The atmosphere is a mixture because it contain many types of gases , liquids and solids.
Cellulose is found in the cell walls of plant cells. It is permable so any substance disolveabls in water can pass through so it dose not act as a barrier. It gives the cell its strength and support. As water moves in to the cell by osmosis it stops the cell from bursting.
Hes trying to infer the density of oak trees (:
