Global climate change is driven by imbalances in the carbon cycle, between processes that generate organic carbon—photosynthesis—and processes that oxidize organic carbon—combustion and cellular respiration. Understanding carbon cycling requires students to trace matter and energy through socio-ecological systems at multiple scales in space and time. However, research on college students’ biological reasoning has produced a troubling finding: Most students taking college biology courses, including science majors, memorize details about processes without understanding and do not use basic principles, including conservation of matter and energy, to reason through biological processes. Students’ difficulties with principled reasoning about biological processes are important because they are based on deep-seated beliefs about how the world works and habitual course-taking practices (e.g. memorization).  
Results/Conclusions

Many students rely on a form of informal reasoning that construes the events of the world as caused by actors (including people, animals, plants, machines, and flames), each with its own purposes and abilities, or by natural tendencies of inanimate materials. For example, force-dynamic reasoning explains the growth of a tree by identifying the actor (the tree), its purpose (to grow), and its needs (sunlight, water, air, and soil). Force-dynamic accounts focus on events at organismal scales and generally do not trace conserved entities such as matter and energy through time.

This approach to reasoning about carbon-transforming processes contrasts sharply with principled scientific discourse, which construes the world as consisting of hierarchically organized systems at different scales. Rather than identifying actors, scientific reasoning sees systems as constrained by fundamental laws or principles, which can be used to predict the course of events. The most fundamental of these principles—conservation of matter and energy—also turn out to be highly problematic for most students, especially since they pay little attention to gaseous products or reactants and conflate chemical potential energy with materials that have chemical potential energy.

When students with these ways of thinking about the world take college biology courses, they tend to focus on the details of processes without recognizing how these processes are constrained by fundamental principles such as conservation of matter and energy. The result is that they memorize and forget rather than learning to think about the processes they study in a principled way. We discuss possible alternative approaches to teaching that make the principles more visible and help students to use them appropriately.