A simple microscopic model for amphiphilic molecules dissolved in water allows us to obtain within the same scheme the molecular structure of membranes and micelles, the equilibrium thermodynamics for these aggregates and the elastic constants of the membranes. We present a study of vesicles with cylindrical geometry which, joined to previous results for spherical vesicles, gives the saddle-splay modulus and the bending rigidity. Finally, the constraint on the exchange of molecules between the inner and the outer monolayers is included in the model, calculating the nonlocal bending curvature.