5) Mechanical Properties Of Biomembranes Flashcards
Thickness
Thickness is measure by x ray scanning 11nm and electron microscope 7nm
Em shows that all biological membranes have 3 layers
2 dark(dense) layers with 2nm thickness each 1 light layer between them with 3nm thickness
Elasticity
The ability of the membrane to support mechanical stress
Membranes are easily deformed if the area remains unchanged
Parameters:
Area compression modulus
Bending modulus
Ultimate strength
Membranes stretch by less than 2% before they tear up
Higher pressure destroys membrane faster
Increasing internal pressure changes the shape of the cell but area doesn’t change
Viscosity
Membranes are regarded as 2D liquids. Can be measured using stokes equation
n=KT/6piDr
d is diffusion constant
r is radius of diffusing molecule
Diffusion constant depend on the microscopic structure of the membrane
Permeability
Biomembranes are
Permeable to water
Poorly permeable to polar substances
Impermeable to small charged particles and ions
Lipids molecular structure
Drawing showing
Hydrophilic head
Hydrophobic tail
Fatty acid tail
Phosphate attached to glycerol and 2 fatty acid tails
Electric charge of polar head
Some lipids are zwitterions as they can carry one positive and one negative charge but have no overall net charge
Structure of non polar tail
Chain of around 14-22 carbon
Can be saturated no double bonds
Unsaturated means double bonds present
Amphiphilic properties of lipids
Molecules that are both hydrophobic and hydrophilic are called amphiphilic
Lipids have highly polar charged heads that is covalently bonded to a long hydrophobic tail that’s not polar
Arranged to minimise the energy of interactions between them head face water and tail face each other
Lipids on the water air interface
Lipids form bilayer that have the heads forming the internal and external faces of the membrane
At physiological temperature lipids are in a mesomorphous state in which the structure is ordered and has fluidity
The mesomorphous state of lipids
Lipids change state by heating
Solid- mesomorphous- liquid
They’re stable in a limited temperature ranges
Phase transition temperature
The characteristic temperature at which lipids melt from solid gel to mesomorphous liquid state
This depends on the length of the fatty acid chain and number of double bonds if the tail is unsaturated
DRAWING
States of lipids in biomembranes
Changing the temp causes shift in the ratio between gel and mesomorphous states that leads to changes in the mechanical properties of the membrane
Confirmations of saturated fatty acid chains
Different conformations are obtained by different rotations around the c-c bond
Trans and gosh are 2 possible types
Kinks and kink diffusion
Bend in the fatty acid chain caused by 2 gosh conformations
Kinks form cavaliers where other molecules can be trapped. Shorten the chain by 0.13nm
Kink can move along the chain and corresponds to a shift of 0.13nm
