The simplest carbohydrates are monosaccharides, or simple sugars.
Monosaccharides consist of a ring (or a chain) of carbon atoms to which hydrogen and
oxygen atoms are attached. The ratio of carbon to hydrogen to oxygen is the same in all
monosaccharides. The number of carbon atoms in the molecule, and the configuration of all
the atoms, determines what type of simple sugar it is. The immediate product of photosynthesis is a monosaccharide called
glucose; other examples of monosaccharides are fructose and galactose. In the plant,
monosaccharides are used as a source of energy, or as building blocks for larger
molecules.
The sugar we are all most familiar with—table
sugar—is sucrose. Sucrose is a disaccharide;
the "di-" indicating that it is made up of two simple sugars (in this case,
glucose and fructose.) The most important commercial sources of sucrose are sugar cane and
sugar beets.
Polysaccharides
are made up of a number of monosaccharides bound together. We said above that
monosaccharides are used as building blocks to create larger molecules—and
polysaccharides are examples of these larger molecules.
The two principle polysaccharides in plants are starch and
cellulose. Both starch molecules and cellulose molecules consist of long chains containing
thousands of glucose molecules attached end to end. The difference between starch and
cellulose lies in how the glucose molecules are attached to each other. This different
configuration of molecules accounts for the very different properties of starch and
cellulose.
Starch is
used primarily as storage for excess food reserve (sugars) and is found in most plants.
Many familiar foods are storage structures for starch, including white or
"Irish" potatoes (swollen stem tips) and sweet potatoes (swollen roots). Starch
is also common in the endosperm
(storage tissue) of monocot seeds and some dicot seeds; this starch provides a ready
source of energy for the germinating embryo. Examples of starch-filled seeds include
grains such as wheat and oats.
Cellulose, you’ll remember from our earlier
discussion, is used primarily for structural support. Unlike starch molecules, once the
glucose molecules are bound into cellulose molecules, they are no longer available as food
sources—for either the plant or us.
Cellulose is very resistant to decomposition. Very few
organisms have the ability to digest or break down cellulose—certain bacteria, fungi,
and other microorganisms; silverfish; and a few others. Still, cellulose is an important
component of a healthy human diet; it is a major constituent of what is commonly known as
dietary fiber.
