I took the term "standard part" in the OP as referring to a relationship between functional units.
In biological terms a functional unit could be a cell, a unicellular organism, a micro-organism, an organ, a macro-organism, an ecosystem etc. i.e a living construct that has evolved to function in its own right.
I would define a "part of a unit" as another unit without which the whole would either not function, or would be functionally impaired.
So, with my definition, a bacterium that is necessary for digestion is part of the whole organism or digestive organ, because, without it, the organism or organ could not function. Similarly, elements of an ecosystem are parts of that living system. Removing part of an ecosystem could cause the whole system to fail.
This would mean biological "parts" can appear to "borrow from one another". The same bacterium can be found in the gut of different organisms (as the spark plug is found in different mechanical devices). The same bird can be found in different ecosystems, etc.