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Insane in the membrane, part 2:  Lipid bilayers

Page history last edited by Ben Dreyfus 4 years, 2 months ago



How did life originate?


Many of the early models of life's origins proposed by biologists included as a crucial step the formation of proto-cellular compartments that could serve as distinct / discrete environments in which chemical reactions could take place. However, the exact structure and mechanism of their formation remained unknown.


In the 1960s laboratory experiments demonstrated that phospholipids could spontaneously assemble into bilayer membranes forming bacteria-sized containers (vesicles). Later experiments demonstrated that such vesicles could also form under simulated early-Earth conditions. Such experiments paved the way for a line of research investigating how these self-assembling membranes could have functioned in the evolution of living cells.


But how exactly does spontaneous membrane formation work?  What are the mechanisms that drive this process? It turns out that an understanding of the combined effects of energy and entropy can help us make sense of this phenomenon.


1.  Last week you explored the entropic contribution to hydrophobic interactions (the entropic reason for why oil and water don't mix).  With your group, write out an explanation for why oil and water don't mix that relies only on the entropic arguments you made last week. (This explanation is incomplete because it does not yet take energy into account.)




Phospholipids have a phosphate ion at one end, and the rest is a hydrocarbon chain (like oil).  The hydrocarbon end is the "hydrophobic" ("afraid of water") end.  The phosphate end is the "hydrophilic" ("water-loving") end.


2.  Gibbs free energy (ΔG = ΔH - TΔS) is a way of quantifying the (sometimes competing, sometimes additive) effects of both energy and entropy.  You can think about what happens when you put hydrophilic molecules into water, and what happens when you put hydrophobic molecules into water, as resulting from both an energetic contribution (in this case, electrical potential energy) and an entropic contribution.  Answer the following questions for the process of oil separating from water that we looked at last week:


  • What is the sign of TΔS for the system during the separation?  How do you know?



  • What interactions contribute to ΔH for the system during the separation? What is the sign of ΔH for each interaction that contributes to the overall ΔH? (Remember that ΔH is due to electrostatic interactions, so you are being asked to identify the various electrostatic interactions that one must consider.) 



  • It turns out that the quantitative value for ΔH upon separation of oil and water under standard conditions is quite close to zero! Why might that be plausible? 



  •  What is the sign of ΔG for the separation of oil and water under standard conditions? What does this tell us about the separation?




3.  As you know, not all substances behave like oil: some substances are soluble in water and dissolve quite easily.  Think about ions (such as phosphate, or sodium chloride, or whatever) that tend to be soluble in water, whereas oil is not.  Answer the following questions for the process of sodium chloride dissolving in water (the system is the water plus the salt):


  • What is the sign of TΔS for the system during the dissolving process?  How do you know?



  • What is the sign of ΔG? How do you know?



  • Can you find the sign of ΔH for the system during the dissolving process?  How do you know? 



  • How does this salt water system differ from the oil/water system?





4.  Using Gibbs free energy, explain what makes some molecules (or parts of molecules) hydrophobic and others hydrophilic. 





5.  Explain why changing the temperature of the system can determine whether a substance dissolves in water or does not.





6.  Putting all this together, explain how phospholipids can spontaneously self-assemble into a lipid bilayer.  Why this particular shape?  (Why not a monolayer, or a trilayer?)  Note that the individual phospholipid molecules are still free to move around within the bilayer, like a two-dimensional liquid; they're not bound together like a solid.




Ben Dreyfus, Vashti Sawtelle, Ben Geller, Julia Gouvea, and Chandra Turpen 1/30/12

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