Teacher-centered lecture-style classes predominate in the Republic of Korea. Typically, classes follow a three-stage structure: introduction, development, and wrap-up. This study aims to analyze concepts teachers present in each stage of photosynthesis classes from a conceptual perspective and how these concepts are connected across each stage. To this end, the semantic network analysis was employed to visualize the concept networks and connection networks of the concepts explained by teachers at each stage. The results revealed that teachers underutilized the introduction and wrap-up stages. Moreover, the concepts presented for each subtopic and their number were similar across stages. Therefore, in order to enhance teaching effectiveness, teachers need to select concepts to be taught for each subtopic in advance. In addition, efforts should be made to arrange and connect the selected concepts appropriately, taking into account the characteristics of each stage.