So, while this piece of pasta made of amylose and this wooden spoon made of cellulose

If you take a bunch of these same glucose molecules and rearrange them in a different way, you get cellulose, which is what plants are made of.

A third class of dietary material, fiber (i.e., non digestible material such as cellulose), seems also to be required, for both mechanical and biochemical reasons, though the exact reasons remain unclear.