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.