Cell Signalling- Communication Within Organisms
What Cell Signalling Actually Is
Cell signalling is how cells talk to each other. That's the whole game. Without it, your body would be a pile of disconnected cells doing nothing useful.
Every organism, from bacteria to humans, relies on these communication networks. Cells release signalling molecules, other cells detect them, and then respond. It's that simple — and that complicated.
The process keeps everything running: growth, metabolism, immune responses, even cell death. When these signals break down, you get cancer, diabetes, neurological disorders. You get dead.
The Four Main Types of Cell Signalling
Cells don't just blast signals everywhere. They use specific delivery methods depending on what needs to happen.
Autocrine Signalling
Cells talk to themselves. They release signals and their own membrane receptors catch them. This is common during development and immune responses.
Why it matters: Cancer cells exploit this. They autocrine signal like crazy, telling themselves to keep growing regardless of external conditions.
Paracrine Signalling
Signals travel short distances. Cells release molecules, nearby cells pick them up. Neighbouring cells only — not the whole organism.
Examples include neurotransmitter signalling at synapses and growth factor distribution in developing tissues.
Endocrine Signalling
Hormones enter the bloodstream. Long-distance communication. This is how the pituitary gland tells your thyroid what to do, or how insulin coordinates glucose uptake across your entire body.
Endocrine signalling is slow — minutes to hours — but the effects last.
Juxtacrine Signalling
Cells must be physically touching. No diffusion, no bloodstream. Direct contact through membrane proteins or gap junctions.
Your immune system uses this extensively. T-cells recognize infected cells by touching them directly.
The Three Components Every Signalling Pathway Needs
Every cell communication system has the same basic parts:
- Ligand — the signalling molecule itself. Hormones, peptides, gases like nitric oxide.
- Receptor — the detector protein on or inside the receiving cell.
- Response — what happens after the signal binds. Gene activation, enzyme activity change, ion channel opening.
Remove any one component and the pathway breaks. This is why some toxins work — they block receptors or mimic ligands.
Types of Receptors: Where Signals Land
Receptors determine what a cell responds to. Get this wrong and you respond to the wrong signals, or none at all.
Membrane Receptors
Water-soluble signals can't cross the cell membrane. They bind to receptors on the surface. The receptor then triggers internal changes — often through a second messenger system.
Examples: G protein-coupled receptors (GPCRs), receptor tyrosine kinases.
Intracellular Receptors
Lipid-soluble signals slip right through the membrane. They bind receptors inside the cell — usually in the cytoplasm or nucleus.
Steroid hormones work this way. So does nitric oxide. The response is typically gene expression changes — slower, but longer-lasting.
Signal Transduction: The Relay Race Inside the Cell
One signal at the membrane triggers a cascade. One molecule activates the next, which activates the next. This is signal transduction.
Amplification happens at every step. One epinephrine molecule at the membrane can eventually cause millions of glycogen molecules to break down.
Key transduction mechanisms include:
- Protein phosphorylation cascades
- Second messengers (cAMP, IP3, calcium ions)
- Ion channel activation
- GTPase switching proteins
Major Signalling Pathways You Should Know
Some pathways come up constantly in biology and medicine. Memorize these.
| Pathway | Signal Type | Primary Function |
| MAPK/ERK | Growth factors | Cell growth and division |
| PI3K/Akt | Insulin, growth factors | Cell survival, metabolism |
| JAK/STAT | Cytokines | Immune responses, hematopoiesis |
| Wnt/β-catenin | Wnt proteins | Development, stem cell maintenance |
| Notch | Delta/Jagged ligands | Cell fate determination |
These pathways are mutated in cancer. PI3K mutations appear in ~30% of breast cancers. MAPK pathway alterations show up in melanoma and colorectal cancer. This is why these pathways get so much research funding.
Why Cell Signalling Matters in Disease
Most drugs on the market target signalling components. Not because it's trendy — because it works.
Cancer: HER2 inhibitors (Herceptin), BCR-ABL inhibitors (Gleevec), VEGF inhibitors block tumour blood supply.
Diabetes: GLP-1 receptor agonists enhance insulin signalling. They don't fix the underlying problem, but they manage it.
Autoimmune diseases: TNF-α inhibitors block inflammatory signalling. This treats rheumatoid arthritis, Crohn's disease, psoriasis.
When signalling goes wrong, the consequences are severe. Mutations in receptor tyrosine kinases cause developmental disorders. Defective G-protein signalling leads to hormonal imbalances. Broken apoptosis signalling lets damaged cells survive and become cancerous.
How Cells Decide: Feedback and Integration
Cells don't blindly respond to every signal. They integrate multiple inputs and use feedback loops to calibrate responses.
Positive feedback: A signal amplifies itself. Oxytocin release during childbirth — each contraction triggers more oxytocin release until delivery completes.
Negative feedback: A response shuts down the signal. Insulin signalling triggers glucose uptake, which reduces blood sugar, which reduces insulin release. Classic homeostasis.
Cross-talk between pathways adds another layer. One pathway can activate or inhibit another. The cell doesn't process signals in isolation — it processes networks.
Getting Started: Studying Cell Signalling
Want to look at cell signalling in practice? Here's how researchers actually do it.
Western Blotting
Detect specific proteins in a cell lysate. You can see if a signalling molecule is present, how much, and whether it's in an activated state (phosphorylated).
Immunofluorescence
Tag proteins with fluorescent antibodies. See where they are in the cell — membrane, cytoplasm, nucleus. Useful for tracking receptor internalization after ligand binding.
Reporter Gene Assays
Attach a reporter gene (luciferase, GFP) to a signalling-responsive promoter. When the pathway activates, the reporter lights up. Quantify the signal.
RNA Interference
Knock down a specific protein. If the pathway breaks, you know that protein was essential. If it keeps working, the protein isn't critical for that specific signal.
CRISPR-Cas9
Permanently mutate a signalling component. Unlike RNAi, this works at the DNA level. You get complete loss of function, not just reduction.
The Bottom Line
Cell signalling is the language cells use to coordinate behaviour. Every organism depends on it. Every disease involves broken communication somewhere in the chain.
You don't need to memorize every pathway. Understand the core concepts — ligand, receptor, transduction, response. Then learn the specifics as they become relevant.
The pathways are interconnected. The biology is messy. But the basic principles are straightforward: cells send signals, other cells receive them, and things happen as a result.