Liver glycogen versus muscle glycogen
Liver glycogen can raise blood glucose because liver cells have glucose-6-phosphatase, the enzyme that removes the phosphate from glucose-6-phosphate (glucose carrying a phosphate), giving free glucose (glucose without the phosphate).
Remember
Muscle has no glucose-6-phosphatase, so its glycogen cannot directly supply blood glucose; the liver has the enzyme, so it can share its glycogen with the whole body.
Memory aid
the liver has the exit door, muscle does not
Picture glucose-6-phosphatase as the exit door that lets free glucose leave the cell.
Glycogen is a branched molecule that stores glucose inside cells. The liver holds about 80 to 100 g of it, and this can be used to raise blood glucose. Skeletal muscle holds a larger store, about 300 to 400 g, but only the muscle itself can use it.
The liver is different because of an enzyme. When glycogen is broken down (glycogenolysis), the product is glucose-6-phosphate, which carries a phosphate group (a charged chemical tag). The phosphate group gives it a charge, so it cannot cross the cell membrane and is trapped inside the cell. To get glucose into the blood, the phosphate group must be removed by glucose-6-phosphatase, which turns glucose-6-phosphate into free glucose (glucose without the phosphate) that can leave the cell.
The liver has glucose-6-phosphatase, so liver glycogen can be turned into free glucose to raise blood glucose between meals. Skeletal muscle lacks it. Its glucose-6-phosphate therefore stays in the cell and goes into glycolysis (the pathway that breaks glucose down to make ATP) to power contraction.
So the liver shares its glycogen with the whole body, while muscle keeps its glycogen for itself.
In von Gierke disease (a glycogen storage disease), glucose-6-phosphatase is missing, so glycogen builds up in the liver and the patient develops severe hypoglycaemia (low blood glucose) on fasting.
Key facts
- Liver store
- about 80 to 100 g of glycogen, which can be used to raise blood glucose.
- Muscle store
- about 300 to 400 g of glycogen, which can only be used by the muscle itself.
- Decisive enzyme
- glucose-6-phosphatase removes the phosphate from glucose-6-phosphate to give free glucose.
- In muscle
- without the enzyme, glucose-6-phosphate stays in the cell and goes into glycolysis to make ATP.
- Shared steps
- glycogen phosphorylase (which breaks the α1-4 bonds in the straight chains) and the debranching enzyme (which breaks the α1-6 bonds at the branch points) act in both liver and muscle, so they do not separate the two tissues.
- Clinical link
- when glucose-6-phosphatase is missing, as in von Gierke disease, glycogen builds up in the liver.
Exam traps
- Muscle holds more glycogen than the liver does, about 300 to 400 g against about 80 to 100 g, so the amount of glycogen does not explain the difference.
- Muscle glycogen does not have a different structure from liver glycogen. Both are mainly α1-4 chains with α1-6 branches, so branching does not decide whether glucose can be exported.
- Muscle cells can break glycogen down, and they do so readily during exercise to meet their own energy needs. The trouble lies in releasing glucose, not in the breakdown.
Read the full notes
Tested in 8 SimpleMed+ questions
Practice it in Metabolism, with the answer and a full explanation after every question.
Practice these questions