In biological phenomena like osmosis, the rate of flow of water molecules in or out of biological compartments depends on the solute concentration and on other forces, like hydrostatic pressure. A similar example is the passive transport of ions in and out the cell membrane. In this paper, we address the problem of faithfully modelling these kind of phenomena with an adequate process calculus. We enhance the ambient calculus stochastic semantics with functional rates, which are calculated by taking into account the volume of ambients and the surrounding environment. A model of osmosis in plant cells will be used as an example to show the new features of our calculus.