The definition does not include homonuclear molecules which is a pain.
Single chemical element, single phase (usually solid), but different 3D structures.
The prototypical examples are the allotropes of carbon.
The most important on in metabolism internals, everything else gets converted to it before being processed in the .
Nutrient that a given species cannot produce and must ingest in its diet.
en.wikipedia.org/w/index.php?title=Nutrient&oldid=1075972831#Essential gives somewhat of an overview:
Mentioned at 100 Greatest Discoveries by the Discovery Channel (2004-2005) medicine episode.
Active compound in pepper.