Therefore, the presence of longer fatty acids decreases the fluidity, makes the membrane more rigid and increases the melting temperature because we have to input more energy to break those lung dispersion forces than in this particular case. Now, what about the double bonds? So let's compare. We have this system in which we have four adjacent phospholipid molecules in which we don't have any double bonds. And such a system is said to be a saturated system. Now, let's suppose we compare this to this case in which we have these for phospholipids that have the same exact length.