Thermodynamics treats with macroscopic systems, where statistical mechanics make sense. In these systems there is an arrow of time following the second law. On the contrary, temporal reversibility is generally seen at microscopic level. What is the limit (magnitude order in number of molecules or other) where a physic system must be seen from a thermodynamic viewpoint? One isolated live cell would be a thermodynamic system: it ages. That also occurs in subcellular systems as mitochondria. But, what's about a discrete sample of cell macromolecules? Make it sense to talk on thermic differences inside a live cell? ¿When a microscopic system should be named macroscopic and to be object of thermodynamics?