Carbohydrates PDF

# Carbohydrates ## Compiled and Edited By Dr. Syed Ismail Associate Professor, SSAC College of Agriculture, VN MKV Parbhani ## Definition Carbohydrates are broadly defined as polyhydroxy aldehydes or ketones and their derivatives or as substances that yields one of these compounds. - Composed of c...

# Carbohydrates ## Compiled and Edited By Dr. Syed Ismail Associate Professor, SSAC College of Agriculture, VN MKV Parbhani ## Definition Carbohydrates are broadly defined as polyhydroxy aldehydes or ketones and their derivatives or as substances that yields one of these compounds. - Composed of carbon, hydrogen, and oxygen. - Functional groups present include hydroxyl groups. - -ose indicates sugar. ## Function of Carbohydrates in Cells - Major source of energy for the cell. - Major structural component of plant cell. - Immediate energy in the form of glucose. - Reserve or stored energy in the form of glycogen. ## Classification of Carbohydrates - Carbohydrates are classified according to the number of subunits that make them up. - **Monosaccharides** - **Oligosaccharides** - Disaccharides - Trisaccharides - Tetrasaccharides - **Polysaccharides** ## Monosaccharides Monosaccharides are simple sugars, or the compounds which possess a free aldehyde (CHO) or ketone (C=O) group and two or more hydroxyl (OH) groups. They are the simplest sugars and cannot be hydrolysed further into smaller units. - Monosaccharides contain a single carbon chain and are classified on the basis of the number of carbon atoms they possess, and as aldoses or ketoses depending upon their groups. ### Classification by Carbon Atoms | Sugar | Structure formula | Aldoses | Ketoses | |--------------|-------------------|------------------------------------------|-------------------------------------| | Triose | C₃H₆O₃ | Glyceraldehydes |Dehydroxy acetone, Erthrulose | | Tetroses | C₄H₈O₄ | Erythrose, Threose | Ribulose | | Pentoses | C₅H₁₀O₅ | Xylose, Ribose, Arabinose | | | Hexoses | C₆H₁₂O₆ | Glucose, Galactose, Mannose | Fructose | ## Monosaccharides - Hexoses ### D-Glucose (dextrose) "Blood Sugar" - The essential energy source for all body functions. - Other names: Dextrose and Blood Sugar. - A component of each disaccharide. ### D-Galactose - Seldom occurs freely in nature. - Binds with Glucose to form sugar in milk: lactose. - Once absorbed by the body, galactose is converted to glucose to provide energy. ### D-Fructose (levulose) "Fruit Sugar" - The sweetest of all sugars (1.5x sweeter than sucrose). - Occurs naturally in fruits and honey "the fruit sugar". ## Steriochemistry Optical isomers (= enantiomers) differ from each other in the disposition of the various atoms or groups of atoms in space around the asymmetric carbon atom. These are, in fact, the mirror image of each other. These may also be likened to left- and right-handed gloves. - One form in which H atom at carbon 2 is projected to the left side and OH group to the right is designated as D-form and the other form where H atom is projected to the right side and OH group to the left is called as L-form (note the use of small capital letters D and L). - For example, the glyceraldehyde has only one asymmetric carbon atom (numbered as 2) and it can, therefore, exist in 2 isomeric forms:  ## D-Aldoses Containing Three, Four, Five and Six Atoms  ## Properties of Monosaccharides 1. **Mutarotation:** When a monosaccharide is dissolved in water, the optical rotatory power of the solution gradually changes until it reaches a constant value. For ex: when D-glucose is dissolved in water, a specific rotation of +112.2° is obtained, but this slowly changes, so that at 24h the value has become +52.7°. This gradual change in specific rotation is known as mutarotation. This phenomenon is shown by the number of pentoses, hexoses, and reducing disaccharides. 2. **Glucoside Formation:** When a D-glucose solution is treated with methanol and HCl, two compounds are formed, these are a - and 6-D-glucosides. Thus, formed glucosides are not reducing sugar and also do not show the phenomenon of mutarotation. 3. **Reducing Power:** Sugars having free or potentially free aldehyde or ketone groups have an ability to reduce the cupric copper to cuprous. Reducing sugar + 2 Cu⁺⁺ → oxidized + 2Cu (cupric) sugar (cuprous) 4. **Oxidation/Reduction:** The alcoholic OH, aldehyde (COH) or keto (C=O) group are oxidized to carboxyl groups with certain oxidizing agents. The oxidation may be brought under mild or with vigorous oxidizing conditions. - With mild oxidant like BrHO: In this group only aldehyde is oxidized to produce gluconic acid (monocarbonic). Ketoses do not respond to this reaction. - **ii. With strong Oxidizing agent like Conc HNO₃:** Both aldehyde or ketone groups are oxidized to yield dicarboxylic acids. - **iii. Oxidation with metal hydroxides:** Metal hydroxides like Cu(OH)₂, Ag OH oxidize free aldehyde or ketone groups of mutarotating sugar and reduce themselves to lower oxides of free metals. - **Reduction:** The aldehyde or ketone group present can be reduced to its respective alcohol with sodium amalgam. For ex: Fructose and glucose give the hexahydric alcohol i.e., Sorbitol and Mannitol. - **Dehydration:** The monosaccharides when treated with Conc H₂SO₄ it gets dehydrated to form 5-hydroxyl-methyl furfural derivative. - **Methylation or Esterification:** The glucosidic and alcoholic OH groups of mono saccharides and reducing disaccharides react with acetylating agents like acetic anhydride in pyridine to form acetate derivatives called esters. ## Carbohydrate Types - **Reducing sugars:** 1. Carbohydrates with a free aldehyde (at C-1) or a free ketone (at C-2) group. 2. They are in hemiacetal or hemiketal form. 3. Do exhibit mutarotation. 4. Do form osazones with phenyl hydrazine. 5. Do form oximes with hydroxylamine. - Examples - Glucose, Fructose, Lactose, Maltose, Cellobiose - **Non-Reducing sugars:** 1. Aldehyde or ketone group is not free but instead utilized in bond formation. 2. They are in acetal or ketal form. 3. Do not exhibit mutarotation. 4. Do not form osazones. 5. Do not form oximes. - Examples - Sucrose, Glycogen, Inulin ## Oligosaccharides These are compound sugars that yield 2 to 10 molecules of the same or different monosaccharides on hydrolysis. Accordingly, an oligosaccharide yielding 2 molecules of monosaccharide on hydrolysis is designated as a disaccharide, and the one yielding 3 molecules of monosaccharide as a trisaccharide and so on. - **Disaccharides** - Sucrose, Lactose, Maltose, Cellobiose, Trehalose, Gentiobiose, Melibiose - **Trisaccharides** - Rhamninose, Gentianose, Raffinose (= Melitose), Rabinose, Melezitose - **Tetrasaccharides** - Stachyose, Scorodose - **Pentasaccharide** - Verbascose - The molecular composition of the 3 legume oligosaccharides (viz., raffinose, stachyose and verbascose) is shown below: - α-Galactose (1-6) α-Glucose (1-2) β-Fructose Raffinose - α-Galactose (1-6) α-Galactose (1-6) α-Glucose (1-2) β-Fructose Stachyose - α-Galactose (1-6) α-Galactose (1-6) α-Galactose (1-6) α-Glucose (1-2) β-Fructose Verbascose  ## Disaccharides - Composed of 2 monosaccharides. - Cells can make disaccharides by joining two monosaccharides by biosynthesis. - **Glucose + fructose = sucrose (table sugar)** - Found naturally in plants: sugar cane, sugar beets, honey, maple syrup. - Sucrose may be purified from plant sources into Brown, White, and Powdered Sugars. - **Glucose + galactose = lactose** - The primary sugar in milk and milk products. - Many people have problems digesting large amounts of lactose (lactose intolerance). - **Glucose + glucose = maltose** - Produced when starch breaks down. - Used naturally in fermentation reactions of alcohol and beer manufacturing.  ## Relative Sweetness of Sugars and Artificial Sweeteners | Sweetener | Sweetness Relative to Sucrose | |--------------------------|--------------------------------| | **Monosaccharides** | | | Galactose | 30 | | Glucose | 75 | | Fructose | 175 | | **Disaccharides** | | | Lactose | 16 | | Maltose | 33 | | Sucrose | 100 | | **Sugar Alcohols** | | | Sorbitol | 60 | | Maltitol | 80 | | Xylitol | 100 | | **Artificial Sweeteners (Noncarbohydrate)** | | | Aspartame | 18,000 | | Saccharin | 45,000 | | Sucralose | 60,000 | | Neotame | 1,000,000 | ## Polysaccharides - Containing 10 or more monosaccharide units attached together. - **Examples** 1. Starch - digestible 2. Glycogen - digestible 3. Fiber - indigestible - **Long chains of glucose units form these polysaccharides.** - Cellulose gives structure to plants, fiber to our diet. - Glycogen is an energy storage sugar produced by animals. - Liver cells synthesize glycogen after a meal to maintain blood glucose levels. ## Types of Polysaccharides ### Starch - The major digestible polysaccharide in our diet. - The storage form of carbohydrate in plants. - Sources: Wheat, rice, corn, rye, barley, potatoes, tubers, yams, etc. - Two types of plant starch: 1. Amylose 2. Amylopectin  ### Amylose - In the form of a straight chain linked together with a-1-4, linkages indicating 300-5,500 glucose units per molecule, molecular wt range from 10⁵ to 10⁶. - Generally it is water-soluble and gives a blue colour with iodine. ### Amylopectins - Contains beside straight chains several branched chains, which are arranged in a-1-4 and β-1-6 linkage units. - One molecule of amylopectin contains 50,000 to 500,000 glucose molecules, molecular wt. range from 10⁷ to 10⁸. - It is insoluble in water and gives a purple colour with iodine.  ### Cellulose - Form cell walls in plant cells. - Also called fiber or ruffage. - Indigestible by humans.  ### Glycogen - The storage form of glucose in the body. - Stored in the liver and muscles. - Found in tiny amounts in meat sources. - Not found in plants. - Not a significant food source of carbohydrate.  ## CONCEPT MAP  ## Summary of Carbohydrates | Carbohydrate | Food Sources | Description | |--------------------|-------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | **Monosaccharides** | | | | Glucose | Fruit juices, honey, corn syrup | | | Galactose | Lactose hydrolysis | | | Fructose | Fruit juices, honey, sucrose hydrolysis | | | **Disaccharides** | | | | Maltose | Germinating grains, starch hydrolysis | | | Lactose | Milk, yogurt, ice cream | | | Sucrose | Sugar cane, sugar beets | | | **Polysaccharides** | | | | Amylose | Rice, wheat, grains, cereals | Unbranched polymer of glucose joined by α-1,4-glycosidic bonds. | | Amylopectin | Rice, wheat, grains, cereals | Branched polymer of glucose joined by α-1,4- and α-1,6-glycosidic bonds. | | Glycogen | Liver, muscles | Highly branched polymer of glucose joined by α-1,4- and α-1,6-glycosidic bonds. | | Cellulose | Plant fiber, bran, beans, celery | Unbranched polymer of glucose joined by β-1,4-glycosidic bonds. | ## Some Clinical Concepts - **Glucose** is the most important energy source of carbohydrates to the mammals (except ruminants). The bulk of dietary carbohydrate (starch) is digested and finally absorbed as glucose into the body. - **Dextrose** (glucose in solution in dextrorotatory form) is frequently used in medical practice. - **Fructose** is abundantly found in the semen which is utilized by the sperms for energy. Several diseases are associated with carbohydrate's e.g., diabetes mellitus, glycogen storage diseases galactosemia. - **Accumulation of sorbitol and dulcitol** in the tissues may cause certain pathological conditions e.g. cataract, nephropathy. - **The non-digestible carbohydrate cellulose** plays a significant role in human nutrition. These include decreasing the intestinal absorption of glucose and cholesterol, and increasing bulk of feces to avoid constipation. - **The mucopolysaccharide hyaluronic acid** serves as lubricant and shock absorbent in joints. - **The mucopolysaccharide heparin** is an anticoagulant (prevents blood clotting). - **The survival of Antarctic fish below -2°C** is attributed to the antifreeze glycoproteins. - **streptomycin** is a glycoside employed in the treatment of tuberculosis. ## Further Readings - Jain, J.L. (2005) Fundamentals of Biochemistry, S. Chand & Company Ltd. Ram nagar, New Delhi, India - Karen C. Timberlake (2012) *Chemistry: An Introduction to General, Organic, and Biological Chemistry* -11th ed. Publishing as Prentice Hall in the United States of America. - Satyanarayana, U. (2007) *Biochemistry*, Books and Allied (P) Ltd, Kolkata, India. # Thank You!