Valerie Obenchain's recommendation:
   I think it would be useful to create an S4 class to hold
   your expression data, maybe called CellGrowth or 
   CellDensity or something similar. You could extend the eSet 
   class, see ?eSet. This would allow you to store the expression 
   data and metadata in one object. You can take advantage of the 
   preexisting accessors, subsetting etc. There is a vignette
   in Biobase, "Notes for eSet developers" that maybe helpful,

   http://bioconductor.org/packages/2.10/bioc/html/Biobase.html

   Data read in with readYeastGrower() and readGenios() would 
   be put into this new class. The functions would become
   methods which would dispatch on the new class. For example,
   fitCellGrowth, baranyi, gompertz, etc. would have methods
   for the CellGrowth class.