Models of biochemical reactions make different assumptions about how molecules interact. At low concentrations (a) each type reactant interacts with an reactant independently of other particles. The “many-body” problem in (a) can be reduced to a single pair representation (b). A more realistic assumption is that the particles interact with each other (dashed lines), as in (c). This can be accounted for by assuming that either (d) the interaction in the reduced pair representation is modified or (e) the diffusion constant depends on the local concentration. (f) The true distribution of macromolecules and reactants inside cells involves many types of molecules.