The cell membrane is essential to the life of the cell and without it, the cell dies. If all the cells in an organism suddenly die, the organism itself dies as well.
The cell membrane provides several vital functions for the cell. Cell walls anchor the cytoplasm and hold the cell's shape. It is a selective, semipermeable membrane that allows the cell to maintain homeostasis by regulating what it allows into the cell. Cells absorb nutrients through the cell membrane and expel waste through the same membrane using both active and passive transport. In plants, fungi and some bacteria, an outer cell wall provides mechanical support for the organism. In complex organisms, the cell wall attaches to the cell walls of other cells to form tissues, organs and ultimately the organism. The cell membrane embeds specific proteins that are molecular signals cells use to communicate with one another. These protein receptors receive signals from other cells as well as the environment. Certain hormones that bind to a cell cause ion channels to open to allow calcium ions to enter the cell. Other embedded proteins on the cell membrane are markers to identify the cell to other cells. These interactions are essential for correct operation of the immune system of an organism.