A plant's cell wall is made of cellulose fibers. What is the purpose of a plant's cell wall?
To support the shape of the plant
Plants do not have bones like some animals do, so they will topple over without bones and a backbone. This is where the role of the cell wall comes in. It is a thick wall that helps to support the plant's shape and gives it the ability to stand without toppling over.
Another function is to act like a semi-permeable membrane. It allows water to get into the plant for use. Also, it allows the plant to have turgor pressure. This allows the plant to become become turgid when it takes in water, causing the cell wall to push externally against the plant, as well as letting it become stable.