Students learn by constructing images or models of the world. These ideas may come from the theories that they create, the knowledge they learn in science class, and from their imagination. These ideas about the world are combined forming something that can be seen as a mosaic. In this mosaic it is possible to find correct, incorrect, naïve, and missing ideas [gaps] in the students' understanding. These images are like running pictures that move from static to more dynamic forms. The students evolving ideas can be represented by a Learning Pathway. Learning pathways represent routes that students move along as they develop understanding in a particular area or domain (Scott, 1991). While in one view learning pathways can be considered peculiar to each individual learner, similarities are found among students just as naïve conceptions and alternative conceptions are prevalent in groups of students. From an understanding of the learning pathway, it is possible to determine the critical path that is a set of investigations that are believed to be necessary for students to move from naïve and alternative conceptions to deep conceptual understanding in a subject area. This is true of the critical pathway suggested in this curriculum that was uncovered after extensive research and classroom trials.

Since this curriculum unit aims for coherent, integrated, and therefore hopefully, retained knowledge, the core concepts should form a causal or explanatory chain that is strongly connected with no gaps from the lungs and stomach to the mitochondria. We call a sequence of lesson topics that makes these connections in a motivating way a critical learning pathway or critical pathway for short. For this reason, the curriculum organizes the curriculum along a chain of lessons comprising a critical pathway. Other lessons and activities deemed enriching but not critical are included as optional activities along the pathway. The critical path in each chapter includes a set of investigations that are believed to be necessary for students to move from naïve and alternative conceptions to deep conceptual understanding in a subject area. A diagram of the particular critical path can be found at the beginning of each chapter.