Why Do Plants Have Cell Walls and Animals Don't?
Plant cells contain cell walls that provide a physical barrier, offer structural support and prevent the cell from bursting under osmotic pressure. They are composed primarily of cellulose and surround the membrane. Animal cells need a greater connectivity and do not require the individual barrier, thus cell walls are unnecessary.
Plant cell walls are composed of both a primary and secondary layer. The primary layer is present as the cell grows, while the secondary develops after a cell has finished growing. Primary cell walls are composed of polysaccharides, the most common being cellulose. Cellulose chains that readily form intramolecular and intermolecular hydrogen bonds. One cellulose chain hydrogen bonds with around 36 other chains to create microfibril. Microfibril have a tensile strength relative to steel and are responsible for the strength of cell walls. The cell wall additionally contains proteins and lignin.
The lack of cell walls in animals allows greater flexibility of the cells, as is necessary for locomotion. The tissues, organs and organ systems present in animals require more physical connection than plant cells to allow the passage of macromolecules between cells. Animals have skin which acts as a barrier to the outside world and offers protection, they do not require a great physical barrier around the individual cells due to this.