Lipid composition influences a membrane’s fluidity, from gel-like to more fluid. This is in addition to other means of making a membrane more fluid, including heating. Sterols increase the rigidity of the membrane, due to their shorter and fatter structure. This can cause problems, such as atherosclerotic plaques in arteries leading to circulatory issues, however cholesterol (a common lipid in the brain) is important in brain tissue, making up $$\frac{1}{2}$$ of the lipids present. Sphingolipids generally have saturated fatty acid tails, conveying a more ordered structure the membrane and thus making it more fluidic. Phospholipids are generally found with unsaturated fatty acid tails, increasing the disorder of the membrane and making them more gel-like. Membrane rafts are formed of more rigid and structured areas of lipids, allowing for variation in the fluidity of the membrane within a single cell. Different cell types will have different lipid composition (such as brain cells having more cholesterol than epithelial, for example). Membrane composition can be different between the two leaflets, with an ATP-dependent flippase catalysing the transfer of lipids. This allows for control of membrane curvature (due to different lipid head-group sizes, or proteins inserting into the different leaflets).
Sterols: increase rigidity, due to short, fat structure Sphingolipids: generally saturated Phospholipids: mainly unsaturated, more disordered structure Fluidity changes to allow transport Rafts: more rigid areas