Phospholipids with embedded proteins make up the membrane surrounding all cells. This membrane, called the plasma membrane, is a fundamental, defining feature of all cells. It is the boundary that defines the space of the cell, separating its internal contents from the surrounding environment. But the plasma membrane is not simply a passive boundary or wall. Instead, it serves an active and important function. The environment outside the cell is changing all the time. In contrast, the internal environment of a cell operates within a narrow window of conditions, such as a particular pH range or salt concentration. It is the plasma membrane that actively maintains intracellular conditions compatible with life.

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In addition, the cells of many organisms have a cell wall external to the plasma membrane. The cell wall plays an important role in maintaining the shape of these cells. In this section, we consider the functions performed by these two key structures.