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• General Principles of Cell Signaling

  • Cells need to sense and respond to their environment
  • Cells must interpret signals to coordinate behaviors
• Individual cells, like multicellular organisms, need to sense and respond to their environment
• A free-living cell must be able to track down nutrients, tell the difference between light and dark, and avoid poisons and predators
• If a cell is to have any kind of "social life", it must be able to communicate with other cells
• Yeast cells of the opposite "sex" detect this chemical mating call and respond by halting their progress through the cell-division cycle and reaching out toward the cell that emitted the signal
• In a multicellular organism, cells must interpret the multitude of signals they receive from other cells to help coordinate their behaviors
• During animal development, cells in the embryo exchange signals to determine which specialized role each cell will adopt, what position it will occupy in the animal, and whether it will survive, divide, or die
• Later in life, a large variety of signals coordinates the animal's growth and its day-to-day physiology and behavior
• In plants, cells are in constant communication with one another to coordinate what happens in its roots, stems, and leaves
• Signal transduction


  The process whereby one type of signal is converted into another
• When you receive a call from a friend on your mobile phone, the phone converts radio signals, which travel through the air, into sound waves, which you hear
• In a typical communication between cells, the signaling cell produces a particular type of extracellular signal molecule that is detected by the target cell
• Target cells possess proteins called receptors that recognize and respond specifically to the signal molecule
• Signal transduction begins when the receptor on a target cell receives an incoming extracellular signal and then produces intracellular signaling molecules that alter cell behavior
• Signals


  • Can act over a long or short range
• Hormones


  Extracellular signal molecules secreted into an animal's bloodstream or a plant's sap
• Endocrine cells


  Cells that produce hormones
• Paracrine signaling


  Signal molecules diffuse locally through the extracellular fluid, remaining in the neighborhood of the cell that secretes them
• Autocrine signaling


  Cells can respond to the local mediators that they themselves produce
• Neuronal signaling


  A message is delivered quickly and specifically to individual target cells through private lines
• Neurotransmitter


  Extracellular signal molecule released at nerve synapses
• Contact-dependent signaling


  Cells make direct physical contact through signal molecules lodged in the plasma membrane of the signaling cell and receptor proteins embedded in the plasma membrane of the target cell
• Examples of signal molecules

  • Hormones
  • Local mediators
  • Neurotransmitters
  • Contact-dependent signal molecules
• A typical cell in a multicellular organism is exposed to hundreds of different signal molecules in its environment
• Whether a cell responds to a signal molecule depends on whether it possesses a receptor for that signal
• Cell-surface receptors


  Receptors that relay messages from extracellular signal molecules across the plasma membrane
• Intracellular receptors


  Receptors that bind to small, hydrophobic signal molecules that can pass through the plasma membrane
• How a cell reacts to a signal depends on the set of intracellular signaling molecules each cell-surface receptor produces and how these molecules alter the activity of effector proteins
• The intracellular relay system and the intracellular effector proteins on which it acts vary from one type of specialized cell to another, so that different types of cells respond to the same signal in different ways
• The extracellular signal molecule alone is not the message: the information conveyed by the signal depends on how the target cell receives and interprets the signal
• A typical cell possesses many sorts of receptors, each present in tens to hundreds of thousands of copies, making the cell simultaneously sensitive to many different signals
• Intracellular relay system


  The intracellular effector proteins on which it acts vary from one type of specialized cell to another, so that different types of cells respond to the same signal in different ways
• A typical cell possesses many sorts of receptors—each present in tens to hundreds of thousands of copies
• Variety of receptors


  Makes the cell simultaneously sensitive to many different extracellular signals and allows a relatively small number of signal molecules, used in different combinations, to exert subtle and complex control over cell behavior
• A combination of signals can evoke a response that is different from the sum of the effects that each signal would trigger on its own
• The intracellular relay systems activated by the different signals interact, so the presence of one signal will often modify the effects of another
• Possible cell responses to combinations of signals

  • Enable a cell to survive
  • Drive it to differentiate in some specialized way
  • Cause it to divide
• In the absence of the proper signals, most animal cells are programmed to kill themselves (apoptosis)
• Fast responses


  Acetylcholine can stimulate a skeletal muscle cell to contract within milliseconds and a salivary gland cell to secrete within a minute or so
• Slow responses


  Cell growth and cell division, when triggered by the appropriate signal molecules, can take many hours to execute