The glucose molecule is a carbohydrate, and is further categorized as a monosaccharide. This means it's a single sugar unit that can be found alone, but is more often found in nature combined with other monosaccharides into short or long chains. There are several sources of pure glucose in nature, some of which are digestible and some of which aren't.
Glucose Syrup
Though of limited culinary utility, glucose syrup--often called dextrose syrup--is a source of pure glucose. Diabetics, who have difficulty controlling blood glucose and have to use injections of insulin to help regulate their sugar levels, may need to use glucose syrup in the event that they become hypoglycemic. Explains Dr. Lauralee Sherwood in her book "Human Physiology," the hormone insulin signals cells to take up glucose from the bloodstream. If a diabetic accidentally uses too much insulin, they can cause the cells to take up too much glucose, leading to a physiological emergency in which brain and other cells aren't getting access to blood sugar. Glucose syrup is an emergency measure for increasing blood sugar quickly.
Fiber
Though indigestible to humans, plant fiber, including wood, is made of glucose. Plants produce glucose through a process called photosynthesis, that's powered by the sun's radiant energy. The plant takes up atmospheric carbon dioxide as well as water from the soil, and combines them to make glucose. Glucose can then either be used for energy by the plant, or can be combined into a long chain of glucose molecules called cellulose. Humans, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry," lack the enzymes to digest cellulose, but many bacteria are capable of breaking apart the glucose units and subsequently using them.
Amylose
Another source of pure glucose--this time, a source that humans can digest--is amylose, which is another name for plant starch. Plants, when they synthesize glucose, don't have to use it for structural material, explain Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry." Instead, they can store it for later use as an energy-providing molecule. The storage form of glucose by plants is more commonly called starch. The starch in some vegetables, like potatoes, and in grains like wheat and oats, is made up of chains of glucose molecules. Humans, upon eating starch, break apart the glucose molecules from one another and absorb them.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
