Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 11.3 Signal Transduction: Cascades of Molecular Interactions Transmit Signals from Receptors to Relay Molecules in the Cell
Signal transduction involves a multi-step pathway in which a signal is relayed through a sequence of molecules, amplifying the message and allowing regulation.
Transduction Pathways
Begins when a receptor is activated by a signaling molecule
The signal is passed along by relay molecules through shape changes
Multistep pathways amplify the signal and allow control and coordination
Key mechanism: phosphorylation (adding phosphate groups to proteins)
Protein Phosphorylation Cascades
Involve sequential activation of protein kinases
Each kinase phosphorylates the next, passing on the signal
Phosphorylation changes protein shape and function—usually activating it
Ends with activation of a protein that triggers the cellular response
Dephosphorylation by protein phosphatases turns off the pathway
This switch system allows rapid on/off control and reuse of kinases
Second Messengers
Small, nonprotein, water-soluble molecules or ions that spread signals quickly
Common examples: cyclic AMP (cAMP) and calcium ions (Ca²⁺)
Work with G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs)
Cyclic AMP (cAMP)
Produced from ATP by adenylyl cyclase when a G protein is activated
Activates protein kinase A, which phosphorylates various proteins
Signal is short-lived: phosphodiesterase breaks down cAMP
Some toxins (e.g., cholera) disrupt this pathway by locking G proteins in active form
Calcium Ions and IP₃
Calcium is stored in the ER, mitochondria, and extracellular fluid
Signal leads to production of IP₃, which opens calcium channels in ER
Released Ca²⁺ acts as a second messenger to trigger various cellular responses
DAG, another messenger, is produced along with IP₃ from a membrane phospholipid
Calcium plays roles in muscle contraction, secretion, and cell division
In a Nutshell
Signal transduction pathways relay messages through phosphorylation cascades and second messengers like cAMP and Ca²⁺. These cascades amplify signals and enable cells to respond precisely and efficiently to external stimuli.