Cell membranes surround every cell you will study. Cell walls made of cellulose are only found around plant cells and a few other organisms. Cellulose is a specialized sugar that is classified as a structural carbohydrate and not used for energy. If a plant cell is like a water balloon, the cell wall is like a cardboard box that protects the balloon. The balloon is protected from the outside world by a structure that provides protection and support.
While many sugars, such as glucose, can dissolve in water (H20), cellulose will not dissolve in water and can form long chains to support plants. When you eat plant material, you can’t even digest and break down cellulose for energy. Cows and other herbivores have special bacteria in their stomachs to digest the cellulose polymers.
While cell walls protect the cells, they also allow plants to grow to great heights. You have a skeleton to hold you up. A 100-foot tall redwood tree does not. It uses the strong cell walls to maintain its shape. For overall support, dense cells in the core of the trunk can let a tree grow very high. Cell walls are slightly elastic for smaller plants, leaves, and thin branches. Winds can push them from side to side and they bounce back. Big redwoods need strength in high winds and sway very little (except at the top).