Figure 2. Determining whether a nutrient can or cannot be catabolized.

A, To establish whether a given nutrient is a source of carbon for a given organism, we first need to determine whether the nutrient can be transported into the cell from the extracellular medium. B, For some nutrients, we can predict if a it does or does not contribute to growth just from knowing to which class it belongs. Complex nutrients are broken down into simple nutrients. See the main text for the description of the enzymes that catabolize fatty acids and purines. C, Any nutrient that is not classified into the nutrient classes in B is classified as G or NG using the logistic model described in the text and Methods.