last lecture on bistability! going to get examples.
assume should do assignment #3 after this.

should read: Ferrell (2002) Curr. Op. Cell Biol. 14:140-148

"lac operon" example is with lactose metabolism in E. Coli bacteria, where
presence of lactose can lead to gene expression for allowing even more lactose
into the cell. when this process starts, it is self-reinforcing and triggers on
(bistable)