These two kinds of molecules act in plants as a support structure, giving the cells the material for their walls, and the ability to maintain their rigidity. It's important to mention that though all plants have cellulose, not all plants have lignin, since this latter molecule is also involved in the production of wood storing carbonic dioxide from photosynthesis in them. We can say too that the function of cellulose is to give the cells capability to form their cell walls.