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Investigation to find out the lowest temperature that kills yeast cells

Asked by airwish | Mar 26, 2007 | AS Level > Biology > Advice
airwish asks:

I'm asked to plan an investigation to find the lowest temperature that kills all the yeast cells.
Mythelene blue solution is used as an indicator to show whether yeast cells are alive or dead.
I'm adding yeast to glucose, to my understanding, this is an anaerobic respiration process where glucose is converted into ethanol and carbon dioxide, however it seems mythylene blue only becomes colourless when oxygen in the solution is used up in respiration. Does it mean the yeast is carring out aerobic respiration then? And when you say respiration, does it mean the respiration of enzymes in yeast? Could you tell me what exactly makes methylene turns from blue to colourless? and is it due to enzyme activity, or general respiration of yeast cells?
Im very much confused!!
Thank you very much.

etutor answers:
Yeast respires aerobically unless oxygen is in short supply - only them does it switch to anaerobic. Methylene blue turns colourless when oxygen is removed from it, so you're talking about aerobic respiration. The oxygen is removed by the yeast cells respiring, but every single reaction in the respiration pathway, is controlled by enzymes. Enzymes CONTROL respiration, but enzymes can't RESPIRE, as they are not living organisms.

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