Demonstrating Carbon Dioxide production in Respiration of Yeast 2.39

Yeast is a single celled fungus that can respire aerobically when oxygen is available and anaerobically in the absence of oxygen.

Aerobic respiration in yeast:

Glucose + Oxygen ======> Carbon dioxide + Water

Anaerobic respiration in yeast:

Glucose ======> Ethanol and Carbon dioxide

Both forms of respiration produce carbon dioxide as a waste product so how could this be demonstrated experimentally?

alcoholic-fermentation-demonstration

This is the simplest set up that could demonstrate this.  Lime water will go cloudy in the presence of carbon dioxide.  Glucose solution is needed to provide the reactant sugar for the yeast to respire.  The oil layer on the top is to prevent the diffusion of oxygen from the air into the Yeast in Glucose solution, this ensuring anaerobic respiration will occur.

What experimental factors could be altered in this set up?

Well assuming you keep the volume and concentration of lime water constant, the time taken for the limewater to go cloudy could be measured under differing conditions: the faster the time, the faster the rate of respiration.  The experimenter could investigate the effect of changing the temperature, the pH or the concentration of glucose solution used.  Make sure you understand how and why changing each of these factors might affect rates of respiration in yeast.

2 comments

  1. Amanda

    Hi, I don’t understand how the carbon dioxide gets into the tube – surely the oil on top of the yeast solution not only keeps oxygen out, but also keeps carbon dioxide in? So why doesn’t the little tube have to go down into the solution?

    • Paul Gillam

      Good question. I guess the concentration gradient for carbon dioxide is so steep that it can diffuse across the oil layer. Or perhaps the oil layer is more permeable to carbon dioxide than to oxygen (but I am not a good enough chemist to see why this might be) You can’t put the little tube into the solution because you need carbon dioxide gas, not dissolved carbon dioxide, to increase the air pressure inside the boiling tube.

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