Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 11.2 Signal Reception: A Signaling Molecule Binds to a Receptor, Causing It to Change Shape
Cell communication begins when a signaling molecule binds to a specific receptor, triggering structural changes that initiate a cellular response.
Types of Receptors
- Most receptors are proteins located in the plasma membrane
- Ligand: a signaling molecule that specifically binds to a receptor
- Ligand binding usually causes a shape change in the receptor
- Shape change may activate the receptor or lead to receptor aggregation
- Receptors can be on the membrane or inside the cell (intracellular)
1. G Protein-Coupled Receptors (GPCRs)
- Largest family of human cell-surface receptors
- Work with G proteins, which bind GTP
- Used by many hormones, neurotransmitters, and sensory systems
- Signal binding → G protein activation → enzyme activation → cellular response
- Pathway shuts off when GTP is hydrolyzed to GDP
- Target for many modern medicines (up to 60%)
2. Receptor Tyrosine Kinases (RTKs)
- Plasma membrane receptors with enzymatic activity
- Function by dimerization upon ligand binding
- Each monomer phosphorylates tyrosines on the other
- Activated receptor can trigger multiple signaling pathways simultaneously
- Abnormal RTK activity is linked to cancer (e.g., HER2 in breast cancer)
3. Ion Channel Receptors
- Also called ligand-gated ion channels
- Ligand binding opens or closes a gate for specific ions (e.g., Na⁺, Ca²⁺)
- Important in the nervous system—e.g., neurotransmitter signaling across synapses
- Ion movement alters cellular activity rapidly
- Some ion channels respond to electrical signals instead (voltage-gated)
Intracellular Receptors
- Located in cytoplasm or nucleus
- Signal molecules must be small or hydrophobic (e.g., steroid hormones, NO gas)
- Ligand binding activates receptor → hormone-receptor complex forms
- Complex enters nucleus and acts as a transcription factor
- Example: Aldosterone receptor in kidney cells activates gene expression to regulate water/salt balance
In a Nutshell
Reception begins when a signaling molecule (ligand) binds to its receptor, causing a shape change that activates the receptor. Receptors may be in the membrane (GPCRs, RTKs, ion channels) or inside the cell (e.g., for steroid hormones), initiating a cascade of molecular events.