Chemical Control: Hormones in Animals

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.

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The Endocrine System

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'.

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Types of Hormones

Hormones can be broadly classified into two types based on their chemical structure, which determines their mechanism of action:

Steroid Hormones

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.

• Examples: oestrogen, testosterone, progesterone, cortisol
• They enter the cell and bind to intracellular receptors (often in the cytoplasm or nucleus).
• The hormone-receptor complex acts as a transcription factor, binding to specific DNA sequences and activating or repressing gene transcription.
• This mechanism means steroid hormones tend to have slow but long-lasting effects because they alter protein synthesis.

Peptide and Protein Hormones

Peptide hormones (e.g. insulin, glucagon, ADH) and protein hormones are water-soluble but lipid-insoluble. They cannot cross the cell membrane.

• They bind to receptors on the cell surface membrane.
• Binding activates a second messenger system inside the cell, most commonly involving cyclic AMP (cAMP).
• The second messenger triggers a cascade of intracellular reactions, amplifying the signal and producing a rapid response.

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The Second Messenger Model (cAMP)

The second messenger model is a key concept in understanding how water-soluble hormones bring about their effects:

1. The hormone (the first messenger) binds to a specific complementary receptor on the target cell surface membrane.
2. This binding activates a G-protein associated with the receptor on the inner surface of the membrane.
3. The activated G-protein in turn activates the enzyme adenylyl cyclase (also called adenylate cyclase).
4. Adenylyl cyclase converts ATP into cyclic AMP (cAMP) -- the second messenger.
5. cAMP activates protein kinase A (an enzyme), which phosphorylates other enzymes, triggering a cascade of reactions.
6. The cascade amplifies the signal -- one hormone molecule can lead to the production of millions of product molecules.

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.

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Major Endocrine Glands and Their Hormones

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

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The Hypothalamus-Pituitary Axis

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:

• TSH (thyroid-stimulating hormone) -- stimulates the thyroid to release thyroxine
• ACTH (adrenocorticotropic hormone) -- stimulates the adrenal cortex
• FSH and LH -- stimulate the ovaries and testes

This creates a hierarchical control system with multiple levels of regulation.

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Adrenaline and the Fight-or-Flight Response

Adrenaline (also called epinephrine) is released from the adrenal medulla in response to stress, fear, or excitement. It prepares the body for rapid action:

• Increases heart rate and stroke volume (increases cardiac output)
• Dilates bronchioles (increases airflow to the lungs)
• Dilates pupils
• Stimulates glycogenolysis in the liver (releases glucose into the blood)
• Redirects blood flow from the gut to skeletal muscles
• Increases metabolic rate

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.

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Exocrine vs Endocrine Glands

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).

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Cell Signalling and Target Cells

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:

• Specificity depends on the complementary shape of the receptor to the hormone (lock-and-key principle).
• Different tissues can respond differently to the same hormone because they have different intracellular signalling pathways.
• The concentration of receptors on a target cell can be up-regulated (increased) or down-regulated (decreased), altering the cell's sensitivity to the hormone.

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Summary

• The endocrine system uses hormones -- chemical messengers transported in the blood -- to coordinate body functions.
• Steroid hormones are lipid-soluble, enter cells, and act as transcription factors.
• Peptide hormones are water-soluble, bind to surface receptors, and use the cAMP second messenger system.
• The hypothalamus-pituitary axis links the nervous and endocrine systems.
• Adrenaline mediates the fight-or-flight response via cAMP signalling.
• Hormones only affect target cells with specific complementary receptors.