Surrounding the cell structure of plant cells is a structure called the cell wall. Plants require more rigid cell walls in order to maintain the cells shape when the amount of water in the cell fluctuates. In addition to providing a structure to the internal shape of the cell, cell walls also protect the cell from its external environment. Cell walls are strengthened by the material cellulose. The cell walls in fungus cells are made of a substance called chitin instead of cellulose.