Now that complete genome sequences are available for a variety of organism, the elucidation of single gene function involved in the metabolism, includes necessarily a better understanding of the cellular responses on all levels of gene products, mRNA, proteins and metabolites. Such knowledge's growth is essential since the observable properties of living organisms, so called phenotypes, are the results of the genotype in juxtaposition with the environmental factors. If much has been done and is

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... in continuous evolution to make clear the mRNA and transcript profiling, considerably less effort has been required to characterize the products of gene expression, the metabolites. Now it is possible to represent cellular processes as metabolic networks and to describe them at different levels of resolution, focusing the attention on the connection map of the metabolic network. Here we describing just the structure of a metabolic network, instead of considering the details of the metabolic flux. For each approach, a number of examples have been given and the potential application have been discussed. Mathematical studies of simple metabolic networks show clearly that, already from an oversimplified approach based only on the analysis of the structure of the network, it is possible to reach important conclusions on the network's design principles. Of course one of the most important constraint on metabolic networks comes from the influences of the external environments. For this reason we focus some examples of the effects of the environments on metabolic networks, making of transcriptomic a new paradigm of the life.