The content knowledge of teachers matters in the classroom. It influences their ability to deliver lessons, select activities for their students and assess students' learning. This study investigates the experiences of biology pre-service teachers who were required to teach physics and earth science concepts during their pre-service teaching. Such experience has been brought about by curriculum change. The new K12 science curriculum in the Philippines follows a spiral progression arrangement of competencies. Thus, a lack of a teachers' expertise with the subject matter to be taught maybe experienced. A qualitative research method was utilised in this study to explore the different areas of concern. Upon analysis, it was found that though pre-service teachers find teaching concepts outside their expertise challenging, they are determined to become effective science teachers in the new K12 curriculum.