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Nov 21 2017 01:12pm
Hiho,

The Professor told us that there is an asymmetrical distribution of (among others) phosphatidylcholine, -serine, -inositol, -ethanolamine, sphingomyeline between the cytosolic leaflet and the exoplasmatic leaflet.
Indeed, -choline is almost exclusively found in the outer leaflet; -serine is, in a steady state cell, exclusive to the inner leaflet; -inositol and -ethanolamine are exclusive to the cytosolic leaflet. Choline is found in the inner leaflet only as an intermediate, that is, it is on its way to being flipped over to the outer leaflet.

Next, the Professor tells us that while the plasmamembrane has an asymmetrical distribution, the endoplasmatic reticulum membrane has a symmetrical distribution of the phospholipids.
This is in contrast with the textbook: it says that the endomembrane has a translocator for -choline, but not for anything else: therefore, the inner leaflet of the endomembrane exists without the presence of -serine, -inositol, -ethanolamine

Can anybody tell me more about the inner- and outer leaflet of the endomembrane?


Rik
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Nov 21 2017 02:41pm
there's also cardiolipin and several other lipids present (since it's a first year cell bio course I doubt he will teach you about the other various lipid structures), as well as enzymes like flippase that allow lipids to transfer between the cytosolic and extracellular side. What specifically did you want to know about the inner leaflet?
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Nov 22 2017 02:03pm
I know of other lipids, I'm concerned about the distribution of just the phospholipids. I am aware of the presence of flippase (which is a translocator).
I'm looking for clarification about why I hear of a symmetrical distribution for phospholipids on one end while I read that they are quite asymmetrically distributed on the other end (and this is due to the presence of a translocator for only phosphatidylcholine, locking out the other phospholipids)
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Nov 22 2017 09:46pm
fundamentally it's a membrane thermodynamics and kinetics principle, as well as the leaflets arranging themselves in a way that is conducive to receptor/channel/pore expression. From an entropy standpoint, lipid symmetry is a much more favorable state than asymmetry. In areas of asymmetry, this is where you will have components like receptors/channels/pores, enzymes, and differing lipids.
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Nov 26 2017 02:12pm
Having symmetry obviously benefits the cell's energy state but achieving that symmetry isn't an energy-free process (hence the existence of translocators, or flippases (and scramblases in some not-so-steady state cells)).
Oh well, I fed the question to a teacher's assistant and she forwarded it to the professor. Who knows, I might become a little smarter than the rest :p
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Nov 28 2017 03:47pm
The professor answered with a review from early 2017 that states there are non-selective translocators within the endoplasmatic reticulum.
#Answered
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