Carbohydrates, or sugars, are a staple of our diet. They are the main sources of energy for humans. Carbohydrates are present in our bread, grains, fruit, and most other foods we eat. They are polymers and made up of monomers.
Carbohydrates are one of the four main classes of macromolecules. Like proteins, and nucleic acids, carbohydrates are polymers (from the Greek polys meaning many and meris meaning part). A polymer is a large molecule consisting of many smaller or identical pieces called monomers. Monomers are linked by covalent bonds to form polymers (Campbell & Reece, 2002).
A dehydration reaction, a type of condensation reaction, joins two monomers together with a covalent bond. In the process, a molecule of water is released because of the bond, hence the names dehydration and condensation. Polymers are broken apart into monomers through hydrolysis (from the Greek hydro meaning water and lysis meaning break). Hydrolysis is the reverse of a dehydration reaction and requires a molecule of water and a specific enzyme.
Carbohydrates are used for fuel and building materials. Carbohydrates include all sugars and their polymers. Carbohydrates generally have a molecular formula that is some multiple of CH 2O or (CH 2O)n. The simplest sugars are monosaccharides (from the Greek monos meaning single and sacchar meaning sugar). Disaccharides are double sugars, containing two monosaccharides joined by a dehydration reaction. Polysaccharides are polymers of many simple sugars. They contain many monosaccharides joined by dehydration reactions. Because of their size, polysaccharides are the only carbohydrates that are considered macromolecules.
Glucose is a very common monosaccharide that is used for cellular respiration among other things. The names of most sugars ends in –ose. The molecular formula of glucose is C 6H 12O 6. You will notice that this is a multiple of CH 2O, (CH 2O)(6). The carbon skeleton of monosaccharides are usually 3-7 carbon atoms long. Glucose and fructose are hexoses (six-carbons long), pentoses have five carbons, and trioses have three carbons. In solution with water, most sugars form rings.
Monosaccharides, particularly glucose, are major sources of energy for the body. They can also be used for carbon skeletons in some simple organic molecules such as amino acids and fatty acids. Monosaccharides not immediately used by the body are incorporated in disaccharides and polysaccharides.
A disaccharide consists of two monosaccharides joined by a glycosidic link. A glycosidic link is two monosaccharides joined by a dehydration reaction with a covalent bond. Maltose is a disaccharide formed by two glucose. Lactose is glucose joined to glacatose. Sucrose, table sugar, is glucose and a fructose. Plants transport sugar as sucrose.
Polysaccharides are usually a few hundred to a few thousand monomers long. They consist of monosaccharides joined by glycosidic linkages. Some polysaccharides serve as storage for sugar and are hydrolyzed as needed. Others serve as building materials for structures that protect cells. The structure of polysaccharides is determined by the monomers that make the polysaccharide up and the position of the glcosidic linkages.
Starch is a storage polysaccharide that is found in plants and is made up entirely of glucose monomers. The monomers are joined by 1-4 linkages (the carbon 1 on one molecule is joined to carbon 4 on another molecule). Starch is helical in shape. Amylose is the more simple type of starch because it is unbranched. Amylopectin is a complex starch that branches. 1-6 linkages at the branch point join the branches. Starch is stored in plants in plastids.
A starch molecule. Note the helical structure (Rusconi).
Animals store glucose as a polysaccharide called glycogen. Glycogen is similar to amylopectin in structure, except it has significantly more branches. Glycogen is stored in the liver and in muscle cells. Glycogen stores are usually depleted in a day if they are not refilled by the consumption of food.
Cellulose is a polysaccharide that is found in plant cell walls. When glucose is submerged in water, it forms a ring. It can form two types of rings depending on how it bonds: the alpha form or the beta form. In glycogen, glucose that is in the alpha form is used. In cellulose, glucose in the beta form is used. Humans and most other organisms do not have enzymes to break down cellulose; however, a little cellulose is still needed in the diet to facilitate smooth intestinal transit. Unlike glycogen, cellulose forms straight polymers that do not branch. These straight molecules run parallel to each other and bond forming units called microfibrils. Microfibirls are used to make cell walls and are very strong. Wood, a very strong material, is made up of cellulose. Fungi are one of the few organisms able to digest cellulose. Some microbes can also break down cellulose like those found in the stomachs of cows.
A cellulose molecule. Note the straight, non-helical structure (Rusconi).
Chitin is a polysaccharide used by anthropods (insects, spiders, crustaceans and other organisms) to build exoskeletons. Pure chitin is leathery; however, it becomes covered with calcium carbonate, a salt, and becomes hard. Many fungi use chitin for cell walls.
Carbohydrates are important: they serve as a source of energy as well as serving as structural components for many organisms. As polysaccharides, carbohydrates form big molecules, which are considered macromolecules.
Campbell, N., & Reece, J. (2002). Biology: Sixth edition. San Francisco: Benjamin Cummings.
Rusconi, J. (n.d.). The Virtual Cell Web Page Chapter 2: The Biomolecules. Retrieved September 18, 2004, from http://personal.tmlp.com/Jimr57/textbook/chapter2/chapter2.htm