Monosaccharides: Definition, Structure & Examples - The simplest carbohydrate unit is called a monosaccharide. In some cases, monosaccharides (as well as other molecules) have identical empirical formulas, but their structural formulas differ, giving each variety unique chemical properties. Such molecules are called isomers.
The simplest units of carbohydrates (which are the building blocks of all complex carbohydrates) are monomers known as simple sugars, or monosaccharides. (The term mono-means one, and the term -saccharide means sugar unit.) Typically, simple sugars consist of three to seven carbon atoms. Of special importance to biologists are two types of monosaccharides: the five-carbon (or pentose) molecules, such as those that are part of DNA and RNA; and the six-carbon (hexose) molecules, such as glucose and fructose. The common ending -ose indicates a sugar.
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| Mnosaccharides Structure |
Glucose and fructose are examples of what are called isomers olecules that have identical empirical formulas but have different properties. We touched briefly on this topic in Chapter 2. The empirical formula for both glucose and fructose is C6H12O6, indicating that each consists of 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen.
In an instance like this, in order to see the differences between the two kinds of molecules, a structural formula is more useful than a general formula. (A structural formula shows the position of the atoms within a molecule.) This type of formula illustrates the significant nature of each isomer by showing variations in molecular arrangement. The differences in the bonds between the carbons change the properties of the chemicals. For example, because of the molecular structure of fructose, it stimulates our taste buds more than glucose, so it tastes sweeter than glucose.
One comment about the notation you will see in a structural formula of carbon molecules. Some monosaccharides, especially those having five or six carbon atoms, form a ring when placed in water, as in the watery interior of living cells. In a structural formula, as a matter of convention, the locations of the carbon atoms are numbered for reference, and the numbers serve as an indication of the placement of the carbons at the intersection of the links in the ring. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 55)
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