A group of students investigated the growth of different varieties of yeast. The students learned that the rate of respiration in a yeast culture is proportional to the biomass of the culture. Respiration rate can be used as a measure of the growth of a yeast culture. Respiration rates can be measured using the redox indicator TTC. • During respiration, hydrogen ions are removed from glucose to reduce hydrogen carriers such as NAD and FAD. • A redox indicator can be used as a hydrogen carrier in experimental conditions instead of NAD or FAD. • The colour change of the redox indicator can be measured using a colorimeter.