After 30 km of a marathon, a runner depletes their muscle glycogen stores. Blood glucose remains normal at this point. Which fact about glycogen metabolism best explains why muscle glycogen depletion causes fatigue but not hypoglycemia?
AMuscle cells store less glycogen than liver cells, so their stores are exhausted before blood glucose is affected
BMuscle cells lack glucose-6-phosphatase and therefore cannot export free glucose to the blood; muscle glycogen is a private fuel reserve consumed locally
CGlucagon signals only the liver to release glucose when blood sugar drops, leaving muscle glycogen unaffected by hormonal signals
DThe phosphorylase enzyme in muscle is less active than in liver, so glucose export to blood is delayed
The tissue-specific logic is essential. Glucose-6-phosphatase, which cleaves the phosphate from glucose-6-phosphate to produce free glucose for export, is present in liver but absent in muscle. Muscle glycogenolysis produces glucose-1-phosphate → glucose-6-phosphate, which enters glycolysis directly to power contractions — it cannot be exported to blood. Liver glycogen, by contrast, maintains blood glucose for the brain and other tissues. Option A may have some truth but doesn't explain the mechanism. Options C and D describe regulatory details that don't address the core metabolic logic.
Question 2 Multiple Choice
Glycogen phosphorylase cleaves glucose residues from glycogen by phosphorolysis (using inorganic phosphate) rather than hydrolysis (using water). What is the key metabolic advantage of this mechanism?
APhosphorolysis is faster than hydrolysis, allowing quicker glucose mobilization during energy crises
BThe product, glucose-1-phosphate, is already phosphorylated and can enter glycolysis without the ATP cost of the hexokinase reaction
CPhosphorolysis prevents glucose from being exported from the cell, ensuring energy stays local
DUsing inorganic phosphate instead of water prevents osmotic disruption of the cell during rapid glycogenolysis
The energetic insight is key: hexokinase normally phosphorylates free glucose at the cost of one ATP before it can enter glycolysis. By releasing glucose-1-phosphate directly, phosphorylase bypasses this cost entirely. After phosphoglucomutase converts glucose-1-phosphate to glucose-6-phosphate, the molecule enters glycolysis without any ATP expenditure at the entry step — a significant efficiency gain during high energy demand. Option A is plausible but incorrect; no evidence suggests phosphorolysis is specifically faster. Option C is a consequence of the absence of glucose-6-phosphatase in muscle, not of the phosphorolysis mechanism itself.
Question 3 True / False
The branched structure of glycogen — with α-1,6 branch points every 8–12 residues — enables faster glucose mobilization than a linear polymer of the same molecular weight would provide.
TTrue
FFalse
Answer: True
Each branch tip is a potential simultaneous site for glycogen phosphorylase to attack. A highly branched glycogen molecule exposes many outer chain ends at once, allowing many phosphorylase enzymes to work in parallel. A linear polymer of the same mass would present only two ends. The trade-off is storage density — branching creates a less compact structure — but for animals that need rapid energy mobilization (unlike plants, which tolerate slower starch degradation), this trade-off is entirely worth it.
Question 4 True / False
Glycogenolysis is simply the reverse of glycogenesis, using the same enzymes and releasing the same intermediates in the opposite direction.
TTrue
FFalse
Answer: False
Synthesis and degradation use distinct enzymes and different chemical mechanisms. Glycogenesis uses glycogen synthase (with UDP-glucose as donor) and branching enzyme to create α-1,6 branch points. Glycogenolysis uses glycogen phosphorylase (phosphorolysis, releasing glucose-1-phosphate) and debranching enzyme (which transfers and hydrolyzes branches). The product of breakdown (glucose-1-phosphate) differs from the activated substrate of synthesis (UDP-glucose). This is a general metabolic principle: parallel but separate biosynthetic and degradative pathways allow independent regulation — cells can accelerate breakdown without simultaneously running synthesis.
Question 5 Short Answer
Why does glycogen have a branched structure rather than a simple linear chain, and how does this branching relate to its physiological function?
Think about your answer, then reveal below.
Model answer: Branching creates multiple simultaneous sites for phosphorylase to attack — each branch tip can be degraded in parallel, enabling rapid glucose release proportional to the number of chain ends. A linear chain exposes only its single terminus at a time. The branched architecture trades maximum storage density for maximum degradation speed, which is appropriate for an animal energy reserve that must respond to sudden demand like muscle contraction or a blood glucose crisis.
This is the core design insight of glycogen as a storage polymer: it optimizes for mobilization speed, not density. Starch, the plant storage polysaccharide, is less branched (amylopectin branches every 24–30 residues vs. glycogen's 8–12) because plants don't sprint. The more branch points, the more outer ends, the more simultaneous enzymatic attacks possible. The 8–12 residue branch frequency in glycogen represents an evolutionary optimization of this speed-versus-density trade-off for animals with high acute energy demands.