Which one of the following statements best describes the mechanism of action of insulin on target cells ?

**Core Concept**
Insulin is a hormone produced by the pancreas that plays a crucial role in glucose metabolism. It regulates blood glucose levels by facilitating glucose uptake in cells, particularly in skeletal muscle and adipose tissue. The mechanism of action of insulin involves a complex signaling cascade that ultimately leads to the uptake of glucose by cells.

**Why the Correct Answer is Right**
Insulin binds to its receptor on the surface of target cells, which triggers a conformational change in the receptor. This activates the insulin receptor tyrosine kinase, which phosphorylates and activates several downstream signaling molecules, including insulin receptor substrate 1 (IRS-1). IRS-1 then recruits and activates phosphatidylinositol 3-kinase (PI3K), which generates phosphatidylinositol 3,4,5-trisphosphate (PIP3). PIP3 activates protein kinase B (PKB/Akt), which in turn phosphorylates and inhibits glycogen synthase kinase 3 (GSK-3). Inhibited GSK-3 leads to the activation of glycogen synthase, resulting in glycogen synthesis and glucose storage. Furthermore, insulin also activates glucose transporter type 4 (GLUT4) translocation to the plasma membrane, facilitating glucose uptake in cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not accurately describe the mechanism of action of insulin. While insulin does have effects on lipid metabolism, its primary role is in glucose metabolism.
**Option B:** This option is incorrect as it implies that insulin directly activates glycogen synthase, whereas insulin actually inhibits GSK-3, which in turn activates glycogen synthase.
**Option C:** This option is incomplete and does not accurately describe the mechanism of action of insulin.

**Clinical Pearl / High-Yield Fact**
Insulin resistance, a hallmark of type 2 diabetes, is characterized by reduced insulin receptor tyrosine kinase activity and impaired PI3K signaling. Understanding the molecular mechanisms of insulin action is crucial for developing effective treatments for diabetes and related metabolic disorders.

**Correct Answer:** C. Insulin binds to its receptor on the surface of target cells, triggering a conformational change that activates the insulin receptor tyrosine kinase.

✓ Correct Answer: C. Insulin binds to a transmembrane receptor at the outer surface of the plasma membrane, which activates the tyrosine kinase that is the cytosolic domain of the receptor.