Final answer:
In the investigation to determine the effect of temperature on yeast metabolism, the mass of sugar and mass of yeast should be held constant, allowing temperature changes to be the sole variable influencing the observed gas volume.
Step-by-step explanation:
To determine the effect of temperature on the metabolism of yeast, it's crucial to control certain variables while conducting the lab investigation. The variables that should be held constant during this investigation are the mass of sugar and the mass of yeast. By keeping these quantities constant, it allows the student to observe the effects of temperature changes on yeast metabolism without the interference from varying amounts of reactants. The volume of gas can change as a result of temperature changes – which is the primary observation in this experiment – and the water temperature is the independent variable that is being manipulated to assess its impact on gaseous volume.
It's important to remember that according to the scientific method and the history of discoveries in gas behavior laws, researchers often held certain variables like the amount of a gas and temperature constant while they varied another to understand its influence on the remaining variables. Since the aim here is to study temperature's influence, it is essential that other factors like the mass of sugar and yeast remain unchanged.