blood cells and the brain. Glycogen stores in the liver can provide glucose when none is available from the gut. The glucose carrier (page 238) allows glucose to enter and leave the liver cells but cannot transport phosphorylated sugars, so glucose-6-phosphate must be converted to free glucose for transport out into the extracellular medium and from there to the blood. To do this, liver has the enzyme glucose-6-phosphate phosphatase, which removes the phosphate group. Muscle also has stores of glycogen, but these are for its own use: it does not have glucose-6-phosphate phosphatase so cannot release glucose into the extracellular medium.

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