Definition: The biological source (organism, tissue or cell type) of an Entity. Usage: Some entities are considered source-neutral (e.g. small molecules), and the biological source of others can be deduced from their constituentss (e.g. complex, pathway). Instances: HeLa cells, Homo sapiens, and mouse liver tissue.
Source:http://www.biopax.org/release/biopax-level3.owl#BioSource