This article develops a theoretical account of how the principles of integration and systemicity jointly structure high-quality instruction in the natural sciences at the secondary level. Rather than treating integration as a mere juxtaposition of topics, the paper frames it as an epistemic design choice that aligns disciplinary concepts, practices, and representations around phenomena whose explanation requires multiple lenses. Systemicity is construed as a way of reasoning about parts, relationships, feedback, and emergence across scales; the argument is that integration becomes coherent only when it is guided by systemic thinking so that content connections serve explanatory purpose rather than thematics. The study synthesizes work from systems theory, learning sciences, knowledge-integration research, and modeling-based inquiry to propose design heuristics that translate theory into classroom practice without fragmenting disciplinary rigor. Methodologically, this is an integrative review with illustrative design analyses of phenomenon-centered sequences in energy, homeostasis, and materials transformations. The discussion shows how coherence arises when crosscutting concepts and epistemic practices are sequenced to support conceptual change, transfer, and disciplinary literacy, and it examines implications for assessment, teacher knowledge, and the use of simulations and community partnerships. The conclusion argues that when integration is organized systemically, students’ explanatory writing, model use, and problem transfer improve because teaching mirrors the structure of scientific inquiry itself.  

Curriculum integration;, systems thinking, conceptual change

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