From Seed to Seed:
Plant Science for K-8 Educators

Carbohydrates. 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 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 of 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 incredibly resistant to decomposition. Very few organisms have the ability to digest or break down cellulose-just 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.