Hormonal communication
BiologyΒ· Unit 16: Control and CoordinationΒ· 25 min read
1. Endocrine vs Exocrine Glandsβ β ββββ± 5 min
Endocrine Gland
Ductless glands that secrete hormones (chemical messengers) directly into blood plasma to target distant organs
Exocrine Gland
Glands that secrete their products through a duct onto an epithelial surface, rather than into the bloodstream
Example:
Salivary glands, sweat glands
Hormonal communication differs from nervous coordination in several key ways that examiners regularly test:
Hormones travel via the blood, while nerve impulses travel along neurons
Hormonal responses are generally slower but longer-lasting than nervous responses
Hormonal communication affects multiple organs, rather than being localized to one area
Classify the following glands as endocrine, exocrine, or both, and explain your answer: (a) Pancreas (b) Salivary gland (c) Pituitary gland
- 1
Recall the core difference: endocrine glands are ductless and secrete into blood; exocrine glands secrete through a duct.
- 2
(a) Pancreas: Both. The exocrine portion secretes digestive enzymes via the pancreatic duct into the small intestine. The endocrine portion (islets of Langerhans) secretes insulin and glucagon directly into blood.
- 3
(b) Salivary gland: Exocrine. It secretes saliva via the salivary duct onto the inside of the mouth.
- 4
(c) Pituitary gland: Endocrine. It is ductless and secretes hormones directly into the bloodstream to target organs.
2. Steroid vs Non-Steroid Hormone Mechanismsβ β β βββ± 8 min
Hormones are grouped by chemical structure, which determines how they interact with target cells:
Steroid Hormones
Lipid-soluble hormones derived from cholesterol that can diffuse across the phospholipid bilayer of cell membranes
Example:
Oestrogen, testosterone, cortisol
Non-Steroid Hormones
Water-soluble hormones derived from amino acids that cannot cross the hydrophobic core of cell membranes, so bind to cell surface receptors
Example:
Insulin, glucagon, adrenaline
Steroid hormones act directly on DNA: 1. Diffuse across the cell membrane into the cytoplasm 2. Bind to an intracellular receptor 3. The hormone-receptor complex enters the nucleus and alters gene transcription 4. New proteins are produced to trigger the response.
Non-steroid hormones use a secondary messenger system: 1. Bind to a cell surface receptor 2. Activate a G-protein, which activates adenylate cyclase 3. Adenylate cyclase converts ATP to cAMP (cyclic AMP), the secondary messenger 4. cAMP activates intracellular enzymes to produce the response.
Explain why non-steroid hormones require a secondary messenger, while steroid hormones do not.
- 1
Relate chemical solubility to the permeability of the cell membrane, which is made of a phospholipid bilayer.
- 2
The phospholipid bilayer is permeable to lipid-soluble substances, but not to polar, water-soluble substances.
- 3
Steroid hormones are lipid-soluble, so they can diffuse directly across the membrane to bind receptors inside the cell, no secondary messenger needed.
- 4
Non-steroid hormones are polar and water-soluble, so they cannot cross the hydrophobic membrane core. They bind to cell surface receptors and use cAMP as a secondary messenger to transmit the signal into the cell.
Test your understanding:
Which molecule acts as the secondary messenger for non-steroid hormone action?
ATP
cAMP
G protein
Adenylate cyclase
Reveal answer
cAMP βCorrect! cAMP is the second messenger that activates intracellular enzymes. G proteins and adenylate cyclase are intermediates in the pathway, not the secondary messenger.
3. Hormonal Regulation of Blood Glucoseβ β β βββ± 7 min
Blood glucose regulation is the most commonly tested example of hormonal communication in CIE exams. The normal set point for blood glucose is ~90 mg per 100 cmβ»Β³ of blood.
Islets of Langerhans
Endocrine tissue in the pancreas that contains alpha cells (secrete glucagon) and beta cells (secrete insulin)
When blood glucose rises above set point (after a meal), beta cells secrete more insulin
Insulin increases cell membrane permeability to glucose, and triggers glycogen synthesis (glycogenesis) from excess glucose in liver and muscle cells
This lowers blood glucose back to set point
When blood glucose falls below set point (during fasting), alpha cells secrete more glucagon
Glucagon triggers breakdown of glycogen to glucose (glycogenolysis) in liver cells, which releases glucose into the blood to raise concentration back to set point
Adrenaline (from adrenal glands) also increases blood glucose during stress by activating glycogenolysis
Explain how hormonal communication returns blood glucose to normal after a person eats a meal high in carbohydrates.
- 1
Carbohydrates are digested into glucose, which is absorbed into the bloodstream, causing blood glucose concentration to rise above the normal set point.
- 2
The rise in blood glucose is detected by beta cells in the islets of Langerhans of the pancreas.
- 3
Beta cells secrete increased amounts of insulin into the bloodstream.
- 4
Insulin binds to cell surface receptors on liver, muscle and adipose cells, increasing their permeability to glucose so more glucose is taken up from the blood.
- 5
Insulin also activates enzymes that convert excess glucose into glycogen for storage via glycogenesis.
- 6
As glucose is removed from the blood and stored, blood glucose concentration falls back to the normal set point, and insulin secretion is reduced to maintain homeostasis.
4. Comparing Nervous and Hormonal Coordinationβ β ββββ± 5 min
This comparison is a very common 4-6 mark question in CIE exams. The key differences are summarised in the table below:
Feature | Nervous Communication | Hormonal Communication |
|---|---|---|
Speed of response | Very fast (milliseconds) | Slow (seconds to days) |
Duration of response | Short-lived | Long-lasting |
Transport | Impulses along neurons | Hormones in blood plasma |
Response range | Localized to specific cells | Widespread across multiple organs |
Signal type | Electrical (along neuron) + chemical (synapse) | Chemical only |
5. Common Pitfalls
Wrong move:
Stating the pancreas is only an endocrine gland
Why:
The pancreas has both exocrine and endocrine functions, which is a common exam trick question
Correct move:
Explicitly state that the pancreas is both, describing the exocrine (digestive enzymes via duct) and endocrine (hormones into blood) roles
Wrong move:
Confusing glycogenolysis and glycogenesis
Why:
The terms sound similar but have opposite effects on blood glucose concentration
Correct move:
Remember: GlycogenolYsis = Glycogen breAks down (raises blood glucose); Glycogenesis = Glycogen is created (lowers blood glucose)
Wrong move:
Claiming non-steroid hormones cross the cell membrane
Why:
Non-steroid hormones are water-soluble and cannot pass the hydrophobic core of the phospholipid bilayer
Correct move:
State that non-steroids bind to cell surface receptors and use a secondary messenger system
Wrong move:
Forgetting that hormones only affect target cells
Why:
Examiners expect you to mention specificity of hormone-receptor binding
Correct move:
Always note that only target cells have the complementary receptor for a specific hormone, so only they respond to the signal
6. Quick Reference Cheatsheet
Concept | Key Fact |
|---|---|
Endocrine vs Exocrine | Endocrine = ductless, secrete into blood; Exocrine = duct, secrete onto surface |
Steroid action | Lipid-soluble, diffuse into cell, bind intracellular receptor, alter gene expression |
Non-steroid action | Water-soluble, bind cell surface receptor, cAMP is the second messenger |
High blood glucose | Beta cells β insulin β glycogenesis β lower blood glucose |
Low blood glucose | Alpha cells β glucagon β glycogenolysis β raise blood glucose |
Hormonal vs Nervous | Slower, longer-lasting, widespread; Nervous = faster, shorter, localized |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 1
Multiple choice on hormone classification
- 2023 Β· 2
Describe mechanism of insulin action
- 2024 Β· 3
Compare nervous and hormonal coordination
Going deeper
What's Next
Hormonal communication is a core foundation for all topics on homeostasis and control in A-Level Biology. Understanding hormone action mechanisms prepares you for more detailed topics like hormonal control of the menstrual cycle, regulation of heart rate, and kidney function. Exam questions frequently link this topic to earlier concepts like cell membrane structure, cell signaling, and homeostasis principles, so mastering it will help you in multiple areas of the syllabus.