Carbohydrates contain three major categories of molecules: monosaccharides (or simple sugars), disaccharides, and polysaccharides.
1. Monosaccharides: This group, also known as simple sugars, is the simplest carbohydrate form. In general, the basic molecular formula is (CH2O)n (carbon and water - carbo-hydrates, get it?), with n generally being between 3 and 7 in living organisms. The most abundant monosaccharide is glucose with n = 6. Thus the formula for glucose would be (CH2O)6 or C6H12O6. . Other common monosaccharides are fructose (n = 6) and galactose (also n = 6). These three molecules have the same molecular formula! How are they different? Simply, the arrangement of the atoms in each of these molecules is slightly different. So although all three molecules are very similar, they are different and distinct. Molecules that have the same molecular formula but different structures are known as isomers. Glucose, fructose, and galactose are all isomers of one another. You should also notice that there is a large amount of oxygen in carbohydrates and you should conclude (correctly) that carbohydrates are polar molecules (and are therefore hydrophilic).
The primary function of monosaccharides is as a source of energy for organisms. In the presence of O2, glucose and other monosaccharides can be broken down to CO2 and H2O with the release of energy. This energy is harnessed by the cell to do work. Additionally, monosaccharides serve as "building blocks" for the formation of di- and polysaccharides.
The formula (CH2O)n represents the simplest form of monosaccharide. Although you will probably not encounter many of the complex forms in this course, they do exist. Examples of the more complex sugars are amino sugars (sugars containing an amino group [-NH2]) and sugar phosphates (sugars containing phosphate groups [PO4]).
2. Disaccharides: As you may have guessed, disaccharides consist of two monosaccharides joined together by a covalent bond. This bond is generally between the number 1 carbon of one monosaccharide and the number 4 carbon of the other molecule. (Biochemists have devised a system for numbering carbon atoms in organic molecules. You do not have to know how this system works other than to understand that each carbon atom is numbered based on its position in the molecule and that the numbering is consistent between different molecules so that the number 1 carbon of every glucose molecule is in the same location, etc.) This particular bond is known as an 1-4 alpha-glycosidic bond. There is an alternate form of a glycosidic bond (the 1-4 beta-glycosidic bond) between two glucose molecules that is structurally different from the alpha form. Although these two molecules look very similar (the only difference being the arrangement of the glycosidic bond), they are very different biochemically, as you will soon see. Bonds can also form between the 1 and 6 carbon of two glucose molecules forming a 1-6 alpha-glycosidic bond. Because of the structure of glycosidic bonds, the two monosaccharides in a disaccharide do not have to be the same. Common examples of disaccharides are sucrose or table sugar (glucose-fructose), maltose (glucose- glucose) and lactose (glucose-galactose).
The primary function of disaccharides is as a nutritional source of monosaccharides. Many of the sugars found in foodstuffs are disaccharides.
3. Polysaccharides: You should realize that one of the glucose molecules of a disaccharide has a free 4-carbon that could form a glycosidic bond with a third monosaccharide. Likewise, the other glucose could also form a second glycosidic bond using the 1-carbon. In fact, both could also form a bond using the 6-carbon. The addition of new monosaccharides could continue indefinitely making a huge molecule forming a long (and branched via the 6- carbon) chain of glucose molecules. This long chain is known as a polysaccharide.
The two major polysaccharides that are important for physiology are starch (made exclusively by plants) and glycogen (made by animals). These two molecules are very similar in that they are polymers of glucose joined by 1-4 alpha-glycosidic bonds. Glycogen tends to be a bit more branched than starch but not quite as long. The major function of starch and glycogen is as a "quick- to-get-at" storage form of glucose. In a simplified fashion, when one consumes more glucose than one needs, the liver (primarily) stores these sugars in the form of glycogen. Then, at some later time, the liver can quickly breakdown the glycogen into glucose that can be used for energy.
Polysaccharides can also be used as structural components. The cell walls of plants (and thus things such as wood and paper) are also formed by long polymers of glucose known as cellulose. Cellulose is different from starch (also produced by plants) in that the glycosidic bonds in cellulose are in the beta configuration. Cellulose is not used as a nutrient because of this one difference. The digestive tract of animals do not produce digestive enzymes capable of breaking beta-glycosidic bonds. Thus, although wood is essentially full of glucose, we cannot eat wood as a source of glucose because we cannot digest it. Other examples of structures that are composed of polysaccharides are the cell walls of bacteria (made up of amino sugars), chitin (the exoskeleton of insects), and chondroitin sulfate, a material found in connective tissue.
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