128 Green Chemistry, 2nd ed
This is obviously an extremely important reaction because it is the one by which
inorganic molecules are converted to high-energy carbohydrate molecules that are in
turn converted to the vast number of biomolecules that comprise living systems. There
are other simple sugars, including fructose, mannose, and galactose, that have the same
simple formula as glucose, C 6 Η 12 Ο 6 , but which must be converted to glucose before
being utilized by organisms for energy. Common table sugar, sucrose, C 12 Η 22 Ο 11 ,
consists of a molecule of glucose and one of fructose linked together (with the loss
of a water molecule): because it is composed of two simple sugars sucrose is called a
disaccharide.
Starch molecules, which may consist of several hundred glucose units joined together,
are readily broken down by organisms to produce simple sugars used for energy and to
produce biomass. For example, humans readily digest starch in potatoes or bread to
produce glucose used for energy (or to make fat tissue). Another chemically very similar
polysaccharide consisting of even more glucose units is cellulose, which comprises much
of the biomass of plant cells. We and other animals cannot digest cellulose directly to use
as a food source but some bacteria and fungi do so readily. Such bacteria living in the
stomachs of termites and ruminant animals (cattle, sheep, moose) break down cellulose
to small molecules that are converted to molecules that can be absorbed through the
digestive systems of animals and utilized as food.
Carbohydrates are potentially very important in green chemistry. For one thing,
they are a concentrated form of organic energy that enables capture of solar energy by
photosynthetic processes. Carbohydrates can be utilized directly for energy or fermented
to produce ethanol, C 2 Η 6 , a combustible alcohol that is added to gasoline or can even be
used in place of gasoline. Secondly, carbohydrates are a source of organic raw material
that can be converted to other organic molecules to make plastics and other useful
materials. The potential for the biosynthesis from glucose of adipic acid used to make
nylon was discussed in Section 5.5.
5.8. Proteins
Proteins are macromolecules that are composed of nitrogen, carbon, hydrogen, and
oxygen along with smaller quantities of sulfur. The small molecules of which proteins
are made are composed of 20 naturally occurring amino acids. The simplest of these,
glycine, is shown in the first structure in Figure 5.8, along with two other amino acids.
As shown in Figure 5.8, amino acids join together with the loss of a molecule of H 2 O for
each linkage formed. The three amino acids in Figure 5.8 are shown linked together as
they would be in a protein in the bottom structure in the figure. Many hundreds of amino
acid units may be present in a protein molecule.
The three-dimensional structures of protein molecules are of the utmost importance
and largely determine what the proteins do in living systems and how they are
recognized by other biomolecules. Enzymes, special proteins that act as catalysts to
enable biochemical reactions to occur, recognize the substrates upon which they act by
the complementary shapes of the enzyme molecules and substrate molecule. There are