A Simple Method for Predicting a Molecule's Biological Properties From Its Polarity

Abstract

The distinction between very polar and less polar substances is a foundation of biochemistry, cell biology, and physiology; it surfaces in multiple concept inventories and elaborations of biological core concepts. However, in our experience, most biology courses do not explicitly teach students how to assess the polarity of any given molecule, thus limiting students’ ability to predict related biological properties such as the molecule’s solubility in bodily fluids, its rate of diffusion through cell membranes, the location of its receptors (at the cell surface or inside the cell), its rate of filtration by the kidneys, and the extent of its persistence in the blood. Here we present a quantitative yet student-friendly method for determining a molecule’s polarity according to the prevalence of polar functional groups. The method calculates a molecule’s “C/fun” ratio—the number of carbon atoms per polar functional group—which correlates closely with the logP value, a widely used indicator of polarity. In addition, the lesson incorporates the Test Question Templates (TQT) framework to provide transparent guidance to both instructors and students on formative and summative assessments of understanding. Our lesson stresses the connections between polarity and the above-mentioned biological properties to help students appreciate the biological utility of understanding polarity. Given its central position in biochemistry and cell biology, polarity might be considered a Threshold Concept, i.e., one that is troublesome (hard to understand), transformative (affecting scientific identity), integrative (connecting other concepts), and irreversible (hard to forget once mastered).