Paper is made ___1___
wood, which is composed of white cellulose. Wood is also
made up of a dark substance known as lignin. Along with
cellulose, lignin also ends up in paper. Newspaper turns
yellow due to the ___2___
of lignin to air and sunlight. Lignin makes wood stiff
and trees stand upright. It is a polymer which acts as a
glue to ___3___ the
cellulose fibers together. Lignin is important for tree
growth as without lignin, a tree could only grow to be
about 6 feet tall. Lignin also helps guard the wood from
pests and other damage.
Newsprint, which must be produced as economically as
possible, has more lignin in it than finer papers. At
the mill, the wood that will be turned into newsprint is
ground up, lignin and all. Lignin ultimately turns paper
yellow because of a process known as oxidation. When
exposed to oxygen in the air, ___4___
lignin molecules begin to change and become less stable.
The molecules will ___5___
more light, giving off a darker color. Newspapers that
are kept completely out of sunlight and air will remain
white. However, after only a few hours of sunlight and
oxygen, the papers will start to change color. |