[zuzannamagdalena]

I am writing a lab report on an experiment whose aim was to investigate the influence of substrate concentration on anaerobic respiration in yeast. The substrate I used was sucrose in the form of crystal sugar.

I decided to include "source of substrate" as a controlled variable, but my Biology teacher remarked it might be irrelevant, saying "Crystal sugar is basically sucrose, and sucrose is sucrose, or C₁₂H₂₂O₁₁ if you like, so I don't see how sugar from different packages could be qualitatively different, and sugar does not "get old" the way yeast does")

Well, I imagine that if you use distilled water (which is supposed to be pure H₂O) in an experiment, you don't worry much about whether you are taking it from the same bottle every time or even whether you are using the same brand (or do you?). Still, I am slightly suspicious about "source of sugar" being completely irrelevant as a controlled variable. However, if I do choose to include "source of substrate" as a controlled variable, I am going to need to include a DETAILED and REASONABLE explanation why it is significant - and I am not quite sure HOW EXACTLY it could affect the results if I use crystal sugar from different packages or of different brands.

All I have is a few intuitions.

Is it really true that "crystal sugar is 100% pure sucrose"?
Even if it is, is it really impossible for crystal sugar from one package to be qualitatively different from crystal sugar from another package (or of another brand, etc.)?
What if I use sugar that was bought ten years ago and sugar that was bought recently? Is it true it doesn't "get old the way yeast does"?
Is it really impossible for environmental factors such as humidity to influence the quality of sugar?
And what if someone at some point added something to a package that was opened a long time ago?

Any clues as to whether the things I mentioned above are relevant? Any other suggestions?

[Borek]

I am with your teacher here. Crystal sucrose has a constant composition and is quite stable. It is possible that there are some small differences, as there are always some impurities present, but I doubt they will give a measurable response.

[jeffrey.struss]

Assuming the crystal sugar is >90% pure (almost a given) I can't see any major effects from using a different provider.