🧬 Types of Cell Signalling Pathways
There are 2 main types based on the type of signal molecule:
🧪 1. Protein/Peptide Signalling – “Outsiders Knock the Door”
Protein/peptide ligands are water-loving (hydrophilic) and can’t enter the cell. So they bind to receptors on the surface of the cell.
🔸 Binding causes the receptor to change shape
🔸 Triggers a signal cascade inside
🔸 Uses second messengers like cAMP
📍 Real-Life Example: Insulin is a peptide hormone. It binds to muscle and fat cells and tells them to absorb sugar from the blood.
🧠 Mnemonic: “Protein = Plasma = Peptide can’t Pass”
→ Peptide/protein hormones can’t pass the membrane – they stay outside!
💊 2. Steroid Signalling – “VIPs Walk In”
Steroid hormones are lipid-loving (lipophilic) and easily pass through the plasma membrane.
🔸 They enter the cell and bind to receptors in the cytoplasm or nucleus
🔸 This hormone-receptor complex binds to DNA
🔸 It changes gene expression (like giving orders to produce a specific protein)
📍 Real-Life Example: Estrogen and testosterone are steroid hormones. They enter cells and help develop male/female features.
🧠 Mnemonic: “Steroids Slip Straight”
→ Steroids slip straight through the membrane!
🧠 Summary Table for Quick Revision:
| Feature | Protein/Peptide Signalling | Steroid Signalling |
| Solubility | Water-soluble (hydrophilic) | Lipid-soluble (lipophilic) |
| Can enter cell? | ❌ No | ✅ Yes |
| Receptor location | Cell membrane | Inside cytoplasm or nucleus |
| Action mechanism | Second messengers (cAMP, IP₃) | Direct gene expression |
| Example | Insulin, Adrenaline | Estrogen, Testosterone |
🎓 Real-Life Example to Remember Everything Together
Imagine:
- A student (ligand) knocks on a classroom door (protein receptor) because he can’t enter. He tells the monitor (second messenger) to give the teacher a message.
- OR a teacher (steroid hormone) has the key and directly enters the classroom (cell), writes on the board (DNA), and changes the lesson (gene expression).