**Question:** Glucagon is secreted by: (PGI Nov 2010)

**Core Concept:**
Glucagon is a hormone produced and secreted by alpha cells in the pancreas islet, primarily responsible for lowering blood glucose levels by stimulating glycogenolysis and gluconeogenesis in the liver. It does this by acting on target cells and organs.

**Why the Correct Answer is Right:**
Glucagon acts on target cells and organs in a way that helps to counteract the effects of insulin, which promotes glucose uptake and storage in cells. Glucagon stimulates glycogenolysis in the liver, converting glycogen into glucose, which is released into the bloodstream. Additionally, glucagon promotes gluconeogenesis, creating glucose from non-carbohydrate sources like amino acids and glycerol. These actions help to lower blood glucose levels. The correct answer is A. Pancreas, as glucagon is secreted by alpha cells within the pancreas islet, specifically targeting liver cells and other cells where it exerts its effects.

**Why Each Wrong Option is Incorrect:**

A) **Pancreas:** While glucagon is indeed secreted by alpha cells in the pancreas, the correct answer is A. Pancreas (option A), not option B (Islets of Langerhans). Alpha cells are a specific type within the islets, not the whole pancreas. The islets of Langerhans are clusters of endocrine cells in the pancreas that produce and secrete hormones like insulin, glucagon, and others.

B) **Islets of Langerhans:** This option combines both alpha and beta cells (producing glucagon and insulin, respectively) within the islets of Langerhans. While glucagon is indeed secreted by alpha cells, the entire islets of Langerhans are incorrect because beta cells also produce insulin.

C) **Liver:** While glucagon does affect liver cells, the correct answer is A. Pancreas, as it is the alpha cells in the pancreas that secrete glucagon. The liver is an organ that responds to glucagon, but the hormone is produced by the alpha cells in the pancreas.

D) **Insulin-secreting cells:** This option is incorrect because insulin-secreting beta cells are different from glucagon-secreting alpha cells. While both insulin and glucagon are produced by different cells in the pancreas, the correct answer is A. Pancreas, focusing on the specific alpha cells that produce glucagon.

**Core Concept:**
Glucagon acts as a counter-regulatory hormone to insulin, helping to maintain glucose homeostasis in response to low blood glucose levels. It does this by stimulating glycogenolysis in the liver, increasing blood glucose levels.

**Why Each Wrong Option is Incorrect:**

A) **Insulin-secreting beta cells:** Insulin is produced by beta cells, not alpha cells, and its primary function is to lower blood glucose levels, primarily by stimulating glucose uptake in tissues like muscles, fat, and liver.

B) **Islets of Langerhans:** Both alpha and beta