8/18/2019 Lecture 11 Sample Problem Answers

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Biosc 1000

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Lecture 11 Sample Problems

1.  The brain burns glucose as its sole metabolic fuel (except during extreme starvation) and

consumes up to 40% of the circulating glucose. In the brain, hexokinase is the rate

determining step of glycolysis – why should this be?

The brain is only going to oxidize glucose for energy so trapping it in the cellis the committed step. Once in, it will be completely oxidized to make ATP.

2.  What is the importance of the lactate dehydrogenase reaction in muscle (pyruvate

!lactate) and the acetaldehyde dehydrogenase reaction in yeast?

 Both oxidize NADH to NAD+ , restoring the oxidized electron carrier so glycolysis

can continue

3. 

A liver biopsy of a boy indicate a deficiency of fructose-1,6-bisphosphatase activity. The boy’s blood glucose levels were normal at a beginning of a fast but decreased

 precipitously. Pyruvate and alanine levels increased and the glyceraldehyde-3-

 phosphate/dihydroxyacetone phosphate ratio increased as well. Why did these symptoms

occur?

The deficiency of fructose-1,6-bisphosphatase blocked the conversion of

 fructose-1,6-bisphoshate to fructose-6-phosphate and the subsequentconversion to glucose-6-phosphate and finally glucose. Because of the block,

 gluconeogenic intermediates accumulate – pyruvate is the product of pyruvate carboxylase and PEP carboxykinase reactions, alanine is the

transaminated form of pyruvate. Glyceraldehyde-3-phosphate is the productof the glyceraldehyde-3-phosphate dehydrogenase reaction.