Part Four - Molecular Biology: The Genome in Action 331

15.  Cell Signaling and Communication 332

15.1 What Are Signals, and How Do Cells Respond to Them? 333 Cells receive signals from the physical environment and from other cells 333 A signal transduction pathway involves a signal, a receptor, transduction, and effects 334 15.2 How Do Signal Receptors Initiate a Cellular Response? 336 Receptors have specific binding sites for their signals 336 Receptors can be classified by location 336 15.3 How Is a Response to a Signal Transduced through the Cell? 339 Protein kinase cascades amplify a response to ligand binding 339 Second messengers can stimulate protein kinase cascades 340 Second messengers can be derived from lipids 341 Calcium ions are involved in many signal transduction pathways 343 Nitric oxide can act as a second messenger 344 Signal transduction is highly regulated 345 15.4 How Do Cells Change in Response to Signals? 345 Ion channels open in response to signals 345 Enzyme activities change in response to signals 346 Signals can initiate gene transcription 347 15.5 How Do Cells Communicate Directly? 348 Animal cells communicate by gap junctions 348 Plant cells communicate by plasmo¬desmata 348