Metabolism chemical-signaling molecule

Introducing chemical potentials for biochemical substrates needs to be done with caution when considering, for example, molecular crowding and signaling molecules with limited copy numbers (Parsegian et al., 2000). This simple chemical system is for cellular metabolic networks, and concentrations replace activities in ideal solutions. 

The secondary metabolites are produced from key intermediates of the primary metabolism pathways. They are often found in limited quantity, can occur transiently in the cell cycle and can be unique for a particular group of organisms or even species. Secondary metabolites typically represent a chemically very diverse group of small molecules (molecular mass <2000 amu) and include (1) products from overflow metabolism as a consequence of nutrient limitation, (2) compounds for defense, (3) regulatory molecules, (4) signaling molecules, or (5) molecules that serve the requirements of evolutionary exploration within the physicochemical space available on this planet. Secondary metabolites can be grouped according to the primary metabolic pathway from which they are derived or in terms of their structural similarity. 

Lipids have been the focus of many MSI studies due to their important role as structural components of cells, signaling molecules, and involvement in disease-related metabolic pathways (13). Their high abundance in tissue and their physical and chemical properties facilitate ionization and MS analysis (6). Lipids are usually detected in positive or negative mode within a mass range of 200-1000 Da. 

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