Lipids and Membranes 5 Flashcards
How does chemical potential vary?
In equilibrium the chemical potential of all identical molecules in different aggregates is the same.
As molecules move, react, dissolve, melt, they will tend to move from high chemical potential to a lower one.
Chemical potential always aims to minimise itself (reach equilibrium).
A ____________ concentration is required for double chain lipids to self-assemble into micelles.
A very low concentration is required for double chain lipids to self-assemble into micelles.
Aggregates form when there is a difference in…
…the cohesive energies between the molecules in the aggregated and monomeric (not aggregated) states.
If the interaction free energy per molecule in aggregates of size N is less than the interaction free energy per monomer, then an aggregate of size N would be energetically more favourable than monomers (aggregation would occur).
What is the critical micelle concentration?
The minimum concentration at which aggregates start to form.
What determines the physical properties of aggregates (e.g. size and size distribution)?
The functional form of the variation of the interaction free energy (or chemical potential) per molecule in aggregates of size N with N.
What determines the preference for the size of aggregates?
The aggregate size for which the interaction free energy (or chemical potential) per molecule is the lowest. If the chemical potential of molecules (proportional to the interaction free energy per molecule) in aggregates of size N has a minimum at a finite value of N, then aggregates of size N would be the preference and multiple aggregates of this size would spontaneously arise.
What are the major driving forces involved in the self-assembly of amphiphiles into ordered structures?
1) The hydrophobic attraction which occurs at the hydrocarbon-water interface and induces the molecules to aggregate (entropic effect due to breakage of H bonds).
2) The hydrophilic, ionic, or steric repulsion of headgroups.
These two interactions act in opposite senses one trying to decrease the interfacial region whilst the other tries to increase it.
Which variable of a lipid molecule leads directly to the spontaneous curvature or preffered curvature of a membrane?
The packing parameter (a.k.a. the shape factor)
Why are most bilayers perfectly flat despite not having a packing parameter = 1?
The monolayers have slight curvature but each leaflet has opposing curvature that cancel each other out.
What are lipid rafts?
The less fluid, more stable phases in cell membranes in which proteins are located?
Describe the cell membrane phases at low temperature and high temperature.
At low temperatures, lipid chains are in an ordered gel phase with an all-trans (straight tails) configuration. In this gel phase, the lipids have high density, highly ordered extended chains, and low mobility.
At high temperatures, lipid chains are in a disordered fluid phase due to rotations around the C-C bonds within the lipid chains. In this fluid phase, the lipids have lower density, higher moblity, and are less well ordered.
What determines the melting temperature of a lipid molecule?
An increase in carbon chain length (number of carbon atoms) causes an increase in melting temperature.
Whereas an increase in the number of double bonds decreases the melting temperature.
What is the role of cholesterol in a cell membrane?
Cholesterol induces ordering of lipid acyl chain tails but retains the liquid-like structure in the place of the bilayer: a liquid-ordered (liquid crystal) phase.
High cholesterol = Liquid-ordered state
Low cholesterol = Liquid-disordered state
Cholesterol moderates the structural order in both liquid and solid phases.
Adding cholesterol to cell membrane causes the properties of liquid and solid phases to converge. It increases chain order of the liquid phase but decreases chain order of the solid phase.
How does cholesterol perform its function?
Cholesterol inserts into cell membrane and prevents the free motion of the chains in liquid phase, ordering the liquid phase. As the temperature decreases, the cholesterol (which can make up to 30% of cell membrane) gets in the way of packing (weakening van der Waals interactions between chains) so chains cannot get close enough to solidify, disordering the solid phase.
