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Cyclists sometimes call it “bonking”; distance runners, “hitting the wall.” Both expressions describe a state of exhaustion in which further exercise is all but impossible. What is the biochemical basis for this condition?

After final exams are finished, tired students will sometimes sleep for 12 or more hours. Although other tissues can use fat stores as a fuel, the brain requires glucose as a fuel. How is the brain supplied with glucose during this long fast?

The answers to both of these questions entail the biomolecule glycogen. Glycogen is a storage form of glucose that can be readily broken down to yield glucose molecules when energy is needed. The depletion of muscle glycogen accounts in part for the feeling of exhaustion after intensive exercise, whereas the parceling out of the liver glycogen stores during a night’s fast allows the brain to continue functioning.

As with any precious resource, glucose should be stored when plentiful. Much of the glucose consumed after an exercise bout or after a night’s sleep is stored as glycogen. The interplay between glycogen breakdown and glycogen synthesis must be highly coordinated to ensure that an organism has glucose when needed.

Not all glycogen metabolism is related to energy needs. The ultimate product of glycogen breakdown—glucose 6-phosphate—can also be processed by a pathway common to all organisms, known variously as the pentose phosphate pathway, the hexose monophosphate pathway, the phosphogluconate pathway, or the pentose shunt. This pathway allows all organisms to oxidize glucose to generate biosynthetic reducing power, NADPH, which is used for the biosynthesis of many biomolecules, including fats. As stated in Section 10, NADPH is also a product of photosystem I in photosynthesis, though this process provides NADPH for plants only. The pentose phosphate pathway can also be used for the metabolism of pentose sugars from the diet, the synthesis of pentose sugars for nucleotide synthesis, and the catabolism and synthesis of less common four- and seven-carbon sugars.

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We begin this section with an examination of how glycogen stores are degraded, or mobilized, and how this mobilization is regulated. Glycogen mobilization is activated during exercise or fasting. We next consider the reverse process: in times of low energy demand and glucose excess, glycogen is synthesized. We will see how glycogen synthesis and degradation are coordinated. Finally, we look at how glucose 6-phosphate, the ultimate product of glycogen breakdown, can be metabolized to provide reducing power and five-carbon sugars.

✓By the end of this section, you should be able to: