Are there basic principles ruling cell metabolism beyond known physical laws? Is it possible to test them quantitatively? In this talk I will consider two of them: (1) the fact that cell metabolism could work in a “critical” state and (2) the notion that cell metabolism maximizes growth and/or in general has “objective functions” that are optimized under constrained resources. I will criticize these putative principles on the basis of my own work on the inference and measurements of metabolic fluxes. While the first is in contradiction with plain thermodynamics, the other is a useful working hypothesis that shall be revised in the light of unavoidable phenotypic heterogeneity within which a trade-off is established. Heterogeneity in fact endows inter-cellular interactions and in turn, means to control the environment. I will provide the experimental example of lactate shuttles among human cells in this respect.