A biosystem, or biological system, is a group of molecules that interact in a biological system. The BioSystems Database serves as a centralized repository of data, and also connects the different biosystem records with associated literature, molecular, and chemical data throughout the NCBI system.
The PubChem BioAssay Database contains bioactivity screens of chemical substances described in PubChem Substance. It provides searchable descriptions of each bioassay, including descriptions of the conditions and readouts specific to that screening procedure. Wang et al provide an overview of the database in this article published in Nucleic Acids Research.
The PubChem Compound Database contains validated chemical depiction information provided to describe substances in PubChem Substance. Structures stored within PubChem Compounds are pre-clustered and cross-referenced by identigy and similiarity groups.
The PubChem Substance Database contains descriptions of samples, from a variety of sources, and links to biological screening results that are available in PubChem BioAssay. If the chemical contents of the sample are known, the descriptions includes links to PubChem Compound. To better understand both PubChem Compound and PubChem Substance, read Bolton et al. in Annual Reports in Computational Chemistry.
PubChem BioActivity Services test compound/substance activity summary across multiple BioAssays. There are three subviews: