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is a Disaccharide found in Milk . It is pivotally composed of a molecule of β-D-galactose and a molecule of β-D-glucose bonded by a β1-4 Glycosidic Linkage .]] Carbohydrates (from ' Hydrate s of Carbon ') or '''saccharides''' ( Greek ''σάκχαρον'' meaning " Sugar ") are simple Organic Compound s that are Aldehyde s or Ketone s with many Hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone Functional Group . Carbohydrates are the most abundant of the four major classes of Biomolecule s, which also include Protein s, Lipid s and Nucleic Acid s. They fill numerous roles in living things, such as the storage and transport of Energy ( Starch , Glycogen ) and structural components ( Cellulose in plants, Chitin in animals). Additionally, carbohydrates and their derivatives play major roles in the working process of the Immune System , Fertilization , Pathogenesis , Blood Clotting , and Development . The basic carbohydrate units are called Monosaccharide s, such as Glucose , Galactose , and Fructose . The general Stoichiometric Formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater.Matthews, C. E.; K. E. Van Holde; K. G. Ahern (1999) ''Biochemistry''. 3rd edition. Benjamin Cummings. ISBN 0-8053-3066-6 Monosaccharides can be linked together in almost limitless ways. Two joined monosaccharides are called Disaccharide s, such as Sucrose and Lactose . Carbohydrates containing between about three to six monosaccharide units are termed Oligosaccharide s; anything larger than this is a Polysaccharide . Polysaccharides, such as Starch , Glycogen , or Cellulose , can reach many thousands of units in length. Many carbohydrates contain one or more modified monosaccharide units that have had one or more groups replaced or removed. For example, Deoxyribose , a component of DNA , is a modified version of Ribose ; Chitin is composed of repeating units of N-acetylglucosamine , a Nitrogen -containing form of glucose. The names of carbohydrates often end in the suffix -ose. MONOSACCHARIDES See Also: Monosaccharide is an aldohexose with the formula (C·H2O)6. The red atoms highlight the Aldehyde group, and the blue atoms highlight the Asymmetric Center furthest from the aldehyde; because this -OH is on the right of the Fischer Projection , this is a D sugar.]] Monosaccharides are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. The general Chemical Formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater. Classification of monosaccharides The α and β Anomer s of glucose. Note the position of the anomeric carbon (red or green) relative to the CH2OH group bound to carbon 5: they are either on the opposite sides (α), or the same side (β). Monosaccharides are classified according to three different characteristics: the placement of its Carbonyl group, the number of Carbon atoms it contains, and its Chiral handedness. If the carbonyl group is an Aldehyde , the monosaccharide is an Aldose ; if the carbonyl group is a Ketone , the monosaccharide is a Ketose . The smallest possible monosaccharide, those with three carbon atoms, are called Triose s. Those with four are called Tetrose s, five are called Pentose s, six are Hexose s, and so on. These two systems of classification are often combined. For example, Glucose is an Aldohexose (a six-carbon aldehyde), Ribose is an Aldopentose (a five-carbon aldehyde), and Fructose is a Ketohexose (a six-carbon ketone). Each carbon atom bearing a Hydroxyl Group (-OH), with the exception of the first and last carbons, are Asymmetric , making them Stereocenter s with two possible configurations each (the -H and -OH may be on either side). Because of this asymmetry, a number of Isomer s may exist for any given monosaccharide formula. The aldohexose D-glucose, for example, has the formula (C·H2O)6, of which all but two of its six carbons atoms are chiral centers, making D-glucose one of 24 = 16 possible Stereoisomer s. In the case of Glyceraldehyde , an aldotriose, there is one pair of possible stereoisomers, which are Enantiomers and Epimer s ( 1,3-dihydroxyacetone , the corresponding ketose is a symmetric molecule with no stereocenters). The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group: if the hydroxyl group is on the right the molecule is a D sugar, otherwise it is an L sugar. Because D sugars are biologically far more common, the D is often omitted. Conformation and Furanose configurations of monosaccharides are named.]] The aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a Hemiacetal or Hemiketal , forming a Heterocyclic ring with an oxygen bridge between two carbon atoms. Rings with five and six atoms are called Furanose and Pyranose forms, respectively, and exist in equilibrium with the straight-chain form. During the conversion from straight-chain form to cyclic form, the carbon atom containing the carbonyl oxygen, called the s. In the ''α anomer'', the -OH substituent on the anomeric carbon rests on the opposite side ( Trans ) of the ring from the CH2OH side branch. The alternative form, in which the CH2OH substituent and the anomeric hydroxyl are on the same side (cis) of the plane of the ring, is called the ''β anomer''. Because the ring and straight-chain forms readily interconvert, both anomers exist in Equilibrium . Use in cells Monosaccharides are the major source of fuel for Metabolism , being used both as an energy source (glucose being the most important in nature) and in Biosynthesis . When monosaccharides are not needed by cells they are quickly converted into another form, such as polysaccharides. DISACCHARIDES , also known as table (left) and Fructose (right).]] See Also: Disaccharide Disaccharides are the simplest polysaccharides. They are composed of two monosaccharide units bound together by a Covalent bond known as a Glycosidic Linkage formed via a Dehydration Reaction , resulting in the loss of a Hydrogen atom from one monosaccharide and a Hydroxyl Group from the other, so the Formula of unmodified disaccharides is C12H22O11. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable. Sucrose , pictured to the right, is the most abundant disaccharide and the main form in which carbohydrates are transported in Plant s. It is composed of one Glucose molecule and one Fructose molecule. The Systematic Name for sucrose, ''O''-α-D-glucopyranosyl-(1→2)-D-fructofuranoside, indicates four things:
Lactose , a disaccharide composed of one Galactose molecule and one Glucose molecule, occurs naturally only in Milk . The Systematic Name for lactose is ''O''-β-D-galactopyranosyl-(1→4)-D-glucopyranose. Other notable disaccharides include Maltose (two glucoses linked α-1,4) and Cellobiose (two glucoses linked β-1,4). OLIGOSACCHARIDES AND POLYSACCHARIDES is a linear Polymer of Glucose mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components of Starch , the other being Amylopectin .]] See Also: Oligosaccharide Polysaccharide Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between two and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Definitions of how large a carbohydrate must be to fall into each category vary according to personal opinion. Examples of oligosaccharides include the disaccharides mentioned above, the trisaccharide Raffinose and the tetrasaccharide stachyose. Oligosaccharides are found as a common form of Protein Posttranslational Modification . Such posttranslational modifications include the Lewis oligosaccharides responsible for Blood Group incompatibilities, the alpha-Gal epitope responsible for hyperacute rejection in xenotransplanation, and O-GlcNAc modifications. Polysaccharides represent an important class of biological Polymer . Their Function in living organisms is usually either structure or storage related. For example, Starch is used as a storage polysaccharide in plants, while in animals Glycogen , which can be broken down much quicker, is used instead. Cellulose and Chitin are examples of storage polysaccharides. Cellulose is used in the Cell Wall s of plants and other organisms, and is the most abundant organic molecule on earth.N.A.Campbell (1996) ''Biology'' (4th edition). Benjamin Cummings NY. p.23 ISBN 0-8053-1957-3 Chitin has a similar structure to cellulose but has Nitrogen containing side branches, increasing its strength. It is found in Arthropod Exoskeleton s and in the cell walls of some Fungi . It has a variety of uses, for example in Surgical Thread s. Other polysaccharides include Callose , Laminarin , Xylan , and Galactomannan . NUTRITION products are rich sources of complex carbohydrates]] Carbohydrates require less Water to digest than Protein s or Fat s and are the most common source of energy. Proteins and fat are vital building components for body Tissue and Cells , and thus it could be considered advisable not to deplete such resources by necessitating their use in energy production. Carbohydrates are not per gram while fats contain 9 kilocalories and alcohol contains 7 kilocalories per gram. Foods that are high in carbohydrates include Bread s, Pasta s, Bean s, Potatoe s, Bran , Rice and Cereal s. Based on evidence for risk of heart disease and obesity, the . Page 769 . ISBN 0-309-08537-3 The . Pages 55-56. ISBN 92-4-120916-X The distinction between "good carbs" and "bad carbs" is an important attribute of Low-carbohydrate Diet s, which promote a reduction in the consumption of grains and starches in favor of protein. The result is a reduction in Insulin levels used to metabolize sugars, and an increase in the use of fat for energy through Ketosis , a process also known as Rabbit Starvation . Classification and Dairy Product s should make up the bulk of carbohydrate consumption. The USDA's ''Dietary Guidelines for Americans 2005'' dispenses with the simple/complex distinction, instead recommending fiber-rich foods and whole grains. DHHS and USDA , ''Dietary Guidelines for Americans 2005'', Chapter 7 Carbohydrates The Glycemic Index and Glycemic Load systems are popular alternative classification methods which rank carbohydrate-rich foods based on their effect on Blood Glucose levels. The Insulin Index is a similar, more recent classification method which ranks foods based on their effects on Blood Insulin levels. This system assumes that high glycemic index foods and low glycemic index foods can be mixed to make the intake of high glycemic foods more acceptable. The World Health Organisation and Food and Agriculture Organization's joint expert report on Diet, Nutrition and the Prevention of Chronic Diseases (WHO Technical Report Series 916) advises carbohydrate consumption of 55-75% carbohydrate, but restricts "Free sugar" intake to 10%. Its definition is "The term "free sugars" refers to all monosaccharides and disaccharides added to foods by the manufacturer, cook or consumer, plus sugars naturally present in honey, syrups and fruit juices." (page 56 of the report; note to Table 6: Ranges of population nutrient intake goals). This is their effective split between simple and complex carbohydrates. METABOLISM See Also: Carbohydrate metabolism Catabolism Catabolism is the metabolic reaction cells undergo in order to extract energy. There are two major Metabolic Pathway s of monosaccharide Catabolism : # Glycolysis # Citric Acid Cycle Oligo/polysaccharides are cleaved first to smaller monosaccharides by enzymes called Glycoside Hydrolase s. The monosaccharide units can then enter into monosaccharide catabolism. CARBOHYDRATE CHEMISTRY Carbohydrates are reactants in many Organic Reaction s. For example:
An artificial carbohydrate is Sorbitol . SEE ALSO
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