  Students' sometimes contradictory ideas about ATP (adenosine triphosphate)
and the nature of chemical bonds have been studied in the biology and chemistry
education literatures, but these topics are rarely part of the introductory
physics curriculum. We present qualitative data from an introductory physics
course for undergraduate biology majors that seeks to build greater
interdisciplinary coherence and therefore includes these topics. In these data,
students grapple with the apparent contradiction between the energy released
when the phosphate bond in ATP is broken and the idea that an energy input is
required to break a bond. We see that students' perceptions of how each
scientific discipline bounds the system of interest can influence how they
justify their reasoning about a topic that crosses disciplines. This has
consequences for a vision of interdisciplinary education that respects
disciplinary perspectives while bringing them into interaction in ways that
demonstrate consistency amongst the perspectives.
