Hormones and the Endocrine System

Spec Mapping — OCR H420 Module 5.1.4 — Hormonal communication, content statements covering the principles of endocrine signalling, the distinction between endocrine and exocrine glands, the major endocrine glands and their hormones, and the role of receptors and second messengers in the cellular response to hormones (refer to the official OCR H420 specification document for exact wording). This lesson lays the conceptual groundwork for the adrenal/pancreas, blood-glucose, and diabetes lessons that follow.

Whereas the nervous system delivers lightning-fast, pinpoint-accurate electrical signals, the endocrine system delivers slower, longer-lasting chemical messages that wash over the entire body in the bloodstream. The two systems complement each other: together they achieve homeostasis, coordinate development, regulate metabolism and respond to stress.

The endocrine concept has a long scientific lineage. Arnold Berthold (1849, paraphrase) showed by castration and transplantation experiments on cockerels that an unknown substance from the testes was required for the development of male secondary sexual characteristics — the first demonstration of an internal chemical messenger. William Bayliss and Ernest Starling (1902, paraphrase) coined the word "hormone" (from the Greek hormao, "I excite") for the gut peptide they called secretin, isolated from duodenal mucosa and shown to stimulate pancreatic exocrine secretion. Frederick Banting and Charles Best (1921, paraphrase) isolated insulin from pancreatic islet extracts and demonstrated its life-saving effect in dogs and then humans with type 1 diabetes — work that won Banting and Macleod the 1923 Nobel Prize. Earl Sutherland (Nobel 1971, paraphrase) discovered cyclic AMP (cAMP) as the second messenger downstream of adrenaline binding to its surface receptor — the first identified intracellular second messenger, and the founder of modern signal transduction biology. Robert Lefkowitz and Brian Kobilka (Nobel 2012, paraphrase) determined the structural basis of G-protein-coupled receptor activation, including the β-adrenergic receptor that responds to adrenaline. Each of these conceptual landmarks now sits inside the framework of OCR Module 5.1.4.

Key Definitions:

• Hormone — a chemical messenger released from an endocrine gland into the blood, travelling to distant target cells where it produces a specific response.
• Endocrine gland — a ductless gland that secretes hormones directly into the bloodstream.
• Exocrine gland — a gland that secretes via a duct onto an epithelial surface (e.g. sweat glands, salivary glands).
• Target cell — a cell bearing specific receptors for a particular hormone.
• Second messenger — an intracellular molecule produced in response to a hormone binding its receptor; it amplifies and spreads the signal.

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Endocrine vs Exocrine Glands

Many students struggle to remember the difference between endocrine and exocrine glands. A simple rule: "exo" means "out" — exocrine glands secrete out through a duct onto an epithelial surface or lumen; "endo" means "in" — endocrine glands secrete in to the blood. The mnemonic is reliable and worth memorising.

Feature	Endocrine	Exocrine
Ducts?	No	Yes
Secretes into	Blood	Epithelial surface / lumen
Example	Pancreas (islets), adrenal, pituitary, thyroid	Sweat glands, salivary glands, pancreas (acini)
Target	Distant cells with matching receptors	Local (the organ containing the duct's opening)
Speed of effect	Seconds to days	Seconds

Note that the pancreas has both endocrine and exocrine components — a classic exam point. Islets of Langerhans (endocrine) secrete insulin and glucagon into the blood; the acini (exocrine) secrete pancreatic juice containing digestive enzymes into the pancreatic duct. The same organ, two functionally distinct cell populations, two completely different secretion routes. Histologically, the islets stain pale against the dark exocrine acini, which is how they were originally identified by Paul Langerhans in 1869 (paraphrase) before their function was understood.

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Major Endocrine Glands and Their Hormones

Gland	Hormone(s)	Main function
Hypothalamus	Releasing factors (e.g. CRH, TRH, GnRH)	Control the pituitary
Anterior pituitary	ACTH, TSH, FSH, LH, GH, prolactin	Control other endocrine glands and growth
Posterior pituitary	ADH, oxytocin	Water balance; uterine contraction / milk ejection
Thyroid	Thyroxine (T3, T4), calcitonin	Metabolism; Ca²⁺ homeostasis
Parathyroid	PTH	Ca²⁺ homeostasis
Adrenal cortex	Cortisol, aldosterone, androgens	Stress response, Na⁺/K⁺ balance
Adrenal medulla	Adrenaline, noradrenaline	Fight or flight response
Pancreas (islets)	Insulin, glucagon	Blood glucose homeostasis
Ovaries	Oestrogen, progesterone	Menstrual cycle, pregnancy
Testes	Testosterone	Spermatogenesis, secondary sexual characteristics
Pineal	Melatonin	Circadian rhythms, sleep

OCR expects detailed knowledge of the adrenal gland and the pancreas, which are covered in the next lesson. Here we focus on the general principles of hormone action.

Endocrine glands of the human body — schematic

Schematic only — not to scale. Pineal gland (circadian) sits inside the brain.

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Two Classes of Hormone: Steroid and Protein

Hormones fall into two broad molecular categories, and their modes of action differ fundamentally.

Steroid Hormones

• Derived from cholesterol, lipid-soluble (synthesised in the smooth ER and mitochondria of cortex cells and gonad cells).
• Examples: cortisol, aldosterone, testosterone, oestrogen, progesterone.
• Can pass directly through the plasma membrane because of their lipid solubility.
• Bind to intracellular receptors (usually in the cytoplasm or nucleus); the hormone-receptor complex then translocates to the nucleus if cytoplasmic.
• The hormone-receptor complex acts as a transcription factor, binding to specific DNA sequences called hormone-response elements and switching genes on or off.
• Effects are slow (hours) but long-lasting, because they involve new mRNA synthesis and new protein synthesis.
• Carried in the blood bound to plasma proteins (e.g. sex hormone-binding globulin, cortisol-binding globulin) — only the free fraction is biologically active.

Protein / Peptide Hormones

• Water-soluble and cannot cross the plasma membrane (peptide hormones are too large; catecholamines are too polar).
• Examples: insulin, glucagon, adrenaline (technically a catecholamine, not a peptide, but acts through the same pathway), ADH, growth hormone, FSH, LH.
• Bind to cell-surface receptors (usually G-protein coupled receptors or tyrosine kinase receptors).
• Receptor activation triggers intracellular signalling via a second messenger (e.g. cAMP, IP3, DAG, Ca²⁺).
• Effects are fast (seconds to minutes) but generally short-lived (second messengers are quickly degraded by phosphodiesterases or buffered).
• Carried free in plasma (or in vesicles for storage in the secreting cell, e.g. insulin in β-cell granules).

Feature	Steroid hormones	Protein / peptide hormones
Solubility	Lipid-soluble	Water-soluble
Receptor location	Inside the cell (cytoplasm / nucleus)	Cell-surface membrane
Mode of action	Direct effect on gene transcription	Second messenger cascade
Speed of response	Slow (hours)	Fast (seconds–minutes)
Duration	Long-lasting	Short-lived
Examples	Cortisol, testosterone, oestrogen	Insulin, glucagon, adrenaline

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Second Messengers — The Cyclic AMP System

Most protein hormones act via the cyclic AMP (cAMP) second messenger system. The classic example OCR expects you to describe is adrenaline acting on a liver cell to raise blood glucose.

Step-by-Step

1. Adrenaline (the first messenger) binds to its specific receptor on the outside of the liver cell membrane.
2. The receptor (a G-protein coupled receptor) undergoes a conformational change that activates an associated G-protein on the inside of the membrane.
3. The activated G-protein activates the enzyme adenylyl cyclase (also spelled adenylate cyclase).
4. Adenylyl cyclase catalyses the conversion of ATP to cyclic AMP (cAMP) — the second messenger.
5. cAMP activates protein kinase A, which phosphorylates other enzymes, including those involved in glycogenolysis.
6. The net result: glycogen is broken down to glucose, which is released into the blood.

flowchart LR
    AD["Adrenaline<br/>first messenger"] --> R[Receptor on membrane]
    R --> G[G-protein activated]
    G --> AC[Adenylyl cyclase]
    AC --> cAMP["ATP → cAMP<br/>second messenger"]
    cAMP --> PK[Protein kinase A activated]
    PK --> GL[Glycogenolysis enzymes phosphorylated]
    GL --> GLU[Glucose released into blood]

Why Second Messengers Matter

The second-messenger cascade achieves enormous amplification. A single adrenaline molecule can cause the production of many cAMP molecules, each of which activates many protein kinase A molecules, each of which phosphorylates many target enzymes. One hormone molecule can therefore lead to the release of thousands of glucose molecules. This is a form of signal amplification and is one reason hormones act in very low concentrations (10⁻¹² to 10⁻⁹ mol dm⁻³).

The adrenaline cascade in detail

flowchart LR
    A["Adrenaline (first messenger)<br/>nanomolar blood concentration"] --> R["β-adrenergic receptor<br/>(GPCR on liver cell membrane)"]
    R --> G["Gs protein activated<br/>(GDP → GTP swap)"]
    G --> AC["Adenylyl cyclase activated"]
    AC --> CAMP["ATP → cyclic AMP<br/>(second messenger; hundreds per receptor)"]
    CAMP --> PKA["Protein kinase A activated<br/>(catalytic + regulatory subunits separate)"]
    PKA --> PHOK["Phosphorylase kinase phosphorylated → active"]
    PHOK --> PHO["Glycogen phosphorylase phosphorylated → active"]
    PHO --> G1P["Glycogen → glucose-1-phosphate"]
    G1P --> G6P["glucose-6-phosphate"]
    G6P --> GLU["Free glucose released into blood<br/>(via glucose-6-phosphatase, liver only)"]