Assuming the cell membrane to have a spherical shape, geometry tells us that the area of the inner leaflet is smaller than the area of the outer due to the difference in radius between them. Does this mean that the inner layer has fewer phosholipids? And, if so, how does it affect the fluidity relation between the layers?

  • $\begingroup$ The protein content and lipid type in each layer is likely to have a bigger impact on the number of phospholipids than anything. $\endgroup$
    – canadianer
    Mar 1, 2015 at 16:53

1 Answer 1


Yes there is a difference in the number of phospholipid molecules because of the curvature. This is very evident when there is a lot of bending as in case of exocytosis (see here) but otherwise I do not think there would be an enormous difference in the number of phospholipids between inner and outer surface of the bilayer.


  • Cell is spherical
  • Cell diameter = 1µm (overall)
  • Thickness of bilayer = 6nm
  • Width of a phospholipid molecule = 1nm

Difference in circumference of outer and inner layers = 2π(6nm) ≅ 37.7nm

So approximately 37 more phospholipid molecules in the outer layer in this cross section of the cell. This is just 0.6% of total number of phospholipids in the outer membrane.

I don't think fluidity will be compromised because of this - only if a layer is overpacked will the fluidity be affected.

  • 2
    $\begingroup$ Don't you have to look at the total area instead of the radius? $\endgroup$
    – March Ho
    Mar 1, 2015 at 17:08
  • $\begingroup$ @MarchHo yeah.. I have to calcuate total area instead of circumference.. just a simplification. I should mention cross section. $\endgroup$
    Mar 1, 2015 at 17:24
  • $\begingroup$ These assumptions are sensible for vesicles, but I'm not sure if they translate well to cells. Have you any evidence that this is true for cells rather than vesicles? $\endgroup$
    – James
    Mar 2, 2015 at 12:38
  • $\begingroup$ @GoodGravy I guess cell is nothing but a huge vesicle. But I did say that this will matter the most in case of vesicles. Yes I agree that there are membrane proteins and cellular shape is not uniform. I can only extrapolate based on these assumptions. I don't think there are any studies for whole cell. $\endgroup$
    Mar 2, 2015 at 13:30

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