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?
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.
There is a syllabus point in the iGCSE Respiration section that asks candidates to know about an experiment that demonstrates heat production in respiration. This must be one of the least interesting experiments ever devised but here goes…..
Respiration is the chemical process occurring in all cells in which food molecules are oxidised to release energy for the cell. Cells need energy for a whole variety of things – active transport of molecules across the cell membrane, muscle contraction, movement of materials around the cytoplasm, cell division, many metabolic reactions etc. In fact, much of the energy released from glucose molecules in respiration is not “useful energy” for the cell but is given off as heat, a waste product. In warm-blooded animals such as humans, this heat energy is used to maintain our body temperature at around 37 degrees Celsius.
How can you demonstrate heat production in respiration?
The germinating seeds in the vacuum flask on the left are respiring because they are alive. The boiled seeds in the vacuum flask on the right will not be respiring because they are dead – boiling will denature all the enzymes needed for metabolism, The thermometer on the left will show a rise in temperature, the one on the right will stay the same. The flask on the right with the boiled seeds is a control. Vacuum flasks are used to insulate the seeds and so prevent heat loss.
The experiment is as simple as that. If the examiners wanted to ask a question on this, I guess they would give you the set up, ask about the design of the experiment, ask about which variables you might control and perhaps what conclusions could be drawn.