The hormone that directly stimulates the hunger center is:
a. Ghrelin
b. Cholecystokinin
c. Insulin
d. Leptin

Verified step by step guidance

1. Step 1: Understand the role of the hunger center in the brain. The hunger center is located in the hypothalamus, specifically in the lateral hypothalamus, which regulates hunger and appetite.
2. Step 2: Review the function of each hormone listed in the options: Ghrelin, Cholecystokinin, Insulin, and Leptin. Each of these hormones plays a role in energy balance and appetite regulation.
3. Step 3: Focus on Ghrelin. Ghrelin is known as the 'hunger hormone' because it is secreted by the stomach when it is empty and directly stimulates the hypothalamus to increase hunger.
4. Step 4: Contrast Ghrelin with the other hormones. Cholecystokinin (CCK) is released by the small intestine and promotes satiety. Insulin regulates blood glucose levels and can indirectly influence hunger. Leptin is secreted by fat cells and signals the brain to reduce appetite when energy stores are sufficient.
5. Step 5: Conclude that the hormone which directly stimulates the hunger center is Ghrelin, based on its specific role in signaling hunger to the hypothalamus.

Key Concepts

Ghrelin

Ghrelin is a hormone produced primarily in the stomach that plays a crucial role in regulating appetite. It is often referred to as the 'hunger hormone' because it stimulates the hunger center in the brain, signaling the body to seek food. Elevated levels of ghrelin are associated with increased appetite and food intake.

Cholecystokinin (CCK)

Cholecystokinin is a hormone released by the small intestine in response to food intake, particularly fats and proteins. It aids in digestion by stimulating the release of digestive enzymes and bile, and it also promotes a feeling of fullness, thereby reducing appetite. Unlike ghrelin, CCK acts to suppress hunger rather than stimulate it.

Leptin

Leptin is a hormone produced by adipose (fat) tissue that helps to regulate energy balance by inhibiting hunger. It signals the brain about the body's fat stores, promoting a sense of satiety when fat levels are adequate. High levels of leptin typically correlate with reduced appetite, while low levels can lead to increased hunger.