The big idea: Feeling hungry or full is not just a feeling — it is controlled by hormones.
A small region of the brain called the hypothalamus is the appetite centre. It listens to hormones that report on the body's energy state.
Three hormones do most of the talking: ghrelin (says 'eat'), and leptin and insulin (say 'stop'). Keeping these in balance is part of homeostasis — keeping the internal environment steady.
- Appetite
- The desire to eat — switched up or down by hormones acting on the brain.
- Hypothalamus
- The region of the brain that acts as the appetite (feeding) centre; it receives the hormone signals and adjusts hunger.
- Ghrelin
- The 'hunger hormone', secreted by the lining of the empty stomach. It stimulates appetite before a meal.
- Leptin
- A hormone secreted by adipose (fat) tissue. It acts on the hypothalamus to suppress appetite, reporting how large the fat stores are.
- Insulin
- A hormone from the pancreas, released after a meal as blood glucose rises; it also helps suppress appetite.
Two jobs, opposite directions: Ghrelin turns appetite UP (you feel hungry).
Leptin and insulin turn appetite DOWN (you feel full).
The exam loves the phrase 'source, target and function' for each hormone — learn all three for leptin especially.
For each appetite hormone the exam wants the same three facts: where it is made (source), what it acts on (target), and what it does (function).
Learn them as a short chain: a hormone is released by an organ, travels in the blood, and acts on the hypothalamus to change hunger.
| Hormone | Source (where it is made) | Effect on appetite |
|---|---|---|
| Ghrelin | Lining of the (empty) stomach | The 'hunger hormone' — rises before a meal and STIMULATES appetite |
| Leptin | Adipose (fat) tissue | Reports fat stores to the brain — high leptin SUPPRESSES appetite ('stop eating') |
| Insulin | Pancreas (after a meal) | Released as blood glucose rises after eating — helps SUPPRESS appetite |
Leptin — the one they ask about most: Source: adipose (fat) tissue secretes leptin.
Target: leptin acts on the hypothalamus in the brain.
Function: it suppresses appetite (makes you feel full).
The logic: more body fat → more leptin → a stronger 'stop eating' signal. So leptin is a long-term signal of how much energy is stored as fat — which is why it is studied in obesity research.
How appetite is regulated, step by step
- Before a meal the stomach is empty, so it releases ghrelin.
- Ghrelin travels in the blood to the hypothalamus, which triggers hunger — you feel like eating.
- After eating, blood glucose rises, so the pancreas releases insulin; insulin helps signal 'full'.
- Fat (adipose) tissue releases leptin in proportion to how much fat is stored.
- Leptin and insulin act on the hypothalamus to suppress appetite — you stop eating.
- Over time, this balance of 'eat' and 'stop' signals keeps body mass roughly steady — homeostasis.
| Signal to the brain | When it is strong | What the hypothalamus does |
|---|---|---|
| Ghrelin (hunger) | Stomach is empty (before a meal) | Triggers the sensation of hunger — you feel like eating |
| Leptin (satiety) | Large fat stores | Reduces appetite — over time this keeps body mass roughly steady |
| Insulin (satiety) | Just after a meal, glucose high | Reinforces the 'full' signal so eating stops |
Signals that say 'EAT'
- Ghrelin from the empty stomach
- Rises before a meal
- Acts on the hypothalamus → hunger
Signals that say 'STOP'
- Leptin from adipose (fat) tissue (long-term)
- Insulin from the pancreas (after a meal)
- Both act on the hypothalamus → appetite suppressed
A memory hook: Ghrelin → Growls (the hungry stomach). Leptin → Less appetite (from your fat Larder). Insulin comes with a meal, so it joins the 'stop' team.
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How this is tested: On Paper 1 a 1-mark item gives a hormone (often leptin) and asks you to match its source, target and function — adipose tissue, the hypothalamus, suppresses appetite.
On Paper 3 a 2-mark Identify asks you to name two appetite-control hormones and the structures that secrete them — pick from ghrelin (stomach), leptin (adipose tissue), insulin (pancreas).
Also on Paper 3, a 2-mark Outline asks you to outline the mechanisms that regulate appetite — name the hypothalamus and at least one 'eat' and one 'stop' hormone.
IB-style question — outline how appetite is regulated
Outline the body mechanisms that regulate appetite in humans. [2]
How to score both marks
- Name the control centre and a 'hunger' signal. The hypothalamus in the brain is the appetite centre; ghrelin, secreted by the empty stomach, acts on it to stimulate hunger before a meal.
- Give a 'stop' signal. After eating, leptin from adipose (fat) tissue (and insulin from the pancreas) act on the hypothalamus to suppress appetite, so you feel full. (Mark 1: hypothalamus + a hunger signal. Mark 2: a satiety/'stop' hormone with its source.)
Final answer
The hypothalamus is the appetite centre. Ghrelin from the empty stomach stimulates hunger; leptin from fat (adipose) tissue and insulin from the pancreas act on the hypothalamus to suppress appetite after a meal.
✓ Why this scores full marks: It names the hypothalamus (the control centre) and gives both directions — a hormone that raises appetite (ghrelin) and a hormone that lowers it (leptin/insulin), each with its source.
An answer that only says 'the stomach makes you hungry' gives one direction and no control centre — it would not score the second mark.
| Thyroxin level | Metabolic rate | Typical symptoms |
|---|---|---|
| Too LITTLE thyroxin | Low (slow) metabolic rate | Tiredness, weight gain, feeling cold, slow heart rate |
| Normal thyroxin | Normal metabolic rate | Energy use balanced — healthy, stable body mass |
| Too MUCH thyroxin | High (fast) metabolic rate | Weight loss, feeling hot, fast heart rate, restlessness |
Spotting a hormone from symptoms — thyroxin: Thyroxin is made by the thyroid gland and sets the body's basal metabolic rate (how fast cells use energy).
If a patient has too little thyroxin, the metabolic rate is low, giving symptoms like tiredness, weight gain and feeling cold.
So a 1-mark Paper 1 question that lists those symptoms is pointing at thyroxin being produced in lower amounts — read the symptoms, then name the missing hormone.