Difficulty: Intermediate | Prerequisites: Basic cell biology, membrane structure, protein synthesis
Tags: cell communication, chemical messengers, signal transduction, gap junctions, neurotransmitters, hormones, paracrines, autocrines, ligands, intercellular signalling, anatomy and physiology
This topic covers how cells in the body talk to one another, which underpins virtually every physiological process you will study from here on: muscle contraction, immune responses, hormone regulation, nervous system signalling. You need a working grasp of cell membrane structure and basic protein synthesis before diving in. The material splits into two halves: the messengers themselves (what they are, how they are made, how they travel) and the receptors and transduction pathways they activate. This document covers the first half.
Cells communicate either directly through gap junctions or indirectly through chemical messengers (ligands) that bind receptors on target cells. Messengers are classified functionally (neurotransmitters, paracrines, autocrines, hormones) and chemically (amino acids, amines, peptides/proteins, steroids, eicosanoids). A messenger's chemical structure determines how it is made, how it travels in the blood, and what kind of receptor it uses.
Gap junction
A channel formed by connexon proteins that directly links the cytoplasm of two neighbouring cells, allowing ions and small molecules to pass freely. Think of it as a direct tunnel between two rooms.
Connexon
The protein unit that forms one half of a gap junction; two connexons (one from each cell) dock together to create the full channel.
Ligand
Any molecule that binds specifically to a receptor. In simple terms, the "key" that fits a receptor "lock."
Neurotransmitter
A chemical messenger released from a neuron at a synapse, acting on an adjacent cell. Think of it as a short-range postal service that only delivers next door.
Paracrine
A local chemical mediator that affects neighbouring cells within the same tissue (e.g. histamine from mast cells). In simple terms, a message that drifts to nearby cells rather than travelling through the bloodstream.
Autocrine
A messenger that acts on the same cell that secreted it, functioning as a self-regulating signal.
Hormone
A chemical messenger secreted into the bloodstream by an endocrine gland, travelling to distant target cells. Think of it as a broadcast message carried by the circulatory system.
Lipophilic ("fat-loving")
Describes molecules that dissolve readily in lipids and can cross cell membranes by diffusion. Steroids and eicosanoids are lipophilic.
Lipophobic / hydrophilic ("water-loving")
Describes molecules that dissolve in water but cannot cross the lipid bilayer on their own. Amino acid messengers and peptides are in this category.
Eicosanoid
A lipophilic paracrine signalling molecule derived from arachidonic acid. Includes prostaglandins, prostacyclins, thromboxanes, and leukotrienes.
Catecholamines
A subgroup of amine messengers derived from tyrosine: dopamine, norepinephrine, and epinephrine.
Prepropeptide
The initial, unprocessed form of a peptide/protein messenger as first synthesised on the rough ER. It is cleaved into a propeptide and then a mature peptide before secretion.
Carrier protein
A blood plasma protein (e.g. transcortin) that binds and transports lipophilic messengers, extending their half-life.
Cyclooxygenase (COX)
The enzyme that converts arachidonic acid into prostaglandins and thromboxanes. Aspirin and ibuprofen work by inhibiting COX.
Direct communication (gap junctions): connexon channels link the cytoplasm of adjacent cells, permitting ions and small molecules to pass. This enables electrical coupling (e.g. cardiac muscle cells beating in sync) and metabolic coupling.
Chemical communication (messengers): a cell releases a ligand that binds a specific receptor on a target cell, triggering an intracellular response. Range and speed vary by messenger class.
Neurotransmitters: released from neurons at synapses; act on adjacent cells. Classic example: acetylcholine (ACh) at the neuromuscular junction.
Paracrines: local mediators affecting neighbouring cells in the same tissue. Example: histamine from mast cells during inflammation.
Autocrines: act on the same cell that secreted them, forming a self-feedback loop.
Hormones: secreted into the bloodstream by endocrine glands; travel to distant targets. Can produce widespread, systemic effects.
The functional class is determined by how the messenger is secreted and how far it travels, not by its chemical structure. The same molecule (e.g. norepinephrine) can act as both a neurotransmitter and a hormone depending on context.
A messenger's chemical structure dictates its synthesis pathway, storage, transport mechanism, and receptor type.
Amino acids: lipophobic; act on plasma membrane receptors. Key examples: glutamate (primary excitatory neurotransmitter in the CNS), GABA (primary inhibitory neurotransmitter), glycine (inhibitory, especially in the spinal cord), aspartate (excitatory in some pathways).
Synthesised in neurons (not obtained from dietary amino acids used for other purposes).
Produced in the cytosol, packaged into vesicles, released via exocytosis.
Amines: derived from amino acids (tyrosine, tryptophan) by enzymatic modification (hydroxylases, decarboxylases). Contain an –NH₂ group.
Catecholamines (from tyrosine): dopamine, norepinephrine, epinephrine.
Other amines: serotonin (from tryptophan), histamine.
Most amines are synthesised in the cytosol and stored in vesicles.
Exception: thyroid hormones are lipophilic amines that can cross membranes and bind nuclear receptors.
Peptides and proteins: the largest class of chemical messengers; hydrophilic.
Peptides: fewer than 50 amino acids. Proteins: 50 or more.
Synthesis: rough ER (as prepropeptides) → Golgi (processed to propeptides, then mature peptides) → packaged into secretory vesicles → released by exocytosis.
Bind plasma membrane receptors because they cannot cross the lipid bilayer.
Steroids: lipophilic; derived from cholesterol.
Examples: cortisol, aldosterone, oestrogen, testosterone.
Synthesised in the smooth ER or mitochondria.
Cannot be stored in vesicles (they would diffuse straight out through the vesicle membrane). Instead, they are synthesised on demand and diffuse out of the cell immediately.
Mostly bind intracellular receptors (cytosolic or nuclear), influencing gene transcription. Some steroids also have membrane receptors.
Eicosanoids: lipophilic paracrines derived from arachidonic acid.
Subclasses: prostaglandins, prostacyclins, thromboxanes (via COX pathway), leukotrienes (via lipoxygenase pathway).
Roles: blood flow regulation, clotting, inflammation, allergic responses.
Synthesis starts when phospholipase A₂ cleaves arachidonic acid from membrane phospholipids.
Clinical link: aspirin inhibits COX, reducing prostaglandin synthesis and inflammation.
Hydrophilic messengers (amino acids, peptides/proteins, most amines): secreted via exocytosis, dissolve freely in plasma, have short half-lives because enzymes in the blood break them down quickly.
Lipophilic messengers (steroids, eicosanoids, thyroid hormones): diffuse through membranes, bind carrier proteins in blood (e.g. transcortin for cortisol), have longer half-lives because the carrier protein shields them from degradation.
The half-life difference matters clinically. Hydrophilic messengers produce rapid but brief effects; lipophilic messengers produce slower-onset but longer-lasting effects.
Aspirin's mechanism of action, one of the most commonly used drugs in the world, comes directly from this material: it inhibits cyclooxygenase, blocking prostaglandin synthesis and reducing pain, fever, and inflammation. Understanding lipophilic versus hydrophilic messenger transport also explains why steroid medications (e.g. prednisone) take longer to kick in but last longer than a shot of epinephrine.
Students often assume that the chemical class of a messenger determines its functional class. It does not. Norepinephrine is chemically an amine, but functionally it can be either a neurotransmitter or a hormone.
Students frequently confuse lipophilic with lipophobic. Remember: lipophilic messengers cross membranes and use intracellular receptors; lipophobic messengers cannot cross and use membrane receptors.
A common error is thinking amino acid neurotransmitters come from dietary amino acids directly. They are synthesised de novo inside neurons.
Students sometimes think steroids are stored in vesicles like peptides. They are not, because their lipophilicity means they would simply diffuse out of the vesicle.
⚠️ Be able to match each chemical class (amino acid, amine, peptide/protein, steroid, eicosanoid) to its synthesis location, storage method, transport mechanism, and receptor type. This is a classic table-format exam question.
⚠️ Know the functional classes (neurotransmitter, paracrine, autocrine, hormone) and be able to assign real examples to each.
⚠️ Understand why steroids cannot be stored in vesicles (lipophilicity, membrane diffusion).
⚠️ Aspirin's mechanism (COX inhibition → reduced prostaglandin synthesis) appears frequently.
True or false: Gap junctions allow hormones to pass directly between cells.
A: False. Gap junctions allow ions and small molecules, not large hormone molecules, to pass.
True or false: Peptide messengers bind intracellular receptors.
A: False. They are hydrophilic and bind plasma membrane receptors.
Fill in the blank: Steroid hormones are derived from __________.
A: Cholesterol.
True or false: Histamine is classified as both a paracrine and an amine.
A: True. Its functional class is paracrine; its chemical class is amine.
Q: Name the four functional classes of chemical messengers and give one example of each.
A: Neurotransmitters (acetylcholine), paracrines (histamine), autocrines (a cytokine acting on the cell that released it), hormones (insulin).
Q: A lipophilic messenger is released from a cell. Describe how it reaches its target and where it binds.
A: It diffuses directly through the cell membrane (no exocytosis needed), travels in the blood bound to a carrier protein, then diffuses into the target cell and binds an intracellular receptor (cytosolic or nuclear).
Q: Why does aspirin reduce inflammation?
A: Aspirin inhibits cyclooxygenase (COX), the enzyme that converts arachidonic acid into prostaglandins. Fewer prostaglandins means less inflammation, pain, and fever.
Q: Explain the difference in half-life between hydrophilic and lipophilic messengers and the reason for it.
A: Hydrophilic messengers dissolve freely in plasma and are rapidly degraded by blood enzymes, giving them short half-lives. Lipophilic messengers bind carrier proteins that shield them from degradation, giving them longer half-lives.
Q: A peptide hormone is synthesised as a prepropeptide. Outline the steps from synthesis to secretion.
A: Synthesised on ribosomes at the rough ER as a prepropeptide → cleaved to a propeptide → processed to a mature peptide in the Golgi → packaged into secretory vesicles → released via exocytosis when the cell is stimulated.
This material connects directly to the endocrine system (hormones, steroid signalling) and the nervous system (neurotransmitters, synaptic transmission). The receptor and transduction content in the companion notes ties into pharmacology, where agonists and antagonists are the basis of drug design. Eicosanoid signalling reappears in immunology and haematology (inflammation, clotting cascades).
cell signalling, intercellular communication, chemical messengers, gap junctions, connexons, ligand, neurotransmitter, paracrine, autocrine, hormone, endocrine, amino acid neurotransmitter, amine messenger, catecholamines, dopamine, norepinephrine, epinephrine, serotonin, histamine, peptide hormone, protein hormone, steroid hormone, eicosanoid, prostaglandin, thromboxane, leukotriene, cyclooxygenase, COX inhibitor, aspirin mechanism, lipophilic, lipophobic, hydrophilic, carrier protein, transcortin, prepropeptide, exocytosis, arachidonic acid, phospholipase A2, GABA, glutamate, glycine