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Thie picture below shows that the phospholipids phosphatidylserine (PS) and Phosphatidylethanolamine (PE) are more likely to be found on the inside of cell membranes than on their exterior. Distribution of phospholipids in cell membranes

Why is this so? What prevents them from creating a shape that is wave-like (illustrated) or some other shape?

Alternative membrane structure

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  • $\begingroup$ Welcome to SE Biology. I have edited your question slightly, especially to define abbreviations. Please finish reading the Tour as it will help you understand how this site operates. $\endgroup$
    – David
    Commented Apr 8, 2022 at 22:26
  • $\begingroup$ I’m not sure if this would help answer your question, but in living organisms the lipid composition of each membrane leaflet is highly regulated. A flippase translocates PS and PE to the cytoplasmic side of the membrane. $\endgroup$
    – canadianer
    Commented Apr 13, 2022 at 0:03

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There seem to be two questions here.

The poster seem to know the answer to the first question — that in the title — that their shape fits into one or other side of a curved membrane. So I would recommend the title be edited to address what appears to be the main concern.

Instead of “Why do these phospholipids insert into one side of an already spheroidal cell membrane?” the concern appears to be the effect of the insertion on the shape of the membrane: “Why doesn’t the insertion of these phospholipids cause a wave (ruffled rug) shape, rather than a spheroid?”

If this is, indeed, the question then the answer would be that the ‘empty spheroidal’ shape of cell membranes is determined by other forces, principally thermodynamics. The main one is that which causes a lipid bilayer in the first place — the polar head being oriented to the aqueous environment, whereas the hydrophobic tails are in contact with one another. This still leaves hydrophobic “edges” to any membrane sheet, which therefore fuse into a hollow spheroid to remove them from contact with water.

So, although I am not a cell biologist, I would hazard that the wedge-shaped phospholipids help relieve bending stresses in a membrane that has already adopted a spheroidal shape for thermodynamic reasons.

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