If networks of chemical reactions are the circuits of biology then catalysts are the switches. But which species should be called catalysts? Chemistry textbooks answer this question when there is a single reaction. For an entire network of reactions , the concept becomes more nuanced, and has been worked out in a recent paper (arXiv:1006.3627). I will discuss this notion of catalysis for reaction networks, and illustrate it with an example from the seesaw gate of Qian and Winfree ( http://www.dna.caltech.edu/DNAresearch_publications.html#SeesawTheory ), which is a motif for engineering large reaction networks out of DNA molecules.
Prerequisites: You should know what a graph is, what a monomial is, what the greatest common divisor of two monomials is, and what DNA is.