The most important difference between plant and animal cells is that plant cells have a cell wall and a cell membrane, while animal cells only have a cell membrane. The cell wall is located outside of the cell membrane in plants, bacteria, fungi and algae. It is a tough, rigid layer that provides structural support and protection for the cell. Many plant cells have both a primary cell wall, which accommodates the cell as it grows, and a secondary cell wall that they develop inside the primary wall after the cell has stopped growing. Secondary cell walls are much thicker and stronger than the primary cell walls, and are the major component in wood.
Plant cell walls are primarily made of cellulose, which is the most abundant macromolecule on Earth. Cellulose fibers are long, linear polymers of hundreds of glucose molecules. The cell wall surrounds the plasma membrane of plant cells and provides tensile strength and protection against mechanical and osmotic stress. It also allows cells to develop turgor pressure, which is the pressure of the cell contents against the cell wall.