O-labeled glucose

[SaintJude]

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Kaplan discrete:

If oxygen-labeled glucose is given to a rat, where will the label first appear?

1. exhaled carbon dioxide
2. exhaled H2O
3. plasma H2O
4. intracellular H2O

 

[MT Headed]

Wikipedia has a good description of glycolysis if your textbook / review materials are lacking.

If you follow the 6 oxygens of glucose as it goes through glycolysis, the first time an original oxygen is kicked off is step 9 when 2PG is converted into PEP and water. This takes place in the cytosol.

I'd go with answer #4.

 

[MedPR]

Kaplan discrete:

If oxygen-labeled glucose is given to a rat, where will the label first appear?

1. exhaled carbon dioxide
2. exhaled H2O
3. plasma H2O
4. intracellular H2O

4, but I don't think we need to know glycolysis this well for the mcat.

 

[SaintJude]

It's actually 1.) exhaled carbon dioxide.

This is a question that requires a true macro-level understanding of glycolysis& respiration (that I didn't have until this question)

Let me share Kaplan's explanation as fodder for discussion:

When radioactively labeled glucose is given to a rat, it will enter the respiration reactions beginning with 1.) glycolysis, 2.) TCA (Krebs) cycle and 3.) e- transport chain. Glycolysis: Glucose breaks down into pyruvic acid during glycolysis. No CO2 is formed during glycolysis. As pyruvic acid forms into acetyl coA, CO2 is released and this may contain the labeled oxygen.

TCA cycle: As acetly coA enters the cycle, more CO2 is formed that, yes, will contain the labeled oxygen. The water that is formed as a result of respiration is formed from the e-transport chain in the mitochondria, combined with hydrogen ions and molecular oxygen, which is taken through the lungs. This molecular oxygen is the final e- acceptor of the chain. Therefore the oxygen in respiratory waste product water comes from the oxygen that is inhaled into the alveoli. It does not come from oxygen that is part of the glucose molecule that is respired. [Now here is where I don't follow the leap in logic] Based on this reasoning, answer choice A.) is correct, as the labeled oxygen could be seen in the exhaled carbon dioxide.

Why the other choices are wrong:

2.) The water that we exhale contains the oxygen that we inhaled through inhalation. Since this oxygen is not part of the glucose, and is thus not labeled, we should not see it.

3.) Plasma water comes from drinking water, or is formed using the oxygen that we inhale in our lungs.

4.) Intracellular water is not produced from glucose.

 

[MedPR]

intracellular water isn't produced from glucose? 🙁

 

[mcloaf]

It's actually 1.) exhaled carbon dioxide.

This is a question that requires a true macro-level understanding of glycolysis& respiration (that I didn't have until this question)

Let me share Kaplan's explanation as fodder for discussion:

When radioactively labeled glucose is given to a rat, it will enter the respiration reactions beginning with 1.) glycolysis, 2.) TCA (Krebs) cycle and 3.) e- transport chain. Glycolysis: Glucose breaks down into pyruvic acid during glycolysis. No CO2 is formed during glycolysis. As pyruvic acid forms into acetyl coA, CO2 is released and this may contain the labeled oxygen.

TCA cycle: As acetly coA enters the cycle, more CO2 is formed that, yes, will contain the labeled oxygen. The water that is formed as a result of respiration is formed from the e-transport chain in the mitochondria, combined with hydrogen ions and molecular oxygen, which is taken through the lungs. This molecular oxygen is the final e- acceptor of the chain. Therefore the oxygen in respiratory waste product water comes from the oxygen that is inhaled into the alveoli. It does not come from oxygen that is part of the glucose molecule that is respired. [Now here is where I don't follow the leap in logic] Based on this reasoning, answer choice A.) is correct, as the labeled oxygen could be seen in the exhaled carbon dioxide.

Why the other choices are wrong:

2.) The water that we exhale contains the oxygen that we inhaled through inhalation. Since this oxygen is not part of the glucose, and is thus not labeled, we should not see it.

3.) Plasma water comes from drinking water, or is formed using the oxygen that we inhale in our lungs.

4.) Intracellular water is not produced from glucose.

This is the sort of question I would've gotten in my 400-level biochem class, so I wouldn't worry about know carbohydrate metabolism in this much detail for the MCAT.

 

[mcloaf]

intracellular water isn't produced from glucose? 🙁

Not directly, the water is produced when you reduce inhaled diatomic oxygen at the end of the ETC.

 

[MT Headed]

Water is kicked off in step 9 of glycolysis when 2PG turns into PEP.

Is Kaplan seriously suggesting the the three oxygens on each pyruvate are all from the original glucose molecule?

 

[SaintJude]

I hate to resurrect an old thread but since I started it, I wanted to provide some closure . I asked a Kaplan teacher, and he told me 2 things that Kaplan wanted me to get from this question

1.) The carbon dioxide generated from the TCA cycle is the same carbon dioxide we EXHALE.

2.) Everything the human body does to deliver oxygen to the tissues comes down to the role as a oxygen as the final electron acceptor in the electon transport chain.

 

[MT Headed]

Those are both good points, though neither has anything to do with answering the question as posed. To truly answer the question would require biochemistry knowledge far beyond the scope of the mcat, (but it's really cool stuff!).