Missed practice Questions

Question 1

During exercise when the body requires as much energy as it can generate, aerobic respiration is utilized until oxygen runs out; however, due to differences in enzyme kinetics, certain reactions are faster than others. Because of this, which of the following substrates would be found in the highest concentration in the cell?
A. Succinyl-CoA B. Isocitrate C. Alpha-ketoglutarate D-Citrate

Answer 1

The question is essentially asking for the rate-limiting step of the Krebs cycle, which is the conversion of isocitrate to a-ketoglutarate. Since this is the slowest step, isocitrate will accumulate when the body struggles to keep up with the energy demands of the cycle. B is correct

Question 2

Oxalocaetate uniformly labeled with carbon-14 i condensed with unlabeled acetyl-CoA. After two complete turns through the TCA cycle, what fraction of the original radioactivity will be found in oxaloacetate? Assume all labeled carbons will be removed from the cycle before and unlabeled carbons.

A. All B. 3/4 C. 1/4 D. 0

Answer 2

First question to answer is “How many carbons are initially radioactively labeled in the citric acid cycle?” then “What fraction of carbons will be labeled after two turns of the citric acid cycle?” Oxaloacetate contains four carbons and acetyl-CoA contains two, so four carbons will be radioactively labeled. We also know that with each turn of the citric acid cycle two carbons are lost via carbon dioxide. Therefore, D is the answer