Applying Beer's Law in the undergraduate cell biology laboratory: examining the mathematical relationship between optical density, cell concentration, and cell size using budding yeast.

Undergraduate students majoring in the life sciences benefit from experience with data analyses that connect mathematical calculations to the biological systems they are studying. Monitoring the optical density and cell number of Saccharomyces cerevisiae liquid cultures allows students to gain quantitative experience generating standard curves and trendlines that capture the relationship between optical density and cell concentration for a given S. cerevisiae strain. Data comparisons across multiple strains can yield insights into the biophysical properties of cells that drive light absorbance and scattering. In this Tips and Tools article, we share a laboratory module that allows students to experience cell biology tools, laboratory measurements, and data analysis to determine the mathematical relationship between optical density and cell concentration in liquid microbial cultures. This module could be integrated into undergraduate classes ranging from general biology to upper-level cell biology or microbiology and can be a starting point for more complex investigations of microbial growth.