Question

How are carbon atoms transferred between molecules?

Answer 1

A case could exist where a covalent bond forms between carbons of two different molecules, A and B, so that they become one molecule. If a different bond then broke in what had been Molecule B so that it dissociated, leaving part of the initial molecule bound to A, the net result would be the "transfer" of one or more carbon atoms from molecule B to molecule A.

But in general, molecules don't "trade" or "transfer" whole atoms. Particularly in biologic systems, where energy barriers involved in bond making and breaking events are, in general, high.