Cellulose

Cellulose is the material plants build their cell walls from, and the most abundant organic polymer on Earth. Wood is about 40 to 50% cellulose, and cotton is around 90%. Each cellulose molecule is a long, straight chain of thousands of glucose sugar units, and neighboring chains bond side by side into strong fibers.

Plants make it from just three elements. Leaves take in carbon dioxide from the air, which supplies the carbon and some oxygen. Roots draw in water, which supplies the hydrogen. Using sunlight, photosynthesis turns these into glucose, and enzymes in the cell membrane string glucose units together and spin the chains into fibers. Industry harvests it by pulping wood: chips are cooked with chemicals that dissolve lignin, the natural glue between fibers, leaving mostly cellulose.

In this build it has two uses. Pressed into a mat of fibers, it becomes paper. Reacted with acetic acid, it becomes cellulose triacetate, the clear film that protects an LCD polarizer. Humans can't digest cellulose, but cows and termites can, thanks to microbes in their guts.