Glycogen and starch are complex branched polymers of glucose that serve as units of glucose storage in animals and plants, respectively. Changes in the structure of these molecules have been linked to changes in their respective functional properties. Enzymatic models of starch synthesis have provided valuable insights into the biosynthetic origins of starch structure and functional properties but have not successfully been applied to glycogen despite the structural similarities between the two polymers. Modifications to biosynthetic models of starch structure were tested for applicability to glycogen. Mathematical evidence is provided showing the necessity (which has hitherto been in doubt) of considering the effects of catabolic (degradative) enzymes in biosynthesis-based approaches that seek to accurately describe the molecular weight distributions of individual chains of glycogen formed in vivo through glycogenesis. This finding also provides future direction for inferring the dependence of enzyme activities on substrate chain length from in vivo data.