Structure of Membranes and Forces

Question 1

Permeability of Biological Membranes

Answer 1

• the cell membrane is easily permeable to small hydrophobic and small uncharged polar molecules
• the membrane is impermeable to larger uncharged polar molecules and ions
• this impermeability to ions allows cells to generate electric potentials to store energy and drive processes

Question 2

Purpose of Membrane Proteins

Answer 2

• generate and regulate potential and chemical gradients
• signal transduction between interior and exterior of cell
• transfer of nutrients and metabolites

Question 3

How does transport occur across the cell membrane?

Answer 3

• passive diffusion / osmosis, very limited
• transmembrane protein channels and transporters, there are either passive or specific/selective
• -passive transporters e.g. aquaporins which just open or close to let water in or out
• -selective transporters transport particular proteins or pump protons/certain ions
• endocytosis, absorption by engulfing in a bulge in the membrane which is then pinched off

Question 4

Cell Wall

Answer 4

• the cell membrane is a chemical barrier but is not very robust, this is the purpose of the cell wall
• the cytoskeleton provides physical strength and the cell membrane contains anchor points for the cytoskeleton
• it controls remodelling of the cytoskeleton and cell movement

Question 5

What is the difference between fat and oil?

Answer 5

• fat is just solid-like oil, there is no compositional difference
• they are both essentially just hydrocarbon chains

Question 6

What is a fatty acid?

Answer 6

• a carboxyl (-COOH) group attached to the end of a hydrocarbon chain
• unsaturation introduces double bonds which reduces flexibility and lowers the melting point

Question 7

Lipids

Answer 7

• ambiphilic molecules composed of a hydrophilic, polar head and hydrophobic hydrophobic hydrocarbon tails
• unsaturation in these tails leads to kinks
• there are 100/1000s of lipid types with different tail lengths, head shapes etc.

Question 8

What happens when lipids are placed in an aqueous environment?

Answer 8

• when placed in an aqueous environment, lipid molecules minimise free energy by organising such that polar head groups are exposed to the water and the hydrophobic tails are shielded
• this creates a liposome/vesicle

Question 9

Forming a Bilayer and Entropy

Answer 9

• forming a bilayer leads to a significant loss of entropy of the lipid molecules so there must be a corresponding increase in entropy somewhere else
• the driving force is the entropy of the solvent molecules
• grouping solute molecules can increase the entropy of solvent molecules

Question 10

Principles of Self Assembly

Answer 10

• in general, hydrophobic tails interact weakly via the van der Waals force and polar head groups containing dipoles and/or charges interact electrostatically or via hydrogen bonding
• the presence of these materials in water also leads to hydrophobic interactions
• the balance of these forces and molecular geometry are what determine type of structures that will spontaneously form

Question 11

Why do most liquids not mix?

Answer 11

-considering a simple lattice model for two solutions, in order for them to mix some of the molecules from liquid A have to swap places with molecules from liquid B
-the energy for mixing should be equal to the energy of interaction between A and B, minus the energy of the interactions A made with A and minus the energy of the interactions of B with B, plus the entropy of mixing
ΔGmixing = ΔHmixing - TΔSmixing
ΔHmixing = 2ΔH_(a-b) - ΔH_(a-a) - ΔH_(b-b)
-this predicts that tendency to mix should increase with T, and the degree of mixing should depend on the enthalpies of mixing i.e. the strength of the bonds broken and formed

Question 12

Driving Force of Self Assembly

Answer 12

• minimisation of free energy
• forming a bilayer results in a significant loss of entropy of the assembling molecules so other changes in the system must contribute for an overall decreases in free energy
• however enthalpic interactions are not dominant (little if any heat flow)
• the driving force is the entropy of the solvent molecules, grouping solute molecules can increase the entropy of solvent molecules
• the entropic driving force due to a gain in entropy of the water molecules is the hydrophobic force

Question 13

Hydrogen Bonding in Water

Answer 13

• unusual properties of water arise mainly from 2 factors:
• -water molecules readily form hydrogen bonds
• -water molecules for 4 hydrogen bonds in a tetrahedral geometry leading to the formation of 3D hydrogen bond structures
• whereas in liquid water a typical hydrogen bond has a lifetime of ~1ps, in the solid phase (ice), hydrogen bonding leads to stable, open tetrahedral networks

Question 14

Hydrophobic Effect - Small Solutes

Answer 14

• microscopically speaking, hydrophobic molecules are NOT repelled by water molecules in fact H-C molecules often interact more strongly with water via van der Waals than each other
• it is just that water molecules have an even stronger mutual interactions
• non-polar solutes can be though of as regions in water where hydrogen bonding cannot occur
• however if the solutes are small enough, a hydrogen bonding network may still form around them with a layer of water molecules with significant orrientational correlations around the solute i.e. a decrease in configurational entropy

Question 15

Hydrophobic Effect - Large Solutes

Answer 15

• for larger, non-polar solutes, an intact hydrogen bonding network around the solute cannot be maintained
• about 1 hydrogen bond per molecule near the surface of the solute is sacrificed and the water-solute interface is shifted slightly away from the solute surface
• due to the fact that under standard conditions, the free energy cost for forming a water liquid-vapour interface is small enough (compared to kT), there exists a thin layer of vapour-like water around the solute
• the free energy of solvation, ΔG, for this case is mainly due to enthalpy instead of entropy and the effect is prorpotional to the surface area of the solute NOT the volume

Question 16

What is the hydrophobic force?

Answer 16

-believed to be an entropic effect caused by the energy cost of the disruption of many hydrogen bonds by a non-polar molecule hence disrupting the quasi-ordered water structure

Question 17

Membrane Lateral Pressure

Answer 17

• steric and electrostatic repulsion between head groups
• interfacial tension between hydrophilic and hydrophobic regions
• repulsion between lipid acyl chains

Question 18

Hydrophobic Force vs Temperature

Answer 18

-free energy transfer of a non-polar compound from a reference state to organic solution e.g. in water:
ΔGtr = ΔHtr - TΔStr
-at room temperature, enthalpy of transfer is negligible, entropy is negative since water tends to form ordered cages around the non-polar molecule
-at high temperatures, these cages are no longer stronger than bulk water and the entropy contribution tends to 0, the enthalpy of transfer however is positive (unfavourable)
-because the temperature dependence of enthalpy and entropy aren’t the same, there exists some temperature at which the hydrophobic effect is maximum

Question 19

List the Current Hydrophobic Force Theories

Answer 19

• vapour bridges
• water structure models
• electrostatic models

Question 20

Hydrophobic Force - Experimental Observations

Answer 20

• initially experiments detected a hydrophobic force range out to 100nm, massively further than Lifshitz-van der Waals forces or entropic forces
• some of this was attributed to experimental error
• more recent experiments have consistently settled on a range of ~10nm, which is still far in molecular terms

Question 21

Hydrophobic Force Theories

Vapour Bridges

Answer 21

• hydrophobic patches pick up air bubbles that bridge two approaching surfaces resulting in a strongly attractive capillary force
• there are any problems with this theory

Question 22

Hydrophobic Force Theories

Water Structure Models

Answer 22

• most complete theory yet

- but only explains the force up to a few nm

Question 23

Hydrophobic Force Theories

Electrostatic Models

Answer 23

• charge / dipole correlations
• relies on peculiar dielectric and long range proton hopping properties of water
• associated energy and entropy can extend over much larger distances than molecular correlations

Question 24

Importance of the Hydrophobic Force in Biology

Answer 24

• strong long-range force
• main interaction responsible for stabilising surfactant micelles and biological membranes
• may also explain protein folding

Question 25

Gibbs Free Energy Equation

Answer 25

ΔG = ΔH - TΔS

Question 26

Chemical Potential

Description

Answer 26

• the potential energy due to chemical bonds in a system
• this energy can be absorbed or released during a phase change or a chemical reaction
• a measure of how easily a chemical reaction will take place
• at equilibrium between phases, the chemical potential of each phase is the same
• as molecules move/react/dissolve/melt they tend to move from a higher chemical potential to a lower chemical potential

Question 27

Chemical Potential

Formal Definition

Answer 27

µ = ∂G/∂Ni

• i.e. chemical potential is the Gibbs free energy per particle
• the chemical potential of a system is the amount by which the gibbs free energy of the system would increase if one extra particle was added to the system with energy and volume fixed

Question 28

Difference Between Chemical Potential at Two Different Concentrations

Answer 28

µ(n1) - µ(n2) = RTln(n1/n2)