Liquid-liquid phase separation of cell membranes exemplifies a biological system leveraging a physical concept to achieve a chemical end. Here, we show that yeast actively tune the transition temperature of their vacuole membranes to be close to the yeast's growth temperature, which implies that the membrane's proximity to the miscibility transition is important for the cell's function. Indeed, in yeast, demixing of vacuole membranes into large, micron-scale domains is correlated with cell survival through extended periods of low nutrients. In living cells and artificial systems alike, phase-separated membranes frequently have excess area (more membrane than is need to enclose the volume), which leads to patterns of dots or stripes.  A persistent open question in the field is what physical mechanisms give rise to these patterns. Here we show which aspects of current theories of pattern formation are supported by our data, and where opportunities lie for developing new models.