What would happen if the phospholipids in the phospholipid bi-layer were reversed, the fatty acid tails now facing outwards and the phosphate heads facing inwards? I'm assuming this will not affect the protein channels, but perhaps the loss of cholesterol in the structure of the bi-layer. Would this then mean that the fluid mosaic model no longer holds?
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$\begingroup$ Why should protein channels not be affected ? Even if they are not reversed, environment around them will change which will have a disastrous effect. $\endgroup$– biogirlDec 19, 2013 at 9:18
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1$\begingroup$ I guess that it true, but the point here is to make the inference in the bi-layer structure after the reversal. What would it's characteristics be, and how would it's functions be altered? $\endgroup$– CloudDec 19, 2013 at 9:22
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$\begingroup$ DO you mean to ask that if we assume nothing in the proteins change(eventhough that isn't realistic), what will be the impact of reversal of the bilayer ? $\endgroup$– biogirlDec 19, 2013 at 9:27
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$\begingroup$ For a start, let's just not consider the protein channels unless absolutely necessary. $\endgroup$– CloudDec 19, 2013 at 9:34
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$\begingroup$ its easy to do this by the way -just mix the lipids into oil $\endgroup$– shigetaAug 13, 2015 at 1:29
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3 Answers
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This would have quite dramatic consequences. The layers are ordered in the way they are, because of their polarity. In the way they are ordered, the hydrophobic tails are inside and directed towards each other, the hydrophilic heads are orientated to the outside and inside. Since both sides of the membrane are surrounded by aqueous solutions, this is necessary to allow a contact between solution and cell membrane and to allow an exchange of molecules between them. If the layers would be oriented the other way, this contact and exchange wouldn't be possible. Proteins channels in the membrane wouldn't be possible as well, since the intermembrane domains are composed preferrably of amino acids with hydrophobic sidechains, while the domains on the outside of the membrane contain more hydrophilic amino acids. A turn like this would need a completely different composition of life - meaning it couldn't be based on water like it is.
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$\begingroup$ Your answer is sufficient to an extent, @Chris, it implying that the fluid mosaic model is made no longer possible. However, I do not understand why the phosphate heads point outwards while the fatty acid tails point inwards if there was to be diffusion of lipids into the cell. It is conventionally known that lipids dissolve into the phospholipid bi-layer, but how would this be possible as there are phosphate 'heads' present? Imagine a lipid wanting to diffuse into the cell, how will it pass through the phospholipid bi-layer if the phosphate heads are hydrophilic in the first place? $\endgroup$– CloudDec 19, 2013 at 10:21
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$\begingroup$ You have to think about how fats (or fatty acids) travel through the blood stream: As small droplets (or micelles) which are organized in the same way: Hydrophilic part outside, hydrophobic part inside. They can fuse with membranes (or are actively taken up) and can so be transported to the cells. $\endgroup$– Chris ♦Dec 19, 2013 at 10:47
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$\begingroup$ Can you explain how fatty acids travel through the bloodstream or provide a source where I can read further on this matter? Are you referring to 'blood lipids'? $\endgroup$– CloudDec 19, 2013 at 11:07
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$\begingroup$ A nice source is the Wikipedia article on blood lipids. $\endgroup$– Chris ♦Dec 19, 2013 at 11:10
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$\begingroup$ I may have misheard previously that lipids "dissolve" into the membrane. Is it sufficient to say that they fit through the gaps in the Phospholipid bi-layer, therefore not repelled by the phosphates? Do protein channels come into affect this process at any stage? $\endgroup$– CloudDec 19, 2013 at 11:44
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Someone has thought this was a very good question and performed an MD simulation on spontaneous bilayer assembly. There, lipids start in random orientations. The ordered bilayers we know and love spontaneously assemble in under 100ns.
So if the lipids were jumbled up (or even reversed), the would probably reform fairly quickly. I wouldn't imagine that it would do the cell much good though...
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If the layer is opposite,then there wont be any cytoplasmic liquid(cytosol) inside the cell, as tail is Hydrophobic.If there is no cytosol,then no function of the cell.Even it becomes difficult to pass substances through the cell when layer is different.
