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This lesson covers hormonal communication in animals as required by the Edexcel A-Level Biology specification (9BI0), Topic 9 -- Control Systems. You need to understand how hormones act as chemical messengers, the structure of the endocrine system, and how hormonal signalling differs from nervous communication.
The endocrine system is a collection of ductless glands that secrete hormones directly into the bloodstream. Hormones are chemical messengers that travel in the blood plasma to target cells, where they bind to specific receptors and trigger a response.
| Feature | Endocrine (Hormonal) System | Nervous System |
|---|---|---|
| Signal type | Chemical (hormones) | Electrical (nerve impulses) and chemical (neurotransmitters) |
| Speed of transmission | Relatively slow (seconds to minutes) | Very fast (milliseconds) |
| Duration of response | Long-lasting (minutes to hours, sometimes days) | Short-lived (usually milliseconds) |
| Specificity | Affects target cells with specific receptors anywhere in the body | Targeted to specific effectors via neurones |
| Mode of travel | Via the bloodstream | Along neurones |
Exam Tip: A common exam question asks you to compare hormonal and nervous communication. Always include at least four points of comparison and use the correct terminology -- 'target cells with specific receptors' not just 'target organs'.
Hormones can be broadly classified into two types based on their chemical structure, which determines their mechanism of action:
Steroid hormones are derived from cholesterol and are lipid-soluble. Because they are non-polar, they can pass directly through the phospholipid bilayer of cell membranes.
Peptide hormones (e.g. insulin, glucagon, ADH) and protein hormones are water-soluble but lipid-insoluble. They cannot cross the cell membrane.
The second messenger model is a key concept in understanding how water-soluble hormones bring about their effects:
| Step | Component | Role |
|---|---|---|
| 1 | Hormone | First messenger; binds to receptor |
| 2 | Receptor | Transmembrane protein; specific to hormone |
| 3 | G-protein | Relay protein; activates adenylyl cyclase |
| 4 | Adenylyl cyclase | Enzyme; converts ATP to cAMP |
| 5 | cAMP | Second messenger; activates protein kinase A |
| 6 | Protein kinase A | Enzyme; phosphorylates target proteins |
Exam Tip: When describing the second messenger model, always mention that the hormone binds to a specific receptor (complementary shape) and that cAMP amplifies the signal. The cascade effect is important -- examiners look for the idea that one hormone molecule leads to a large cellular response.
| Gland | Hormone(s) | Key Function |
|---|---|---|
| Hypothalamus | Releasing hormones (e.g. TRH, GnRH) | Controls the anterior pituitary |
| Anterior pituitary | FSH, LH, TSH, ACTH, growth hormone | Controls other endocrine glands |
| Posterior pituitary | ADH, oxytocin | Water balance, uterine contraction |
| Thyroid | Thyroxine (T4) | Metabolic rate |
| Adrenal medulla | Adrenaline (epinephrine) | Fight-or-flight response |
| Adrenal cortex | Cortisol, aldosterone | Stress response, ion balance |
| Pancreas (islets of Langerhans) | Insulin (beta cells), glucagon (alpha cells) | Blood glucose regulation |
| Ovaries | Oestrogen, progesterone | Female sexual development, menstrual cycle |
| Testes | Testosterone | Male sexual development |
The hypothalamus is the key link between the nervous system and the endocrine system. It receives nervous input from the brain and responds by releasing releasing hormones (or inhibiting hormones) that act on the anterior pituitary gland.
The anterior pituitary then secretes tropic hormones (hormones that stimulate other glands), such as:
This creates a hierarchical control system with multiple levels of regulation.
Adrenaline (also called epinephrine) is released from the adrenal medulla in response to stress, fear, or excitement. It prepares the body for rapid action:
Adrenaline is a water-soluble hormone that acts via the second messenger pathway (cAMP). Its effects are rapid but relatively short-lived.
Exam Tip: Adrenaline is secreted by the adrenal medulla (not the cortex). The adrenal cortex secretes cortisol and aldosterone. Do not confuse these two regions.
| Feature | Endocrine Glands | Exocrine Glands |
|---|---|---|
| Ducts | No ducts (ductless) | Secrete via ducts |
| Secretion destination | Directly into the blood | Into a cavity or onto a surface |
| Examples | Thyroid, adrenal, pituitary | Salivary glands, sweat glands, pancreas (exocrine portion) |
The pancreas is unique because it has both endocrine and exocrine functions. The islets of Langerhans are the endocrine portion (secreting insulin and glucagon), while the acinar cells are the exocrine portion (secreting digestive enzymes into the pancreatic duct).
A hormone can only affect a target cell -- a cell that has specific receptors with a shape complementary to the hormone. Non-target cells lack these receptors and are unaffected.
Key points: