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MCAT Question A Day - 7/17/13 - Answer!

When exercising, skeletal muscles can continue to carry out glycolysis and produce small amounts of ATP, even though glucose is not arriving via the blood rapidly enough. Which of the following best explains this phenomenon?

A.  Glycolysis is anaerobic and doesn’t require oxygen.
B.  Extra glucose molecules can be supplied from stored glycogen within the muscle.
C.  Extra glucose molecules can be supplied from stored glycogen within the liver.
D.  All of the above

The correct answer is (B). Each glycogen molecule stored in a muscle cell is made of thousands of monosaccharides (glucose) linked together. When needed, each glycogen can be broken down to supply these glucose molecules for the muscle cell to use in glycolysis. The cell does not have to wait for glucose molecules to be delivered by the blood. Glycogen stored in the liver can also be broken down to glucose, but it still would have to arrive at the muscle cells via the bloodstream. Although choice (A) is a true statement, it is not relevant to why glucose is still available.