This study focuses on exploring pre-service science teachers' conceptual integration understandings in explaining the subject of metabolism with the concepts of physics and chemistry. Action research was employed in this study. Nine pre-service science teachers taking the General Biology II course participated. Participants were taught metabolism considering conceptual integration. Data were collected via paper-and-pencil questionnaire administered before and after the intervention. The results showed that the pre-service science teachers could not understand physics and chemistry concepts at a sufficient level while explaining metabolism. Although the activities and practices were executed for conceptual integration in the action plan, only three participants achieved complete conceptual understanding for better conceptual integration at the end of the course in the first question. Thus, future studies could design interventions to be effective in conceptual integration. Further research could also design experimental investigations to examine how conceptual integration is necessary for establishing the relationship between science and other fields.