Cells, particularly cells in multicellular organisms, must be able to communicate with one another. Cells accomplish this through cellular signals. In extracellular signaling, one cell acts as the signal cell and secretes a signal molecule. The signal molecule binds to a receptor on a target cell and initiates a signal cascade, during which the target cell translates the signal into a physical cellular response. Some signals, such as the hormones secreted by endocrine cells, must travel long distances through the bloodstream and extracellular space to reach their targets. Other signals, such as neurotransmitters, are secreted very close to their target cells. Sometimes cells secrete signals that they themselves respond to, and sometimes the signal cell must be in contact with the target cell. Signal cascades are often a series of many steps, and the signal can be turned on and off by molecular switches.
At A Glance
Signal molecules and target cells enable cells to respond to changes in their environments.
- Instead of entering the bloodstream, some signals may diffuse directly to neighboring cells, travel between adjacent cells, or even target the same cell that released them.
- The binding of signal molecules to receptors elicits specific cell responses. Signals travel throughout the entire body, but only to target cells that have the appropriate receptors.
- An extracellular signal molecule reaches a target cell, which converts the signal to an intracellular signal.
Hormones and neurotransmitters are two types of the many different kinds of signal molecules.
- Intracellular signaling proteins can switch on or off, controlling steps of many cellular pathways.