Figure 12.

Relaxation of leaflet tensions in a lipid bilayer which is composed of two phospholipids and cholesterol (orange): The top row shows a bilayer membrane with two lipid components (blue and red) that do not undergo flip-flops from one leaflet to the other. The bilayer is tensionless in the sense that the bilayer tension $\Sigma ={\Sigma }_{ll}+{\Sigma }_{ul}$ is (close to) zero. However, the upper leaflet of the bilayer is compressed by a negative leaflet tension ${\Sigma }_{ul}<0$ whereas the lower leaflet is stretched by a positive leaflet tension ${\Sigma }_{ll}>0$, as indicated by the schematic springs on the left and on the right of the bilayer. As a third component, cholesterol (orange) is added to both leaflets so that they initially contain the same number of cholesterol molecules, as depicted in the middle row. After the cholesterol has been redistributed by flip-flops, both leaflets have attained a tensionless state as indicated by the relaxed springs. The cartoon at the bottom also indicates that the two tensionless leaflets typically differ in the preferred areas that they would assume in a symmetric bilayer [27].