Biology Subject Test 1 Item 34

[Simran1031]

Hey guys,
this type of question always gives me a hard time.. i was wondering if someone could explain this to me.
Item 34 Kaplan bio subject test

if a 18O-labeled glucose given to a rat, where will the label appear?
a. exhaled co2
b. plasma h20
c. exhaled h20
d. intracellular h20.
the answer is a. the explanation talks about glycolysis and how o2 is the final oxygen acceptor and is the one that comes out during expiration..but im confused..

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[futuredoctor10]

Good question! I am not sure... the glucose subject to aerobic respiration (gluc + 02) will produce CO2 and H20 but I dont know how you know that the labeled glucose ultimately becomes exhaled CO2

 

[Simran1031]

yea..this type of question has historically tripped me up..

 

[G1SG2]

Whenever I see glucose in a Bio passage, I usually think of the combustion of glucose with oxygen/aerobic respiration. Doing so would make answer choice a (exhaled co2) seem appropriate, as then one would think of TCA.

 

[futuredoctor10]

so to answer this question... is my reasoning right?
Glucose labeled in a rat would undergo aerobic respiration.

Glucose + O2 --> CO2 + H20

Exhaled CO2 contains the labeled "oxygen" from the glucose, because in the TCA cycle (aka Krebs cycle) the carbon dioxide molecules are "products" of the intermediate reactions of the cycle?

 

[G1SG2]

Seems correct. The labeled glucose in glycolysis probably yielded two labeled pyruvates, and then led to the byproduct CO2, which carried the label.

 

[rnicholsaz]

The end result of glucose in oxidative conditions is CO2 and water. The CO2 is released in the Krebs cycle. The water is produced by O2 being the final electron receptor in oxidative phosphorylation, but doesn't use any of the original glucose molecule to make water.

 

[futuredoctor10]

Thanks pookiez88 and rnicholsaz - that makes a lot of sense and helped clarify the question completely!

 

[Simran1031]

thanks for responding everyone!