Organisms have evolved a tremendous diversity of different lipid molecules that they utilize to construct their membranes. Lipids control the physical properties of membranes, such as their viscosity, bending modulus, or ability to phase separate. Organisms ranging from bacteria to deep sea animals regulate lipid biosynthesis in order to maintain the physical properties of membranes in response to low temperature or high hydrostatic pressure. I will present recent experiments in which we have used synthetic biology approaches to understand what cellular functions are controlled by one such property, membrane viscosity. This work has yielded an unexpected link between membrane lipid composition and cellular respiration, which carries implications for the distribution of lipids between and within organisms.  I will then present recent data on how highly fluidizing lipids mediate the behavior of an animal model system (the fruit fly) and its interactions with its primary food source (yeast).