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?
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.