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Glucose-6-phosphatase is an enzyme that is involved in the final step of glycogenolysis and gluconeogenesis pathways. It catalyzes the hydrolysis of glucose-6-phosphate (G6P) to glucose and inorganic phosphate. The enzyme is found in the endoplasmic reticulum of hepatocytes (liver cells), renal tubules, and enterocytes (intestinal cells). However, glucose-6-phosphatase is not present in muscle cells.

In liver cells, the primary function of glucose-6-phosphatase is to regulate blood glucose levels by releasing glucose into the bloodstream from glycogen stores during fasting. When blood glucose levels are low, glycogen stored in the liver is broken down to glucose-6-phosphate through glycogenolysis. Glucose-6-phosphatase then converts glucose-6-phosphate to glucose which is then released into the bloodstream through the liver cells' Glucose Transporter 2 (GLUT2) transporter. In addition to glycogenolysis, glucose-6-phosphatase also plays a role in gluconeogenesis. This process takes place when glycogen stores are depleted or during prolonged fasting, and generates glucose from non-carbohydrate sources like lactate, amino acids, and glycerol. The final step of gluconeogenesis also involves the conversion of glucose-6-phosphate to glucose by glucose-6-phosphatase enzyme.

Glucose-6-phosphatase is not present in muscle cells. Instead, muscle cells rely primarily on glucose uptake from the bloodstream. Muscle cells store glucose as glycogen and utilize it for energy during exercise in the form of glucose-6-phosphate. Since muscle cells don't contain glucose-6-phosphatase, they cannot release glucose back into the bloodstream. Instead, they simply convert glucose-6-phosphate to glucose-1-phosphate, which is then used in glycolysis to produce ATP, the cellular energy currency. To summarize, the function of glucose-6-phosphatase in liver cells is to maintain blood glucose levels by converting glucose-6-phosphate to glucose during glycogenolysis and gluconeogenesis and subsequently releasing it into the bloodstream. In contrast, glucose-6-phosphatase is not present in muscle cells.