Cellulose is a complex polysaccharide, meaning it is a giant chain of many simple sugars. The specific repeating sugar building blocks are a common form of glucose. These individual glucose molecules are joined by rigid chemical links called beta-1,4-glycosidic bonds.
Because of how these specific chemical bonds form, every alternating glucose unit must flip upside down. This required flipping creates incredibly long, straight chains of molecules that do not coil. These straight chains pack very tightly together next to one another in neat rows.
They form strong chemical connections called hydrogen bonds between the adjacent parallel chains. This tight packing creates rigid microfibrils that give towering trees their amazing structural strength. A common student misconception is that animals can easily digest all plant material for energy.
In truth, humans and most animals lack the specific enzyme needed to break those beta bonds. We simply pass cellulose through our digestive tracts completely unaltered as healthy dietary fiber. Animals like cows and termites can only digest grass or wood because special bacteria live in their stomachs. These helpful gut microorganisms produce the cellulase enzyme required to chop the tough chains back into glucose.