Hi ALeksandar. I could not imagine a cell membrane with a highly rigid structure, may be the more flexibility by means of less saturation in their structure could help cell membrane.

On the other hand, it could be that unsaturated and saturated lipids have differences of solidification at lower temperatures, so the membranes should be dinamic, and more Saturated fats the more solid they are are room temperature. May be it helps you to think about it in some way, or to give you some clues.

May be a real expert on this matter could help better than me.

greetings.

Hi Aleksandar, "why" is a always good question to ask, but is very difficult to answer satisfactorily.

My attempt of an answer is that the cell membrane acts like a fluid that accommodates proteins that float on it like icebergs.

Lipids in the membrane have to be flexible. Methylene groups between double bonds in the fatty acid chain afford some flexibility. Plus conjugated double bonds in fatty acids are more likely to be oxidized.

This my attempt to answer "the why". It is probably unsatisfactory.

Hi Javier and Steingrimur,

Thank you for your answers. This is exactly what I suspected but wasn't sure about it. Is there any literature that addresses this question? Specifically why aren't there conjugated double bonds in phospholipids.

Best,

Alex

Hi Michal,

Thank you for your answer and the article you suggested. I was aware that conjugated lipids can incorporate into biological membranes, but I am more interested in the physico-chemical "exclusion mechanism" , for example, can a liposome be made entirely of conjugated amphiphatic lipids and function like a biological membrane? Also, there are examples of carotenoid molecules (which are heavily conjugated) incorporated into the lipid bilayer of thylakoid membranes and they influence the physico-chemical properties of the membrane making it less fluid.

But my question is: is it physically and chemically possible for the membrane to be entirely made of conjugated lipids and still function like a normal biological membrane? My sense is that this is not possible because of lack of conformational isomerism in conjugated systems but I want to read other people's opinions on this subject.

Best,

Alex

Hi Aleksandar, personally I think it is improbable to have a non-flexible phospholipid membrane.

But, you probably know about lipid rafts, which I think is nature's way of making patches of different lipid compositions to accommodate proteins of specific functions.

http://www-3.unipv.it/webbio/anatcomp/freitas/2012-2013/Simons2000_rafts_sign_transduc_FIGUREs.pdf

Well, Aleksandar, your question "is it physically and chemically possible for the membrane to be entirely made of conjugated lipids and still function like a normal biological membrane" is quite ill-defined because you do not define what you mean by "function like a normal biological membrane". If you mean that it should be able to allow a cell to continue living as normal, then almost certainly not. Most likely a cell whose all lipids have conjugated double bonds will die, e.g. due to oxidative stress, the failure to optimize the lipid environment requirements of different membrane proteins, and the failure to provide essential (non-conjugated double bond PUFAs) to cells. After all, the conjugated double fatty acids are cytotoxic (Article Cytotoxic effect of conjugated trienoic fatty acids on mouse...

I suspect that the reasons related to the chemical reactivity account for the rarity of conjugated double bond fatty acids and for the reason that there is very little evolutionary sense in making lots of conjugated double bonds. After all, a key point for introducing double bonds into lipids is to introduce kinks that decrease the chain melting temperature of membrane lipids, so one can have thicker membranes that nevertheless remain fluid at temperatures where proteins do not denaturate. So, except perhaps for some poorly know special purposes, there probably are very few reasons to make more rigid conjugated double bonds, that probably behave with respect to melting a lot like saturated fatty acids but are far more reactive chemically, including susceptible to oxidation.

But you were probably not deferring to actual biological functions of a biological membrane, since you are talking about a liposome membrane functioning like a biological membrane. So what did you mean?