A "Pulse-chase" experiment using 14C-labeled carbon sources is carried out on a yeast extract maintained under strictly anaerobic conditions to produce ethanol. The experiment consists of incubating a small amount of 14C-labeled substrate (the pulse) with the yeast extract just long enough for each intermediate in the fermentation pathway to become labeled. The label is then "chased" through the pathway by the addition of excess unlabeled glucose. The chase effectively prevents any further entry of labeled glucose into the pathway.
(a) If [1-14C]glucose (glucose labeled at C-1 with 14C) is used as a substrate, what is the location of 14C in the product of ethanol? Explain.
(b) Where would 14C have to be located in the starting glucose to ensure that all the 14C activity is liberated as 14CO2 during fermentation to ethanol? Explain.