Glucose is a carbohydrate, meaning that it's composed of the elements carbon, hydrogen and oxygen. It's one of the most common organic, or carbon-based, molecules in nature, and is the primary source of energy for many living organisms. There are many sources of glucose in nature, both in food items and elsewhere.
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The significance of the glucose molecule in nature is far-reaching. It is not only a very important nutritional molecule and a source of energy for cells, it is also structural. Plants synthesize glucose by combining carbon dioxide with water, using the sun as a source of energy. Much of the structural material of a plant -- cellulose -- is made up of glucose, says Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry."
Many organisms rely on glucose from food sources for energy. Many also store glucose in the form of a long chain of glucose molecules to meet energy needs when food supplies are scarce. Humans store glucose in the liver as a chain called glycogen, says Dr. Lauralee Sherwood in "Human Physiology." Plants store glucose in a similar long chain of sugar units, called amylose, or starch. Other animals can eat plants for food because of the stored amylose.
Humans obtain glucose from a plant's stored amylose when they consume plants. Amylose doesn't taste sweet, despite the fact that it's made up of units of the sugar glucose. Humans can't digest cellulose, the glucose-based structural material of a plant, because they lack the enzymes, explains Dr. Sherwood. Additional sources of glucose in food include sugars, like table sugar and milk sugar. Both table sugar and milk sugar consist of glucose combined with a different sugar molecule.
Because some animals eat grass and leaves, it's common to assume that they can digest the cellulose that makes up the bulk of the plant material. This is not true, however. Animals that are able to "digest" cellulose actually rely on a symbiotic relationship with bacteria in their digestive tracts, says Drs. Mary Campbell and Shawn Farrell in "Biochemistry." The bacteria can digest the cellulose, and the animal eating the grass, in turn, digests the bacteria.
While only some sources of glucose in nature can provide energy to humans -- sugar and starches are digestible, cellulose is not -- indigestible glucose is nevertheless important to the human diet. Cellulose is more commonly called dietary fiber, and it helps improve health in several ways. It increases digestive bulk and assists in the mechanical function of the digestive system, absorbs cholesterol and toxins, and helps promote a feeling of fullness that prevents overeating.