Radioactive isotopes of various elements can be used to label molecules for use in studies of metabolic pathways. For example, 14C is a radioactive isotope of carbon and naturally occurs at much lower rates than non-radioactive 12C (i.e. most molecules

Question

contain 12C carbon atoms). A cell can be fed a nutrient molecule labeled with the radioactive isotope and scientists can detect where the radioactive isotope ends up. For each of the following scenarios, trace the radioactive isotope to where it ultimately would end up after moving through the cellular respiration, fermentation, or biosynthetic pathways. Only consider these metabolic pathways; do not consider any other cellular processes.
A. E. coli is fed 14C-labeled glucose and grown under aerobic conditions.
B. E. coli is fed 14C-labeled glucose and grown under anaerobic conditions (fermentation).
C. E. coli is fed 14C-labeled fatty acids and grown under aerobic conditions.
D. E. coli is fed 14C-labeled acetyl CoA and grown under conditions that promote biosynthetic pathways.

What will be an ideal response?

Answer 1

In scenario A, the 14C glucose would enter the glycolysis pathway where it would be split into 2 molecules of pyruvic acid. These would then move through the intermediate step and the Krebs cycle. During these pathways, carbon molecules are released as carbon dioxide; eventually all the carbons in glucose are released as CO2 so the radioactive isotope would ultimately be found in CO2. In scenario 2, the 14C-labeled glucose will enter glycolysis to become 2 molecules of pyruvic acid. The lack of oxygen will cause E. coli to use fermentation. Pyruvic acid will be reduced by electrons from NADH. Depending on which fermentation pathway is used, the 14C-label will eventually be found in the waste products of fermentation: various organic acids, alcohols, or CO2. In Scenario 3, fatty acids are broken down by beta-oxidation to form acetyl CoA which will enter the Krebs cycle. Carbons from acetyl-CoA will be released as CO2, so the radioactive carbon isotope will be found in CO2. In scenario 4, acetylCoA can be diverted from entering the Krebs cycle and instead be used to make fatty acids. In this case, the radioactive carbon isotope will be found in fatty acids and lipids. AcetylCoA can also enter the Krebs cycle then various intermediates can be diverted to make amino acids, so some radioactive carbon may end up in amino acids and proteins.