THE INSIDE STORY OF CELL COMMUNICATION
Cells communicate by sending and receiving signals. Signals may come from the environment, or they may come from other cells. In order to trigger a response, these signals must be transmitted across the cell membrane. Sometimes the signal itself can cross the membrane. Other times the signal works by interacting with receptor proteins that contact both the outside and inside of the cell. In this case, only cells that have the correct receptors on their surfaces will respond to the signal.
Once inside the cell, the signal continues on its way. Its ultimate destination depends on the nature of the signal, with some signals traveling to the nucleus or to other structures inside the cell. Signals most often move through the cell by passing from protein to protein, each protein modifying the next in some way. Collectively, the proteins that relay a signal to its destination make up a signaling pathway. A signaling pathway can have few or many steps. Some signaling pathways branch out in different directions, sending signals to more than one place in the cell. As a signal is transferred from protein to protein, it can also be amplified. By dividing and amplifying a signal, the cell can convert a small signal into a large response.
Once a signal reaches its target molecule (usually a protein), it works to change the behavior of the cell. Depending on the signaling molecules involved, the cell can respond in a variety of ways.
Each cell receives a complex combination of signals which simultaneously trigger many different signaling pathways. Each step in a signaling pathway provides an opportunity for cross-talk between different signals. Through cross-talk, the cell integrates information from many different signaling pathways to initiate an appropriate response.