Types, Roles, and Functions of Carbohydrates - Term Paper Example

?Introduction Carbohydrates are organic substances with a general formula of Cn (H2O)n n?5) and basically, they are hydrates of carbon. Carbohydrates are compounds comprising of three different atoms chemically combined. These atoms include carbon, oxygen and hydrogen. Strong covalent and hydrogen bonds join the atoms within their structures. Carbohydrates comprise of a wide range of substances broadly distributed and available in nature. In plants, carbohydrates account to over 75% of the dry weight, which is mainly in form of lignin, cellulose and starch (Pigman, 63). Structural composition of carbohydrates Structurally, carbohydrates exist as polyhydroxy or alcohol groups containing ketone or aldehyde functional groups (Engel, Gary and Reid, 115). These functional groups are responsible for the distinct chemical and physical properties of the different types of carbohydrates. Normally, carbohydrates are made of monosaccharides, which are the basic and the simplest units. Therefore, carbohydrates are polymers, consisting of monosaccharide monomers. The chemical and physical properties of carbohydrates depend on the type of monosaccharide monomers, the length of the polymer chain and the method used to join the basic units (Linhard and Bazin 55). Carbohydrates are divided into different categories depending on the length of the carbon chain. These categories include monosaccharide, disaccharides, oligosaccharides and polysaccharides. Monosaccharides are single and the simplest carbohydrate molecules, comprising of five or six carbon chains (Linhard and Bazin 57). Examples of monosaccharide include five carbon (pentose) sugars such as xylose, arabinose and ribose. Hexoses, which are six carbon sugars, are the most common simple sugars. They include fructose, mannose, glucose and fructose. Green plants through photosynthesis process naturally synthesize glucose. During photosynthesis, carbon dioxide and water combine in presence of light energy to form glucose (Voet, et al, 164) 6CO2 + 6H2O + energy> C6H12O6 + 3O2 The monosaccharides synthesized from the green plants undergo polymerization reactions to form disaccharides and other complex carbohydrates. Other methods natural processes that produce glucose include chemosynthesis in autotrophic bacteria and biosynthesis (McKee and McKee, 306). The presence of many chiral centers on the structure of glucose results into formation of two structural conformations, which could be either enantiomer or diastereomer. Enantiomers are mirror images. Glucose exhibits two enantiomeric structures namely, D and L glucose. Both of these structures demonstrate different physical and chemical properties (Pigman, 79) Disaccharides consist of two monosaccharide molecules that undergo polymerization reaction forming the longer chain carbohydrate (Timberlake, 127). Polymerization reaction is building up process, where small units (monomers) link together to form a complex molecule (polymer). Glycosidic bonds link the two-monosaccharide molecules after undergoing polymerization reaction. Polymerization process leads to formation of disaccharides, oligosaccharides and polysaccharides. These reactions occur between hydroxyl group of two different molecules leading to formation of covalent glycosidic bonds (Engel, Gary and Reid, 217-228). Examples of disaccharide include sucrose, maltose and lactose. Sucrose is formed when fructose and glucose undergo polymerization reactions. Therefore, fructose and glucose are sucrose monomers. Similarly, maltose is formed from the reaction of two glucose molecules while lactose from glucose and galactose molecules. Hence, glucose molecules are maltose monomers. Oligosaccharides comprises of about three to ten monosaccharides that are linked together after undergoing polymerization reactions. Examples include fructo- oligosaccharide and galacto-oligosaccharide (Pigman, 162). Polysaccharides comprise of long carbon chains formed by numerous monosaccharide units. The large number of number of monosaccharide monomers in polysaccharides result to formation of three-dimensional structure (Linhard and Bazin 130). Examples of polysaccharides include cellulose, glycogen, starch and lignin among others. Glycogen consists of glucose monomers. This polysaccharide is the preferred method of storing excess glucose in the mammalian blood (McKee and McKee, 119). . According to Voet, et al glucose is usually stored in the liver and in the muscles. Starch is the preferred method of storing excess glucose in plants. This compound is three dimensional in structure just like other polysaccharides. Digestive animals that feed on it digest the carbohydrate easily (72). Cellulose is another important polysaccharide, which consists of long linear chains of the glucose monomers. The polysaccharide is an important structural compound of woody plants and other types of vegetation and it forms an important structural unit for the plant cell wall. Because of its tough fibers, the digestive enzymes of many mammals are incapable of digesting cellulose and hence it forms an important component of roughage (Pigman, 77). However, organisms capable of producing cellulase enzyme have the capacity of digesting the polysaccharide. Lignin is another type of polysaccharide that forms an important structural component of wood, cobs hulls, husks among other plant structures (Pigman, 104) Lignin is a highly indigestible compound and hence it is of little nutritional value to animals. Properties of carbohydrates The physical properties of carbohydrates depend on the type of monosaccharide that makes up the structure, glycosidic linkages, the available functional groups, molecular weight and branching along the structure (Linhard and Bazin 58). Monosaccharides, disaccharides and oligosaccharides are soluble in water and polar solvents forming sweet tasting solutions. Solubility decreases with increase in molecular weight. Therefore, monosaccharides are the most soluble while polysaccharides are the least soluble. Starch is a highly branched polysaccharide and hence it does not dissolve in polar solvents. Viscosity and surface activity of carbohydrates is directly proportional to the molecular weight of the compound and the length of the carbon chain. Polysaccharides have the highest molecular weight compared with other carbohydrates and hence they are the most viscous. The high viscosity of polysaccharides enhances their suitability in manufacture of thickeners and high viscosity substances especially in the food industry (Linhard and Bazin 59). Carbohydrates have different ranges of crystallization. Sucrose is possibly one of the easiest carbohydrates to crystallize. Cellulose has a tendency of forming crystals but not as readily as sucrose. Besides these two carbohydrates, the others are difficult to crystallize and they exist as amorphous solids. Most carbohydrates exhibit a hygroscopic property (Linhard and Bazin 59). Water accounts to 2-10% of the total dry weight of carbohydrates. Naturally, carbohydrates are relatively stable molecules. However, reducing sugars have aldehyde functional groups that make them sensitive to oxidation (Timberlake, 244). Oligosaccharides and polysaccharides contain functional groups sensitive to acidic- glycosidic additions. However, the ease of hydrolysis in these polysaccharides depends on the type of saccharide units and their position in the chain (Pigman, 126). Carbohydrates contain many chiral centers and hence they are optically active. They are either enantiomers or diastereomers with particular optical orientation. Reducing sugars such as glucose and fructose demonstrate mutarotation when in aqueous solutions. However, most carbohydrates are poor absorbers of light in the visible spectra and in the ultra violet region (Linhard and Bazin 60) Chemical properties of carbohydrates The chemical properties of a carbohydrate depend on the type of monosaccharide residue present, the available functional groups, the position of linking with another functional group and the configurations (Linhard and Bazin 58). Carbohydrates have different functional groups besides the adelhydes and the common secondary and primary hydroxyl groups. Additional functional groups include free amino groups. The amino group in carbohydrates can undergo various reactions, including acylation, sulfonation and phosphorylation (Linhard and Bazin 59). Depending on the functional groups, present carbohydrates undergo series of reactions including hydrolysis, reduction and oxidation. Acid catalyzed hydrolytic reaction is the most common chemical process that carbohydrates undergo. Oligosaccharides and polysaccharides undergo chemical changes when subjected to acid catalyzed hydrolytic process. This process cleaves the glycosidic bonds in these carbohydrates. The conditions required for hydrolytic reactions vary depending on the size of the carbohydrate molecule. Monosaccharides require mild conditions but complex polysaccharides are normally subjected to concentrated mineral acid solutions and higher temperatures (Pigman 155) When a monosaccharide undergoes hydrolysis reaction, it forms a glycoside. Oligosaccharides and polysaccharides undergo solvolytic reaction in presence of hydrogen chloride to produce methyl glycoside products. When a disaccharide such as maltose is subjected to acid catalyzed hydrolytic reaction, glucose is formed. Enzymes are sometimes used to initiate hydrolytic reactions in carbohydrates. Naturally, starch undergoes hydrolytic reaction through fermentation reaction to form maltose (Engel, Gary and Reid, 298). . Carbohydrates undergo oxidation process readily depending on the functional group on the carbon chain. The aldehyde group in the monosaccharide is readily oxidized to form aldonic acid. Carbohydrates are divided into reducing and non-reducing sugars depending on how the react with Benedicts, Tollen’s or Fehling solutions. When monosaccharides react with one of these solutions, they become oxidized, reducing the respective oxidizing copper and silver ions (Voet, 240),. Therefore, glucose and other monosaccharides act as reducing agents. Bioenergetic functions of carbohydrates Carbohydrates play a critical role in bioenergetics. In addition, they are used as structural compounds and regulation of cell biochemistry and physiology. Carbohydrates are used as energy reserves for living organisms. Animals store glucose in form of glycogen while plants store it in form of starch. Starch and glycogen consists of highly branched polysaccharides and the compact structure ensures maximum storage of energy in a limited space. The stored energy reserves in plants and animals are easily converted into glucose for cellular metabolism through enzyme-catalyzed reactions (McKee and McKee, 96) The conversion of stored energy into glucose occurs when the body is in need of increasing its energy supply, to meet various demands such as growth and development of new cells. Plants are primary food producers and through photosynthesis, they supply man with carbohydrates, which are ingested to release glucose to provide the body with energy. Oligosaccharides play an important physiological role in the body. They are used as important structural component for cell membranes and protein surfaces. In addition, oligosaccharides promote the growth of beneficial microorganism in the digestive canal, which is important for the aiding digestion. D- Ribose sugar, a pentose monosaccharide is an important structural constituent of DNA, RNA, ATP, NADP and NAD important bio-molecules that play critical physiological and biochemical processes in the body (Engel, Gary and Reid 84) Work Cited Engel, Thomas, Gary Drobny, Reid Philip. Physical Chemistry for Life Sciences. Chicago: Prentice Hall, 2007. Linhard, Robert and Bazin, Helene. “Properties of Carbohydrates.” 1999. 11 December, 2011. http://www-heparin.rpi.edu/main/files/papers/263.PDF McKee, Trudy and McKee, James. Biochemistry: The Molecular Basis of Life. Oxford: Oxford University Press, 2008. Pigman, Horton. The Carbohydrates: Chemistry And Biochemistry,1B. 2nd ed. New York: Academic Press, 1980. Timberlake, Karen. Chemistry: An Introduction to General, Organic and Biological Chemistry. 11 ed. New York: Prentice Hall, 2011. Voet, Donald, et al. Fundamentals of Biochemistry: Life at the Molecular Level. 3rd ed. New York: Wiley, 2008. Read More