Many of today’s pressing societal problems require solutions that span scientific fields to successfully address their impacts. However, it is unclear if university students are being adequately prepared to wrestle with these complex interdisciplinary problems by their discipline-specific science courses. We evaluated ways that undergraduate science students leveraged knowledge from introductory chemistry, biology, and physics courses when crafting explanations about interdisciplinary phenomena. We did this by analyzing student interviews of biology majors that had previously taken courses in biology, chemistry, and physics and constructing a framework that characterized their different reasoning strategies. From these interviews, we identified three broad frames students used in their explanations: i) a colloquial frame, where students used personal experience to explain the phenomena in everyday language; ii) a partial scientific frame, where students attempted to use scientific ideas in their explanations, often mixing colloquial and scientific language; and iii) a scientific frame, where students accurately drew upon scientific ideas and language to express their ideas. We then used these frames to characterize the different structural scales in which students situated their explanations, the kinds of models they developed when explaining the phenomena, and how closely the analogies they drew upon aligned with the phenomena they were explaining. This work will help guide the development of explicit instructional methods that equip students with the skills needed to address interdisciplinary phenomena.