Mixing theory allows the use of partial differential equations to describe complex biological systems. This formalism makes it possible to model the development of a photosynthetic microalgae biofilm (communities of micro-algae agglomerated on a surface and entangled in a self-produced extracellular matrix) or the functioning of the large intestine. The purpose of the talk will be to introduce this formalism and then present these two applications. In each case, the context and issues will be specified. Eventually, the numerical schemes and simulations will be commented.