Glucose-induced acidification in yeast cultures

We present an investigation (for A-level biology students and equivalent) into the mechanism of glucose-induced extracellular acidification in unbuffered yeast suspensions. The investigation is designed to enhance understanding of aspects of the A-level curriculum that relate to the phenomenon (notably glucose catabolism) and to develop key skills in scientific enquiry. We demonstrate that glucose-induced extracellular acidification (monitored by pH electrodes linked to a computer) is inhibited by glycolytic and TCA cycle inhibitors. Dimethylsuccinate, which forms succinate after entering cells, also induces acidification that is inhibited by malonate, an inhibitor of succinate oxidation. The yeast plasma membrane has a proton pump that extrudes protons from the cell. However, inhibitors of the pump do not affect glucose-induced acidification during 20 minutes of monitoring pH, indicating that under the experimental conditions employed, the pump does not have a significant role in the process. Taken together, the data indicate that glucose-induced acidification of the medium occurs as a result of its catabolism and subsequent outward diffusion of the CO₂ derived from respiration and/or fermentation. A discussion is presented on how the investigations developed skills of scientific enquiry in undergraduate trainee teachers who carried out some of the experiments and on how the investigations can be used to enhance these skills in A-level students.