The activities of virtually all cells are influenced by their surroundings. Cells receive large amounts of information from numerous sources in their surroundings. One source of information is the physical environment. Another key source of information is other cells—neighboring cells, nearby cells, and even very distant cells. Both unicellular and multicellular organisms sense information from their surroundings and respond by changing their activity or even dividing.

In this section, we take a look at general principles of cell communication, including how cells send and receive signals and how a cell responds after it receives a signal. These mechanisms first evolved in unicellular organisms as a way to sense and respond to their environment, and also as a way to influence the behavior and activity of other cells. As result, the basic principles apply to all cells, both prokaryotic and eukaryotic, and to both unicellular and multicellular organisms.