An unsolved problem is how do cell receptors know when a concentration of nutrient or toxin is outside the cell? We have discussed quorum sensing in bacteria, in relation to infection. Yeast need to sense the presence of a mate.-This article discusses the problem: -http://sciencenow.sciencemag.org/cgi/content/full/2009/928/2- The following article discusses the orgin of life issues and decided that replicating molecles have an advantage over catalyzed reactions. Still a very complex issue.-http://www.physorg.com/news173351870.html