The Relationship Between Food Concentration, and Respiratory Activity In this experiment different concentrations of sucrose were tested to determine which leads to the most respiratory activity in yeast. Yeast is a heterotrophic anaerobic fungus which lacks chlorophyll. Yeast is used commercially to ferment the sugars of wheat, barley, and corn to produce alcohol, and in the baking industry to raise or expand dough. Yeast or alcoholic fermentation is the anaerobic process of respiration by which sugars, such as glucose and sucrose, are converted into ethanol and carbon dioxide (CO2