholipids
through typically hydrogen or ionic bonds. The weaker interactions mean they can more
readily dissociate from the membrane.
Membrane proteins are difficult to solubilise and detergents are used to solubilise them.
Can be used to analyse the interiors of phospholipid bilayers. Freeze
the cell in water immobilising the cell. Fracturing then occurs causing
Freeze Fracture Electron fractures in the ice. These fractures occur in points of weakness on the
Microscopy protein. This usually occurs down the centre of the phospholipid
bilayer where the hydrophobic core is. A thin layer of metal is then
added allowing the sample to be viewed under a TEM.
FRAP (Fluorescence Recovery Live cell visualisation detects movement of lipids and proteins in
After Photobleaching) living cells.
The protein is marked with a fluorescent antibody.
This can be viewed under a microscope.
The cell should then be bleached with a lazer so they are no longer
fluorescent. Photobleaching - bleach with light so sample can no
longer fluoresce.
Fluorescence recovery - measure how and when sample recovers
fluorescent properties. Photo bleached molecules replaced by non
photo bleached molecules over time bc of diffusion/movement of
biomolecules.
Biological Membranes 2
