Cellular Messaging The Thomson's gazelle in
Figure 1.1 is fleeing for its life, seeking to escape the predatory cheetah nipping at its heels. The gazelle’s heart is racing, its breathing accelerated and its muscles performing at their highest level. These physiological functions are all part of the “fight-or-flight” response, driven by hormones released from the adrenal glands at times of stress -in this case, when the gazelle first sensed the cheetah. Hormonal signaling and the subsequent response by cells and tissues throughout the gazelle’s body illustrate how cell-to-cell communication allows the trillions of cells in a multicellular organism to “talk” to each other, coordinating their activities.
Communication between cells is essential not only for multicellular organisms such as gazelles and oak trees but for many unicellular organisms as well.
In studying how cells signal each other and how they interpret the signals they receive, biologists have discovered some universal mechanisms of cellular regulation, additional evidence for the evolutionary relatedness of all life. The same small set of cell-signaling mechanisms shows up again and again in diverse species, in biological processes ranging from hormone embryonic development to cancer. The signals received by cells, whether originating from other cells or from changes in the physical environment, take various forms, including light and touch. However, cells most often communicate with each other by chemical signals. For instance, the fight-or-flight response is triggered by a signaling molecule called epinephrine. In this chapter, we focus on the main mechanisms by which cells receive, process, and respond to chemical signals sent from other cells. We will also take a look at apoptosis, a type of programmed cell death that