Six Year 10 biology students from two schools in Hong Kong were interviewed to give explanations about biological phenomena related to these topics: enzymes and metabolism as well as the breathing system and transport system in humans. Their responses were coded (activity, condition, entity, organisation, and property) based on a framework of mechanistic reasoning originally developed to study students’ reasoning in physics and chemistry. The relationships between components were reflected in the links relating two components in specific order. The link quality was graded based on its relevance and accuracy in explaining target phenomena. The links were chained with arrows to construct reasoning diagrams to reflect the complexity of student reasoning.

Results were compared across the topics and across three student ability groups. Individual analyses of the reasoning of each of the six students about the topics were also conducted. Among the five reasoning components, activity appeared most frequently in the students’ explanations across the topics. There was not a pattern between the quantity of links and student ability; however, the link with quality increased with student ability. The complexity of reasoning did not demonstrate any pattern with student ability, but the chaining between links of the reasoning diagram of a particular interviewed item helped distinguish between student abilities. Suggestions for teaching and researching mechanistic reasoning were made.