True about Insulin:
**Core Concept**
Insulin is a hormone secreted by the beta cells of the pancreatic islets that plays a crucial role in glucose metabolism by facilitating glucose uptake in cells, thereby lowering blood glucose levels. It acts by binding to the insulin receptor, a transmembrane receptor tyrosine kinase, on the surface of target cells.
**Why the Correct Answer is Right**
The correct answer involves understanding the mechanism of insulin action. Insulin binds to the insulin receptor, which triggers a cascade of intracellular signaling events. This leads to the activation of various downstream effectors, including protein kinase B (Akt), which ultimately results in the translocation of glucose transporter type 4 (GLUT4) to the cell surface, allowing glucose to enter the cell. This process is essential for maintaining normal blood glucose levels.
**Why Each Wrong Option is Incorrect**
**Option A:** Insulin is secreted by the alpha cells of the pancreatic islets. This is incorrect because insulin is actually secreted by the beta cells of the pancreatic islets.
**Option B:** Insulin acts by increasing glucose production in the liver. This is incorrect because insulin actually decreases glucose production in the liver by inhibiting gluconeogenesis.
**Option C:** Insulin receptor is a ligand-gated ion channel. This is incorrect because the insulin receptor is a transmembrane receptor tyrosine kinase, not a ligand-gated ion channel.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that insulin resistance, a condition characterized by decreased insulin sensitivity, is a hallmark of type 2 diabetes. Insulin resistance can be caused by various factors, including obesity, physical inactivity, and genetic predisposition.
**Correct Answer:** D.