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Monosaccharides are a type of simple carbohydrate, or simple sugar. The word comes from the Greek manos, or single, and sacchar, sugar. The designation "carbohydrate" means that organic material is a combination of carbon and water, like sugars. A common example is glucose, which is found in the blood and can be expressed by its chemical makeup as C6(H2O)6.
These carbohydrates are differentiated from disaccharides and polysaccharides by the number of rings the chemical compound has. They are single ringed carbohydrates, while disaccharides like sucrose, also known as table sugar, have two rings. A special bond called a "glycisidic bond" joins these compounds together to form a disaccharide. In this case, sucrose is formed when fructose and glucose are bonded. Polysaccharides, like glycogen, are made of many monosaccharides joined together.
There are classifications based on how many carbon atoms these simple sugars have. The classifications by carbon atoms are as follows: Triose- 3, Tetrose- 4, Pentose- 5, Hexose- 6, Heptose- 7, Octose-8, Nonose- 9, and Decose- 10. The example above, glucose, is a hexose monosaccharide since is has six carbon atoms.
They are also classified as aldehyde or ketone, depending on what type of carbonyl group is contained in the formula. An aldehyde has a carbon bonded between hydrogen and oxygen, while a ketone has a bond between carbon and oxygen. These are expressed in formulaic mode as aldehyde (-CHO) or ketone (-CO-).
There are about 20 types of monosaccharides that occur naturally and about 50 that are formed synthetically. Some of the most well-known are glucose, fructose, and galactose.
These carbohydrates are absorbed into the body through the walls of the intestine. They are passed into the bloodstream and are stored to be used for energy later on. If they are stored long enough, certain carbohydrates will turn to fat.
Some, most notably fructose, have gotten attention for this ability to turn to fat. Studies in 2008 showed that the body tended to treat fructose differently than other carbohydrates. Most carbohydrates pass through the liver, where that organ decides whether to store or burn the product. The studies showed that fructose bypassed the liver and went immediately into the metabolism. Whatever was not burned immediately turned to glycogen, a type of body fat.
Like any carbohydrate, monosaccharides provide an invaluable service. They provide energy and balance to the human body and other living organisms. As with any nutritional consumption, people should consume them in moderation.
@sapphire12, while it is true that foods with only monosaccharides are often less nutritious, it is important to remember that the absorption of monosaccharides in your body will still give you energy, especially important for people who exercise often or otherwise burn a lot of calories each day. However, the structure of monosaccharides does mean it is important to avoid overconsumption of calories, though it is a concern with even more complex carbohydrates.
What we see from monosaccharides' structure is that the simpler the sugar, the less you can get from it nutritionally, and the easier it is for you to gain weight from eating that sugar. This is also what is meant when people refer to "simple" carbohydrates versus "complex" carbohydrates. Complex carbohydrate foods also have fiber, while most simple carbohydrate foods, or monosaccharide foods, are made up almost entirely of simple sugars and little else.