Carbohydrates and Sugars

• The unit focuses on the characteristics, classification, and functions of biomolecules, including carbohydrates, proteins, nucleic acids, and hormones
• It explores the structural differences between DNA and RNA
• It also describes the role of biomolecules within a biosystem

Biomolecules

• Biomolecules interact to constitute the molecular logic of life processes
• Vitamins and mineral salts assist in the function of organisms
• Living systems are composed of non-living atoms and molecules
• Biochemistry studies what goes on chemically within a living system

Carbohydrates

• Carbohydrates are primarily produced by plants, for a large group of naturally occurring organic compounds
• These were considered hydrates of carbon
• However, not all compounds fitting the formula are carbohydrates, and some carbohydrates don't fit the formula
• Carbohydrates contain specific functional groups
• Carbohydrates are defined as optically active polyhydroxy aldehydes or ketones or compounds producing such units on hydrolysis
• Sugars are sweet-tasting carbohydrates
• Sucrose is common in homes, lactose in milk, and saccharides are simply sugars

Classification of Carbohydrates

• Carbohydrates are classified based on hydrolysis behavior:

Monosaccharides

• This is a carbohydrate that cannot be further hydrolyzed to a simpler unit of polyhydroxy aldehyde or ketone
• About 20 monosaccharides are known

Oligosaccharides

• These yield two to ten monosaccharide units upon hydrolysis
• Further classified (disaccharides, trisaccharides, etc.) based on the number of monosaccharides produced
• Disaccharides are most common, yielding same or different monosaccharide units
• Sucrose yields one glucose and one fructose, while maltose yields two glucose molecules

Polysaccharides

• These yield large numbers of monosaccharide units upon hydrolysis
• Common examples include starch, cellulose, glycogen, and gums
• Polysaccharides aren't sweet, and are also called non-sugars
• Carbohydrates are classified as reducing or non-reducing sugars
• Reducing sugars reduce Fehling's solution and Tollens' reagent
• All monosaccharides are reducing sugars

Monosaccharides: Further Classifications

• They are also classified by number of carbon atoms and functional groups present.
• Aldoses contain an aldehyde group
• Ketoses contain a keto group
• Number of carbon atoms in the monosaccharide is indicated in the name

Glucose Preparation

• Glucose occurs freely and in combined forms in nature
• It's found in sweet fruits and honey
• Ripe grapes contain substantial amounts
• It is prepared as follows:
• From sucrose (cane sugar): Boiling sucrose with dilute HCl or H2SO4 in alcoholic solution yields equal amounts of glucose and fructose
• From starch: Commercial glucose is obtained by hydrolyzing starch with dilute H2SO4 at 393 K under pressure

Structure of Glucose

• Glucose is an aldohexose, also known as dextrose
• It serves as a monomer, the constituent of larger carbohydrates (starch, cellulose)
• Glucose is the most abundant organic compound on Earth
• Assignment of Glucose Structure:
• Molecular formula = C6H12O6
• Prolonged heating with HI forms n-hexane, with all six carbons linked in a straight chain
• Reaction with hydroxylamine forms an oxime, and reaction with hydrogen cyanide gives cyanohydrin
• Glucose reacts with hydroxylamine to form oxime
• These reactions confirm presence of a carbonyl group
• Oxidation by a mild oxidizing agent like bromine water yields a six-carbon carboxylic acid (gluconic acid)
• Indicates that a carbonyl group is present as an aldehyde group
• Acetylation of glucose with acetic anhydride gives glucose pentaacetate
• This confirms the presence of five -OH groups
• Because it exists as a stable compound, five -OH groups are attached to different carbon atoms
• Oxidation with nitric acid yields dicarboxylic acid, saccharic acid
• This indicates the presence of a primary alcoholic (-OH) group
• Fischer determined the exact spatial arrangement of -OH groups, correctly represented as I (configuration)
• Gluconic acid represented as II and saccharic acid as III

Glucose and Stereoisomers

• Correctly referred to as D(+)-glucose
• D refers to the configuration whereas (+) represents dextrorotatory nature of the molecule
• D and L have no relation with optical activity of the compound, and are not related to the letters 'd' and 'l'
• The letters D or L placed before a compound's name indicates the compound's relative configuration of a particular stereoisomer with respect to the configuration of another compound that is known
• In carbohydrates, this refers to relation with a particular isomer of glyceraldehyde
• Glyceraldehyde contains one asymmetric carbon atom, existing in two enantiomeric forms:

Glyceraldehyde Isomers

• Isomers include (+)-Glyceraldehyde and (-)-Glyceraldehyde (+) Isomer of glyceraldehyde has ‘D’ configuration (+) Isomer has -OH group on the right hand side Those chemically correlated to D(+) isomer of glyceraldehyde are said to have D-configuration Those correlated to L(-)-isomer of glyceraldehyde are said to have L-configuration L(-) isomer has -OH group on the left hand side To assign configuration of monosaccharides, the lowest asymmetric carbon atom (as shown below) which is compared As in (+) glucose, -OH on the lowest asymmetric carbon is on the right side which is comparable to (+) glyceraldehyde, so (+) glucose is assigned D-configuration Other asymmetric carbon atoms aren't considered for this comparison Glucose and glyceraldehyde structures written with the most oxidised carbon at the top

Cyclic Structure of Glucose

• One of the —OH groups may add to the —CHO group and form cyclic hemiacetal structure.
• The structure of glucose can be explained by the following
• Despite having the aldehyde group, glucose does not give Schiff's test and it does not form the hydrogensulphite addition product with NaHSO3.
• The pentaacetate of glucose does not react with hydroxylamine indicating the absence of free CHO
• It was found that glucose forms a six-membered ring in which —OH at C-5 is involved in ring formation
• Glucose exists in two crystalline variations

Isomers

• Isomers consist of a and B. a-form of glucose is obtained by crystallization from concentrated solution of glucose at 303 K while the B-form is obtained by crystallization from hot and saturated aqueous solution at 371 K. The two cyclic hemiacetal forms of glucose differ in the configuration of the hydroxyl group at C1, called anomeric carbon

Fructose

• It's an important ketohexose, obtained with glucose from sucrose hydrolysis
• A natural monosaccharide found in fruits, honey, and vegetables
• In pure form it is a sweetener
• Molecular formula C6H12O6
• On the basis of its reactions, it was found to contain a ketonic functional group at carbon number 2 and six carbons in straight chain as in the case of glucose
• Also exists in two cyclic forms (D – (–) – Fructose), obtained by addition of —OH at C5 to the (C=O) group
• The cyclic structures of two anomers of fructose are represented by Haworth structures

Disaccharides

• Hydrolysis with dilute acids or enzymes yields two molecules of same or different monosaccharides
• • Two monosaccharides joined by an oxide linkage, losing a water molecule
• That linkage between two monosaccharide units through oxygen atom is called glycosidic linkage
• Sucrose: Hydrolysis yields equimolar mixture of D-(+)-glucose and D-(-) fructose
• These two monosaccharides are held together by a glycosidic linkage between C1 of a-D-glucose and C2 of B-D-fructose
• the reducing groups of glucose and fructose are involved in glycosidic bond formation, sucrose is a non reducing sugar.
• Sucrose is dextrorotatory but it undergoes a change in the sign of rotation, from dextro (+) to laevo (−) and the product is named as invert sugar.

Maltose

• Composed of two a-D-glucose units, with C1 of one glucose (I) linked to C4 of another glucose unit (II)
• Free aldehyde group at C1 can be produced, showing reducing properties
• Lactose is commonly known as milk sugar
• It is composed of B-D-galactose and B-Dglucose

Polysaccharides

• Contain numerous monosaccharide units joined by glycosidic linkages
• They store food and provide structure in nature
• These act as the food storage or structural materials in nature.

Starch

• A main storage polysaccharide of plants
• Important dietary source for humans
• High starch content in cereals, roots, tubers, and some vegetables
• Polymer of a-glucose, consisting of amylose (water soluble, 15-20% of starch) and amylopectin
• Chemically, amylose is a long unbranched chain with 200–1000 a-D-(+)-glucose units held together by C1–C4 glycosidic linkage
• Amylopectin is insoluble in water, comprising 80-85% of starch
• It's a branched chain polymer of a-D-glucose units, with chain formation by C1-C4 glycosidic linkage and branching by C1-C6 glycosidic linkage

Cellulose

• Occurs exclusively in plants, the most abundant organic substance in plant kingdom
• A straight chain polysaccharide composed only of B-D-glucose units joined by glycosidic linkage between C1 of one glucose unit and C4 of the next glucose unit.

Glycogen

• Carbohydrates that are stored in animal body
• It's similar to amylopectin, more highly branched
• Found in liver, muscles, and brain
• Enzymes break glycogen down to glucose if the body needs glucose

Importance of Carbohydrates

• Essential for life in both plants and animals, forming the major portion of food
• Honey used as a source of energy
• Carbohydrates store molecules - starch in plants and glycogen in animals.

Nucleic Acids

• D-ribose and 2-deoxy-D-ribose are present in nucleic acids
• They are present in biosystem in combination with proteins and lipids
• Nucleic acids are responsible for the transfer of characters from parents to offsprings
• Nucleic acids are polymers of nucleotides
• Nucleotides are composed of a base, pentose sugar and phosphate moiety
• The two types of nucleic acids are DNA and RNA
• DNA consists of 2-deoxyribose sugar as compared to RNA which has ribose
• DNA contains thymine with uracil in RNA being the alternative fourth base
• Three types of ribonucleic acids are noted, mRNA, rRNA and tRNA

Proteins

• Proteins are the most abundant biomolecules with the function of protecting the body from infection
• They are Polymers of α-amino acids
• They are connected to each other by peptide bond (or peptide linkage)
• Peptide linkage is an amide formed between -COOH groups and –NH2 group. The reaction proceeds through the combination of the amino group of one molecule with the carboxyl group of the other
• Proteins are polymers of amino acids

Amino Acids

• Amino acids can be classified as acidic, basic or neutral
• Equal number of amino and carboxyl makes it neutral
• Number of amino more that carboxyl makes it basic
• Number of carboxyl are more compared to amino that makes it acidic
• Amino acids which can be synthesized in the body are known as non-essential acids
• Those that cant must be obtained through diet and known as essential acids

Structure of Proteins

• Amino acids are joined together by a chemical link called a peptide linkage
• A peptide linkage results from the reaction of the amino group of one amino acid with the carboxyl group of another
• The product is known as a peptide
• If a third amino acid combines to a dipeptide, the product is called a tripeptide. A tripeptide contains three amino acids linked by two peptide linkages
• They can be classified into fibrous proteins and globular proteins

Fibrous and Globular Proteins

• Fibrous proteins polypeptide chains run parallel and are held together by hydrogen and disulphide bonds, forming fiber-like structure (ie. keratin)
• Globular protein polypeptide chains coil around to give spherical shape, that are usually soluble in water (ie. insulin)

Primary Structure of Proteins

• Proteins may have one or more polypeptide chains
• Each polypeptide has amino acids linked in a specific order
• Any change creates a different protein.

Secondary Structure of Proteins

• Proteins can exist in two different structures viz. a-helix and B-pleated sheet structure
• These structures arise due to the regular folding of the polypeptide chain do to hydrogen bonding

Tertiary Structure of Proteins

• Represents overall folding of the polypeptide chains
• Further folding of the secondary structure
• Gives rise to two major molecular shapes
• Stabilized by hydrogen bonds, disulphide linkages, van der Waals and electrostatic force attraction

Quaternary structure of Proteins

• Some of the proteins have two or more polypeptide chains and referred them as sub-units
• Arrangement of these sub-units with respect to each other known as quanternary structure

Denaturation of Proteins

• Found in a biological system with a unique three-dimensional structure and biological activity is called a native protein
• Physical change in temperature or chemical change in PH is called denaturation

Enzymes

• Biocatalysts that speed up chemical reactions
• Enzymes are needed in only small quantities for the progress of a reaction

Vitamins

• Organic compounds required in small amounts in the diet
• Can be synthesized by plants (sometimes by gut bacteria), but not by the body
• Classified as fat-soluble (A, D, E, K) or water-soluble (B group, C)

Hormones

• Act as intercellular messengers, transported via bloodstream from endocrine glands to target sites
• Chemically, can be steroids, polypeptides, or amino acid derivatives
• Act as intercellular messengers

Explore reducing sugars, aldoses vs. ketoses, and monosaccharide structures. Understand glucose's properties and sucrose hydrolysis. Learn about enzymes and glucose production from starch.