lipids are arranged within the membrane with polar head towards the outer side and non polar tails towards inner side, this ensures that the non polar tail is protected from aqueous environment.

My question is why should we protect non polar part ,will it destroy in contact with polar part? What should be the correct reason for bilayer arrangement?

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    $\begingroup$ Where did you get that quote from? As I mention in my answer, I dislike using the term "protected". It's misleading. Furthermore the quote also uses "inner" and "outer" in an unhelpful way as "inside" and "outside" often refer to compartmentalisation of the cell/organelle/vesicle/thing with a membrane. For example if something is on the inner leaflet of a eukaryotic cell, it's on the cytoplasmic side. The quote in the question would make that sound like it was deep in the membrane. $\endgroup$ – James Jun 3 '16 at 8:27
  • $\begingroup$ The issue here is the word protected I think. There is no intentional protection going on, simply the tails are repelled from water (but not each other) and the heads are attracted to water. $\endgroup$ – Troyseph Jun 3 '16 at 11:32

What should be the correct reason for bilayer arrangement?

I'll answer your second question first, but there is an almost identical question on this site already: Why do cells have a bilayer?

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There is water on the extracellular and intracellular side of the membrane. What's actually happening at a molecular dynamics level is the self-association of the hydrophobic lipid tail groups driven entropically by water. In other words the polar (hydrophilic) head-groups "prefer" interacting with the water (called the interfacial region) and the the hydrophobic tail groups "prefer" not interacting with the water. With those two preferences in play, the lipid bilayer formation we know and love emerges.

why should we protect non-polar part, will it destroy in contact with polar part?

To directly address the first part of the question: no, nothing would be destroyed. The word "protect" isn't appropriate (it's a bit too anthropomorphic for my taste!). Here is a video showing the bilayer spontaneously assemble in a molecular dynamics simulation. Read the more thorough 2003 journal article for an idea of early MD simulations of the bilayer formation. As you can see nothing "bad" happens when the water collides with the lipid tails and the lipids aren't destroyed.

Interesting read: MEMBRANE LIPIDS OF THE PAST AND PRESENT. Good animations and explanations of different membrane formations.

For an academic perspective, I'd recommend a couple of reviews: Cournia et al., 2015 and Gerit et al., 2008.


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