Because nutrients are the building blocks of cellular parts, a lack of nutrients can reduce the ability of cells to build new molecules that protect them from environmental stress. Dorival Martins and Ann English (2014) investigated this phenomenon with Saccharomyces cerevisiae yeast exposed to hydrogen peroxide. The researchers grew three cultures of yeast on a nutrient-rich solution and three cultures of yeast on a nutrient-poor solution. They then exposed each yeast culture to 0.2mM hydrogen peroxide and measured catalase activity (U/mg). Required:
Identify the independent variable.