Cellulose contains only glucose units in its molecular structure but insoluble in water explain.
Cellulose consists of large number of glucose molecules (1,250–12,500 glucose residues per molecule) linked together by β-1,4-glycosidic bonds. β-1,4-glycosidic bonds may be formed by taking out a molecule of water from the glycosidic -OH group on carbon atom 1 of one β-D-glucose molecule and the alcoholic -OH group on carbon atom 4 of the adjacent β-D-glucose molecule. These β-glycosidic bonds create a linear, flat, ribbon-like structure. This structure makes the cellulose very rigid.
Each glucose unit in cellulose has many hydroxyl (-OH) groups. These -OH groups form an extensive inter- and intra-molecular hydrogen bonding between cellulose chains. These extensive hydrogen bonding creates a very stable and rigid structure that water molecules cannot easily penetrate and interact with the glucose molecules and prevent it to dissolve in water.
Although, cellulose is insoluble in water but absorbs some water molecules and adds to the bulk of the faecal matter and makes it easier to pass through the intestines.
Cellulose is fibrous, tough, white solid, insoluble in water but soluble in ammoniacal cupric hydroxide solution (Schweitzer’s reagent).
NOTE: Cellulose when hydrolysed by heating with dilute acids, gives D-glucose. Cellobiose is formed in case of incomplete hydrolysis. The cattle, goats, and other ruminants have digestive enzymes (Cellulases) capable of hydrolyzing cellulose into glucose. Consequently, these can feed directly on cellulose. Humans and many other mammals does not have the necessary enzymes in their digestive tract, and they cannot use cellulose as foodstuff.