Carbohydrates Topic 3 PDF

**CARBOHYDRATES** Carbohydrates are the most abundant biomolecules on earth. Oxidation of carbohydrates is the central energy-yielding pathway in most non-photosynthetic cells. **Definition:** Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis...

**CARBOHYDRATES** Carbohydrates are the most abundant biomolecules on earth. Oxidation of carbohydrates is the central energy-yielding pathway in most non-photosynthetic cells. **Definition:** Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. carbohydrates have the empirical formula (CH~2~O)n. There are three major classes of carbohydrates: 1. Monosaccharides ================== Monosaccharides, or simple sugars, consist of a single polyhydroxy aldehyde or ketone unit. The most abundant monosaccharide in nature is the six-carbon sugar D-glucose, sometimes referred to as dextrose. 2. Oligosaccharides =================== Oligosaccharides consist of short chains of monosaccharide units, or residues, joined by characteristic linkages called glyosidic bonds. The most abundant are the disaccharides, with two monosaccharide units. Example: sucrose (cane sugar). 3. Polysaccharides ================== The polysaccharides are sugar polymers containing more than 20 or so monosaccharide units, and some have hundreds or thousands of units. Example: starch. Polysaccharides are of two types based on their function and composition. Based on function, polysaccharides of two types storage and structural. A. Storage polysaccharide - starch. B. Structural polysaccharide - cellulose. General properties of carbohydrates =================================== - - - - - Physical Properties of Carbohydrates ==================================== - - - - Biological Importance ===================== - - - - - - - - - - - **Monosaccharides** - - - - Classification of Monosaccharides ================================= Monosaccharides are classified in two ways. (a) First of all, based on the number of carbon atoms present in them and (b) secondly based on the presence of carbonyl group. The naturally occurring monosaccharides contain three to seven carbon atoms per molecule. Monosaccharides of specific sizes may be indicated by names composed of a stem denoting the number of carbon atoms and the suffix -*ose*. For example, the terms *triose*, *tetrose*, *pentose*, and *hexose* signify monosaccharides with, respectively, three, four, five, and six carbon atoms. Monosaccharides are also classified as aldoses or ketoses. Those monosaccharides that contain an aldehyde functional group are called aldoses; those containing a ketone functional group on the second carbon atom are ketoses. Combining these classification systems gives general names that indicate both the type of carbonyl group *and* the number of carbon atoms in a molecule. Thus, monosaccharides are described as aldotetroses, aldopentoses, ketopentoses, ketoheptoses, and so forth. Glucose and fructose are specific examples of an aldohexose and a ketohexose, respectively.  **Trioses** Trioses are "Monosaccharides" containing 3 carbon atoms. The molecular formula of triose is C~3~H~6~O~3~ Characteristics: - - - -  Tetroses ======== Tetroses are "Monosaccharides" containing 4 carbon atoms. The molecular formula of tetrose is C~4~H~8~O~4~ Characteristics: - - - - - **Pentoses** Pentoses are "Monosaccharides" containing 5 carbon atoms. It is an important component of "nucleic acid". The molecular formula of Pentose is C~5~H~10~O~5~ Characteristics: - - - - - **Hexoses** Hexoses are "Monosaccharides" containing 6 carbon atoms. The molecular formula of Hexose is C~6~H~12~O~6~ Characteristics: - - - - - Structure of Monosaccharides ============================ - - **2. Cyclic or Ring Structure:** Here the atoms are arranged in the form of a ring. Haworth (1929) proposed this formula and hence the name Haworth's Projection Formula. The sugar molecules exist in two type of rings which are as follows -- (a)Furanose Ring -- 5 membered ring (b)Pyranose Ring- 6 membered ring  **Properties of Monosaccharides** 1.Colour - colourless 2.Shape - crystalline 3.Solubility -- water soluble 4.Taste - sweet 5.Optical activity -- Optically active. (a) Dextrorotatory ('d' form) and (b) Levorotatory ('l' form) 6.Mutarotation -- The change in specific rotation of an optically active compound is called mutarotation. +1120 +52.50 +190 α-D-glucose β -D-glucose 7\. Glucoside formation - Glucose + Methyl alcohol = Methyl glucoside 8. Esterification -- ====================  9\. Reducing agents -- Monosaccharides reduce oxidizing agent such as hydrogen peroxide. In such reaction, sugar is oxidized at the carbonyl group and oxidizing agent becomes reduced. C~6~H~12~O~6~ + 2 Cu(OH)~2~→C~6~H~12~O~7~ + Cu~2~O + 2H~2~O **Disaccharides** Disaccharides consist of two sugars joined by an **O**-glycosidic bond. The most abundant disaccharides are sucrose, lactose and maltose. Other disaccharides include isomaltose, cellobiose and trehalose. The disaccharides can be classified into: 1. Homodisaccharides ==================== **2. Heterodisaccharides**. +-----------------+-----------------+-----------------+-----------------+ | **Hommodisaccha | **Maltose** | **Isomaltose** | | | rides** | | | | | | **(malt sugar | | | | | )** | | | +=================+=================+=================+=================+ | **structure** | 2α-glucose | 2 α-glucose | | +-----------------+-----------------+-----------------+-----------------+ | **Type of | α-1-4 | α1-6 glucosidic | | | bond** | glucosidic bond | bond | | +-----------------+-----------------+-----------------+-----------------+ | **Anomeric | Free | Free | | | Carbon** | | | | +-----------------+-----------------+-----------------+-----------------+ | **Reducing | Reducing | Reducing | | | Property** | | | | +-----------------+-----------------+-----------------+-----------------+ | **Produced by** | It is produced | by the | | | | from starch by | hydrolysis of | | | | the action of | some | | | | amylase | | | | | | polysaccharides | | | | | such as dextran | | +-----------------+-----------------+-----------------+-----------------+ **Heterodisaccharides**: are formed of 2 different monosaccharide units **Heterodisaccharides** **Sucrose** -------------------------- ---------------------------------------------------------------------------------------------------- -- **Composition** α-D-glucose+ β--D-fructose **Type of bond** α-1-β-2 glucosidic bond OR β 2-α-1 fructosidic bond **Anomeric C** no free aldehydeor ketonegroup **Reducing property** is not a reducing sugar **Composition** α-D-glucose**+**β--D-fructose **Anomeric C** nofreealdehydeorketonegroup **Effect of hydrolysis** The hydrolysis of sucrose to glucose and fructose is catalysed by sucrose (also called invertase), **Present in** Table sugar Cane sugar, beet sugar   Polysaccharides =============== Polysaccharides contain hundreds or thousands of carbohydrate units. - Polysaccharides are *not* reducing sugars, since the anomeric carbons are connected through glycosidic linkages. - Nomenclature: **Homopolysaccharide-** a polysaccharide is made up of **one type** of monosaccharide unit **Heteropolysaccharide-** a polysaccharide is made up of more than **one type** of monosaccharide unit Starch ====== - - - - - - Structure of Amylose Fraction of Starch - - - - **Structure of Amylopectin Fraction of Starch** - - Glycogen ======== - - - - - - Cellulose ========= - - - - - - - - - -  CHITIN ====== - - - - - - The following table is the **list of biologically important polysaccharides** and their functions. Polysaccharides are complex carbohydrates. Name of the Polysaccharide Composition Occurrence Functions ---------------------------- --------------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------- ---------------------------------- Starch Polymer of glucose containing a straight chain of glucose molecules (amylose) and a branched chain of glucose molecules (amylopectin) In several plant species as main storage carbohydrate storage of reserve food Glycogen Polymer of glucose Animals (equivalent of starch) Storage of reserve food Cellulose Polymer of glucose Different regions of plant, in sieve tubes of phloem Cell wall matrix Inulin Polymer of fructose In roots and tubers (like Dahlia) Storage of reserve food Pectin Polymer of galactose and its derivatives Plant cell wall Cell wall matrix Hemicellulose Polymer of pentoses and sugar acids Plant cell wall Cell wall matrix Lignin Polymer of glucose Plant cell wall (dead cells like sclerenchyma) Cell wall matrix Chitin Polymer of glucose Bodywall of arthropods. In some fungi also Exoskeleton Impermeable to water Murein Polysaccharide cross linked with amino acids Cell wall of prokaryotic cells Structural protection Hyaluronic acid Polymer of sugar acids Connective tissue matrix, Outer coat of mammalian eggs Ground substance, protection Chrondroitin sulphate Polymer of sugar acids Connective tissue matrix Ground substance Heparin Closely related to chrondroitin Connective tissue cell Anticoagulant Gums and Mucilages Polymers of sugars and sugar acids Gums - bark or trees. Mucilages - flower Retain water in dry seasons **TESTS FOR CARBOHYDRATE** The following are the tests to identify the presence of carbohydrates. 1. Molisch's test 2. Fehling's test 3. [[Benedict's test]](https://byjus.com/chemistry/benedicts-test/) 4. Tollen's test 5. Iodine test ### **(a) Molisch's Test:** [Molisch's test](https://byjus.com/chemistry/molischs-test/) is a general test for carbohydrates. This test is given by almost all of the carbohydrates. In this test, concentrated sulfuric acid converts the given carbohydrate into furfural or its derivatives, which react with α-naphthol to form a purple coloured product. The chemical reaction is given below.  ***Note:** The appearance of purple or violet ring confirms the presence of carbohydrate.* ### **(b) Fehling's Test:** This test is given by reducing sugars. To the aqueous solution of carbohydrate fehling's solution is added and heated in water bath. The formation of red precipitate confirms the presence of reducing sugars. The copper ions present in fehling's solution in +3 state is reduced to +2 oxidation state and in alkaline medium it is precipitated as red [cuprous oxide](https://byjus.com/chemistry/copper-oxide/). The chemical reaction is given below. Fehling\'s Test ***Note:** The appearance of red precipitate confirms the presence of carbohydrates.* ### **(c) Benedict's Test:** This test is given by reducing sugars. in an alkaline medium, sodium carbonate converts glucose to enediol and this enediol reduces cupric to cuprous forming cuprous hydroxide. This solution is kept in sodium citrate and on boiling, red precipitate of cuprous oxide is formed. The chemical reaction is given below. ***Note:** The appearance of red precipitate confirms the presence of carbohydrates.* ### **(d) Tollen's Test:** This test is given by reducing sugars. Carbohydrates react with Tollens reagent and forms a silver mirror on the inner walls of the test tube. This confirms the presence of reducing sugars. Silver ions are reduced to metallic silver. The chemical reaction is given below. Tollen's Test***Note: **The appearance of silver mirror confirms the presence of reducing sugars.* ### **(e) Iodine Test:** This test is only given by starch. Starch reacts with [iodine solution](https://byjus.com/chemistry/iodide/) forms complex blue colour solution. On heating the blue colour disappears and on cooling the blue colour reappears. The chemical reaction is given below.\ ***Note: **The appearance of blue colour solution confirms the presence of starch.* ### **STEREOISOMERS** Stereoisomers have the same molecular formula and chemical bonds but they have different spatial arrangements. That is, they have the same connectivity but differ in the way in which the constituent atoms are oriented in space. They can be divided into configurational stereoisomers and conformational stereoisomers. The precise specification of the spatial arrangement of the groups in a configurational isomer is called its configuration, and in a [conformational isomer](https://www.sciencedirect.com/topics/chemistry/conformational-isomer), its conformation. **Configurational isomerism** - This type of isomerism is non-superimposable and non-interconvertible by rotation around single bonds. - They can be interconverted by breaking and making bonds. - These are of two types that are Enantiomers(optical isomers), and Diastereomers. **1. Enantiomers** - In this, two isomers are mirror images of each other. - It is also known as inversional isomerism. - For example, the enantiomers of lactic acid is given below: https://byjus-answer-creation.s3.amazonaws.com/uploads/10443\_Chemistry\_628393717d19069219f3de09\_EL\_Stereoisomerism-22\_031961\_a.jpg\_img\_upload\_solution\_2022-08-10%2012:35:59.923568.png **2. Diastereomers** - These do not mirror images of each other. - These are also known as geometrical isomers. - For example, diastereomers of But-2-ene is shown below:  **Conformational isomers** - These are non-superimposable but easily interconvertible by rotation about single bonds. - Cycloalkane and alkanes show this type of isomerism. - For example, the conformational isomers of ethane is shown below: https://byjus-answer-creation.s3.amazonaws.com/uploads/10443\_Chemistry\_628393717d19069219f3de09\_EL\_Stereoisomerism-22\_031961\_c.jpg\_img\_upload\_solution\_2022-08-10%2012:36:46.377048.png ### **OPTICAL ISOMERISM** To define optical isomerism, it is a case where the isomers exhibit identical characteristics in terms of molecular weight and chemical and physical properties as well. However, they differ in their rotation effect of polarized light. Optical isomerism mainly occurs in substances that have similar molecular and structural formulas, but they can't be superimposed on each other. To keep it simple, we can say that they are mirror images of each other. Alternately, it can also be found in substances with an asymmetric carbon atom. Optical isomers are molecules that are **non-superimposable mirror images** of each other and they are not identical. Molecules with a **chiral carbon** show this kind of isomerism. Chiral carbons, also known as **asymmetric** carbons, refer to a carbon atom that is bonded to **four different** functional groups.  Each carbon in this diagram has four different groups bonded to it. They have the same **structural formula** and are arranged so that they are **non-superimposable mirror images** of each other. This makes them optical isomers. Typically, optical isomerism is exhibited by the stereoisomers that rotate the plane of polarized light. If the same plane of polarized light traveling through an enantiomer solution rotates in the clockwise direction, the enantiomer is then said to exist as (+) form, and if the plane of polarized light rotates in the anti-clockwise direction, the enantiomer is known to exist in (-). For example, an enantiomer of alanine (otherwise called amino acid), which rotates the plane of polarized light in a clockwise and anti-clockwise direction, can be represented as (+) alanine and. (-) alanine respectively. The rotation extent of plane-polarized light by the two enantiomeric forms is precisely the same whereas, the direction of rotation is opposite. Furthermore, if the two enantiomer pairs are present in an equal amount, then the resultant mixture is known as a racemic mixture. It means 50% of the mixture exists in (+) form, and the remaining 50% exist in (-) form. Since the racemic mixture rotates the plane of polarized light equally towards the opposite direction, the net rotation remains zero. Therefore, the racemic mixture is optically inactive. **EPIMERS** Epimer in stereochemistry specifies one of a pair of **stereoisomers**.  At the stereogenic centre, two isomers present in the molecule differ, while the rest remains identical. A molecule may contain numerous stereocenters leading to several stereoisomers. Epimers Epimers are carbohydrates that differ in the location of the -OH group in one location. Both D-glucose and D-galactose are the best examples. D-glucose and D-galactose epimers create a single difference at C-4 carbon. They are not enantiomers, they are just epimers, or diastereomers, or isomers. Epimers -- Example ------------------ Below, stereoisomers illustrate the D and L configurations of glucose. Here, glucose is referred the D and L on the basis of last chiral carbon atom.  **DEXTROROTATORY AND LAEVOROTATORY** Dextrorotatory and laevorotatory are terms used to describe the optical rotation of a substance. Optical rotation refers to the ability of a compound to rotate the plane of polarized light. The direction of rotation can be either clockwise (to the right) or counterclockwise (to the left). Dextrorotatory and laevorotatory compounds have different characteristics, which are explained below: \- Dextrorotatory: A compound is dextrorotatory if it rotates the plane of polarized light in a clockwise direction. \- Laevorotatory: A compound is laevorotatory if it rotates the plane of polarized light in a counterclockwise direction.\ \ https://lh4.googleusercontent.com/gP9bQe1TIrnUXZf6Tv5GmuMEFjMX1wm81RRF9QQhjb5APvCRazRGeU2JL0J8mrs8dbXGRCEnb\_s0LG-hAeV\_UwZJtZxld-MTndgDY7Ybvyp\_lonsn2fpGHcQV\_TDCBSy1NfrUKlg - The above structures show two different forms of glucose structure.  - The D-glucose is dextro glucose, which has an optical rotation towards the right side, meaning it is dextrorotatory. - The dextro glucose structure turns plane-polarised light in a clockwise direction. - The L-glucose is levo glucose, which has an optical rotation towards the left side, meaning it is levorotatory. - The levo glucose twists plane-polarised light in an anti-clockwise direction. - L and D glucose are mirror images of each other. - Since they are mirror images of each other, they are called enantiomers. The levo form is the enantiomer of the dextro form of glucose.