Cells must be able to adapt to changes in the external environment. Cells in multicellular organisms must even be able to communicate with one another and cooperate in order to carry out functions. In fact, all of your body’s activities (from flexing a muscle to releasing stomach acid) rely on cellular communication or cell signaling. In this unit, we will discuss the ways in which cells talk to one another and respond to the environment. Note that this is a large and complex unit with a number of new terms.