This study investigates an experienced science teacher’s pedagogical content knowledge (PCK) regarding digestive system in the framework of Magnusson, Krajcik and Borko (1999).  PCK, which is put forward by Shulman (1986), is an accepted theoretical framework that leads researchers to better understanding of teachers' knowledge of teaching (Abell, 2007). PCK expresses how teacher's content knowledge and pedagogy can be demonstrated according to the characteristics and needs of students (Shulman, 1986). Later, Magnusson et al. (1999) demonstrated a comprehensive PCK model which includes orientations towards science teaching, knowledge of science curriculum, knowledge about students’ understanding of specific science topics, knowledge of assessment in science, and knowledge instructional strategies for teaching science. Orientation towards teaching science demonstrates the knowledge and beliefs of the teacher about the aims and objectives of teaching for science teaching at a given class level. In short, it represents the teacher's point of view on science teaching. Knowledge of science curriculum divided into two parts which are knowledge of goals and objective, and knowledge of specific curricular program. Knowledge of goals and objective represents the knowledge of teachers on the aims and objectives of the subject to be taught to the students. Otherwise, knowledge of specific curricular program includes the teacher's knowledge of the aim of science curriculum, and material knowledge in teaching of specific subjects. Knowledge of students’ understanding of science consists of two parts which are knowledge of requirements for learning which includes students’ needs to learning and knowledge of areas of students’ learning difficulties which includes teachers’ knowledge about the issues such as misconceptions that the student has difficulty in learning. Knowledge of assessment in science also includes two sections that are knowledge of dimensions of science learning to asses that expresses the knowledge of measuring what students have learned on a particular subject and knowledge of methods and assessments includes specific activities and instruments. Finally, knowledge of instructional strategies consists of two parts which are knowledge of subject-specific strategies that represent knowledge about how to enact science, and knowledge of topic-specific strategies demonstrate teachers’ knowledge about specific strategies about particular science unit. Teachers may not have the same deep knowledge in each case for each component of the PCK; thus, this is the indication that the PCK is topic-specific (Magnusson et al., 1999).

Due to the topic specific nature of PCK, digestive system is selected as the research topic. Digestive system, as a part of human body systems, is a complex biological system that consists of subsystems which are interacting with each other (Ben-Zvi Assaraf, Dodick, & Tripto, 2013; Hmelo-Silver & Roger Azevedo, 2006). Digestive system, as a complex system, includes different organs, and organization and interaction among them. Furthermore, it occupies an important place at both middle school science and high school biology curriculum. It is also acknowledged as a difficult topic to understand for students, and teach for teachers (e.g. Tekkaya, Ozkan & Sungur, 2001). Students’ poor understanding and misconceptions of the digestive system is also revealed by researchers (e.g., Dry, 1998; Prokop & Fanèovièová, 2006).  For instance, although students understand the place where digestion begins, they have difficulty to understand the formation of feces and the passage of nutrients into the blood as a result of the completion of digestion (Teixeria, 2000). Accordingly, current study explored following research questions:

1. What is the science teachers’ PCK (knowledge of science curriculum, knowledge of students, knowledge of instructional strategies, knowledge of assessment strategies) regarding digestive systems?

In this study, one of the qualitative research designs case study (Merriam, 2009) was adopted. Data were collected from experienced science teacher by means of semi-structured interviews include Content Representation (CoRe) questions (Loughran, Mulhall, & Berry, 2004), and researcher field notes were obtained during classroom observations and analyzed by inductive method. Pre-interview questions consist of PCK components proposed by Magnusson et al., (1999). Purposive sampling is applied in this study because criteria decision is crucial to get detailed description (Merriam & Tisdell, 2016). Thus, decided criteria are that teacher should be graduated from elementary science education to be educated both pedagogy and subject matter, be still working to be observed during classroom, be experienced (more than five years experience is accepted as experienced (Creswell, 2007), to have developed PCK (Abell, 2007). The participating teacher, accordingly is working at a well-known private school in Ankara, capital of Turkey, has 16 years of experience and graduated from science education program. Post-interviews were prepared by comparing pre-interviews and observation reports. It helps to reveal more information about differences between planned and enacted instruction. Member checking which is controlling results to decrease misunderstanding used to increase internal validity (Stake, 2010). The researcher is in the classroom as long as the topic is taught and records the observation data without any intervention to the teacher. Using both observation and interviews during study provide triangulation which increases internal validity (Merriam & Tisdell, 2016). Audio was recorded throughout the course with the permission of the teacher. In addition, field notes were written by the researcher.

Participating teacher is found to be knowledgeable about knowledge of curriculum especially constrains about digestive system and she seems to have mastered knowledge of students because she is aware of the difficulties they face during learning and their misconceptions. She is admitted as well-informed about knowledge of instructional strategies by indicating many instructional methods both subject-specific and topic-specific in the classroom as well as knowledgeable about knowledge of assessment. For example, regarding knowledge of curriculum, teacher emphasizes that the systems have a complex structure and that the relationship between the systems should be understood. Thus, according to the new changing science curriculum, all systems in our body to 6th grade students may cause problems at their level. The teacher is not sure about the order in which the systems are given in the curriculum, but claims that the order should be circulation, respiration, excretion and digestion. She does not state objectives given in curriculum instead saying that ‘students should know organs, the structure of the organs and how they work’. Related to knowledge of students, the teacher states that, students have misconception that “Digestion occurs in the stomach.” She also explains that students have difficulty in conceptualizing physical and chemical digestion, especially the action of bile in small intestine. Concerning knowledge of instructional strategies, she mentions that she planned the lesson in line with constructivist approach and used 5E and 7E learning cycle models while teaching digestive system. As far as knowledge of assessment strategies are considered, she generally evaluates their students with questioning during classroom and by multiple-choice and open-ended questions at the end of the unit. The results of this study reveal the teaching of the digestive system, one of the complex systems, by the experienced science teacher, and sheds light on the deficiencies in teacher education studies.

Abell, S. K. (2007). Research on science teacher knowledge (Chapter 36). In S.K.Abell and N.G. Lederman (Eds.), Research on Science Teacher Education, pp.1105-1149, New York: Routledge. Ben-Zvi Assaraf, O., Dodick, J., & Tripto, J. (2013). High school students’ understanding of the human body system. Research in Science Education, 43, 33–56. doi:10.1007/s11165-011- 9245-2 Creswell, J. W. (2007). Qualitative Inquiry and Research Design: Choosing among Five, Traditions. London, UK: Sage Dry, H. A. (1998). Student misconceptions in science: the human digestive system. Hmelo-Silver, C. E., & Azevedo, R. (2006). Understanding complex systems: Some core challenges. The Journal of the learning sciences, 15(1), 53-61. Loughran, J., Mulhall, P., & Berry, A. (2004). In search of pedagogical content knowledge in science: Developing ways of articulating and documenting professional practice. Journal of research in science teaching, 41(4), 370-391. Magnusson, S.J., Borko, H., & Krajcik, J.S. (1999). Nature, source, and development of pedagogical content knowledge for science teaching. In J. Gess- Newsome & N. Lederman (Eds.), Examining Pedagogical content Knowledge (pp.95- 132). Boston, MA: Kluwer Press. Merriam, S.B. (2009). Qualitative research: A guide to design and implementation. San Francisco, CA: Jossey-Bass. Merriam, S.B., & Tisdell J.E (2016). Qualitative research: A guide to design and implementation. San Francisco, CA: Jossey-Bass. Prokop, P., & Faněoviěová, J. (2006). STUDENTS'IDEAS ABOUT THE HUMAN BODY: DO THEY REALLY DRAW WHAT THEY KNOW?. Journal of Baltic Science Education, (10). Shulman, L. S. (1986). Those Who Understand: Knowledge Growth in Teaching. Educational Researcher, 15(2), 4-14. Stake, R. E. (2010). Qualitative research: Studying how things work. Guilford Press. Tekkaya, C., Özkan, Ö. & Sungur, S. (2001). Biology Concepts Perceived as Difficult by Turkish High School Students. Hacettepe University Journal of Education, 21, 145-150. Teixeira, F. (2000). What happens to the food we eat? Children’s conceptions of the structure and function of the digestive system. International Journal of Science Education, 22(5), 507-520.