Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
Why do we use plants?
In addition to food, medicine, and industrial chemicals, we mostly use plant materials for their structural qualities.
Plants use four types of molecules to keep their shape and give strength to their cell walls. This strength is what holds up trees such as giant redwoods against gravity. The four molecules are lignin, cellulose, hemicellulose, and pectin, in order of how strong the molecules make the cell walls.
Lignin is a huge molecule, and it links together many other molecules. It is what makes wood a good structural material.
Cellulose is also a huge molecule, a polymer, made up of long chains of thousands of molecules of the simple sugar glucose.
Hemicellulose is made up of a number of different simple sugars all chained together. It is a more random molecule than cellulose—it doesn’t crystallize, and has less strength.
Pectin is another large molecule made up of many different simple sugars all linked together. Pectins are why unripe fruits are hard. As the fruit ripens, the pectins break down, and the soft fruit can be eaten by animals that distribute the seeds for the plant.
Cellulose and lignin are the two most abundant organic polymers on earth. We use them together in wood products to build our houses and furniture and to make paper.
The lignin in newsprint paper is what makes the paper yellow with age. More expensive papers are made by removing almost all of the lignin, leaving mostly cellulose, which makes a nice white paper.
Cotton and flax are plant fibers used for making cloth and paper. Linen (made from flax fibers) is made of lignin and cellulose. Cotton is 95 percent cellulose.