Using your knowledge of fatty acid biosynthesis, provide an explanation for the following experimental observations:

(a) The addition of uniformly labeled [14C] acetyl-CoA to a soluble liver fraction yields palmitate uniformly labeled with 14C.

(b) However, the addition of a trace of uniformly labeled [14C]acetyl-CoA in the presence of an excess of unlabeled malonyl-CoA to a soluble liver fraction yields palmitate labeled with 14C only in C-15 and C-16.