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.