Cells are the smallest form of life, and are the functional and structural units of all living things. As the cell diagram shows, there is a cell wall which is a rigid non-living layer that is found outside the cell membrane and surrounds the cell. Then there is a middle lamella that separates one cell from another. The cell diagram shows a cell membrane, which physically separates the intracellular space from the extracellular environment. The cell membrane is composed of a double layer of special lipids called phospholipids that in return, consist of a hydrophilic head and hydrophobic tail. In the cell diagram, a cell membrane is strong enough to provide the cell with mechanical support and flexible enough to allow cells to grow and move.