Re: Would saccharomyces cerevisiae breakdown sucrose or glucose faster?

The question really cannot be answered.  It depends completely on the 
previous history of the organism whether S. cerevisiae or anything else.  
Assuming that the organism can metabolize sucrose to start with, as not 
all bacteria or fungi can, the enzymes necessary to do so may need to be 
induced.  Same is true for all glycolytic enzymes.  If the cell has been 
growing on something like succinate, then glycolysis is minimal and it 
won't initially grow on either sucrose or glucose or fructose.  All the 
enzymes have to be induced first.  In this case, you wouldn't be able to 
tell any difference between growth rate on the various sugars.

If we assume that the cell is adapted to one or the other sugar, and then 
measure growth rate and growth yield (mass of cells), I don't think there 
would be a signficant difference in either rate or yield.  You're talking 
about a very small part of the overall process.  If there was any 
difference in rate, it would be swamped by all the other steps involved.

If you limited yourself to asking whether sucrose, fructose or glucose 
would make phosphoenolpyruvate (PEP) faster than one of the other sugars, 
then you might find a small difference, but not anything that you'd really 
be interested in.  And any one of the sugars might be the "fastest" or 
the "most".  It depends not only on the sugar, but the level of the 
membrane transporters for each and the actual amount of enzyme specific to 
each step that was already in the cell.  For example, even though glucose 
would take a couple steps more that fructose possibly, if the levels of 
the enzymes specific to glucose versus fructose were much higher, then 
glucose would likely show a faster increase in PEP.  Also, even if they 
were equal, fructose (and glucose) can be shunted off into other pathways 
besides glycolysis.  If these pathways were active, they might reduce the 
amount of fructose rapidly and thus decrease the amount available for 
glycolysis.

In other words, it's complicated and depends on the particular organism 
and that organism's metabolic potential and its history of growth.