CARBOHYDRATES The word "carbohydrate" comes from the Greek word sakharon, which means "sugar". In chemistry, carbohydrates are a common class of simple organic compounds. A carbohydrate is an aldehyde or a ketone that has additional hydroxyl groups. The simplest carbohydrates are called monosaccharides, which have the basic structure (C·H2O)n, where n is three or greater. Two monosaccharides link together to form a disaccharide. Monosaccharides and disaccharides are called sugars and typically have names ending with the suffix -ose. More than two monosaccharides link together to form oligosaccharides and polysaccharides. In everyday usage, the word "carbohydrate" refers to any food that contains a high level of sugars or starch. In this context, carbohydrates include table sugar, jelly, bread, cereal, and pasta, even though these foods may contain other organic compounds. For example, cereal and pasta also contain some level of protein. It is a group of organic compounds occurring in living tissues and foods in the form of starch, cellulose, and sugars. The ratio of oxygen and hydrogen in carbohydrates is the same as in water i.e. 2:1. It typically breaks down in the animal body to release energy. Originally the term carbohydrate was used to describe compounds that were literally “carbohydrates,” because they had the empirical formula CH2O. Carbohydrates have been classified in recent years on the basis of carbohydrate structures, not their formulae. Such aldehydes and ketones are now known as polyhydroxy. Cellulose, starch, glycogen and m are amongst the compounds that belong to this family. The general formula for carbohydrates is Cx(H2O)y. Definition of Carbohydrates in Chemistry “Optically active polyhydroxy aldehydes or polyhydroxy ketones or substances which give these on hydrolysis are termed as carbohydrates”. The Functions of Carbohydrates Carbohydrates serve several biochemical functions: • Monosaccharides serve as fuel for cellular metabolism. • Monosaccharides are used in several biosynthesis reactions. • Monosaccharides may be converted into space-saving polysaccharides, such as glycogen and starch. These molecules provide stored energy for plant and animal cells. • Carbohydrates are used to form structural elements, such as chitin in animals and cellulose in plants. • Carbohydrates and modified carbohydrates are important for an organism's fertilization, The chirality of the carbohydrate • Aldose - monosaccharide in which the carbonyl group is an aldehyde • development, blood clotting, and immune system function. Classification and Nomenclature of carbohydrates The carbohydrates are divided in to three major classes depending upon whether or not they undergo hydrolysis, and if they do,on the num Types of Carbohydrates Types of Carbohydrates – Monosaccharides, Disaccharides & polysaccharides The different types of carbohydrates can be classified on the basis of their behavior on hydrolysis. They are mainly classified into three groups: 1. Monosaccharides 2. Disaccharides 3. Polysaccharides The simple carbohydrates include single sugars (monosaccharides) and polymers, oligosaccharides, and polysaccharides..l Monosaccharides • Simplest group of carbohydrates and often called simple sugars since they cannot be further hydrolyzed. • Colorless, crystalline solid which are soluble in water and insoluble in a non-polar solvent. • These are compound which possesses a free aldehyde or ketone group. • The general formula is Cn(H2O)nor CnH2nOn. • They are classified according to the number of carbon atoms they contain and also on the basis of the functional group present. • The monosaccharides thus with 3,4,5,6,7… carbons are called trioses, tetroses, pentoses, hexoses, heptoses, etc., and also as aldoses or ketoses depending upon whether they contain aldehyde or ketone group. Examples: Glucose, Fructose, Erythrulose, Ribulose. Three characteristics are used to classify monosaccharides: • Number of carbon atoms in the molecule • Location of the carbonyl group • Ketone - monosaccharide in which the carbonyl group is a ketone • Triose - monosaccharide with 3 carbon atoms • Tetrose - monosaccharide with 4 carbon atoms • Pentose - monosaccharide with 5 carbon atoms • Hexose - monosaccharide with 6 carbon atoms • Aldohexose - 6-carbon aldehyde (e.g., glucose) • Aldopentose - 5-carbon aldehyde (e.g., ribose) • Ketohexose - 6-carbon hexose (e.g., fructose) A monosaccharide is D or L, depending on the orientation of the asymmetric carbon located furthest from the carbonyl group. In a D sugar, the hydroxyl group is on the right the molecule when written as a Fischer projection. If the hydroxyl group is on the left of the molecule, it is an L sugar. Oligosaccharides • Oligosaccharides are compound sugars that yield 2 to 10 molecules of the same or different monosaccharides on hydrolysis. • The monosaccharide units are joined by glycosidic linkage. • Based on the number of monosaccharide units, it is further classified as disaccharide, trisaccharide, tetrasaccharide etc. • Oligosaccharides yielding 2 molecules of monosaccharides on hydrolysis is known as a disaccharide, and the ones yielding 3 or 4 monosaccharides are known as trisaccharides and tetrasaccharides respectively and so on. • The general formula of disaccharides is Cn(H2O)n-1and that of trisaccharides is Cn(H2O)n-2 and so on. • Examples: Disaccharides include sucrose, lactose, maltose, etc. • Trisaccharides are Raffinose, Rabinose. Polysaccharides • They are also called as “glycans”. • Polysaccharides contain more than 10 monosaccharide units and can be hundreds of sugar units in length. • They yield more than 10 molecules of monosaccharides on hydrolysis. • Polysaccharides differ from each other in the identity of their recurring monosaccharide units, in the length of their chains, in the types of bond linking units and in the degree of branching. • They are primarily concerned with two important functions ie. Structural functions and the storage of energy. • They re further classified depending on the type of molecules produced as a result of hydrolysis. • They may be homopolysaccharidese, containing monosaccharides of the same type or heteropolysaccharides i.e., monosaccharides of different types. • Examples of Homopolysaccharides are starch, glycogen, cellulose, pectin. • Heteropolysaccharides are Hyaluronic acid, Chondroitin. Examples of Carbohydrates • Monosaccharides: glucose, fructose, galactose • Disaccharides: sucrose, lactose • Polysaccharides: chitin, cellulose Sugars and Non-sugars: Monosaccharides and oligosaccharides are crystalline solids, soluble in water and sweet to taste. They are collectively known as sugars. The polysaccharides, on the other hand, are amorphous, insoluble in water and tasteless. They are called non-sugars. Reducing and Non Reducing Sugars : The carbohydrates may also be classified as either reducing or non-reducing sugars. All those carbohydrates which have the ability to reduce Fehling solution and Tollen’s reagent are known as reducing sugars. Whereas others are non-reducing sugars as they can not reduce a Fehling solution and a Tollen’s reagent. Except sucrose all monosaccharides and disaccharides are reducing sugars. REFERENCES Organic Chemistry-III by Alok Bariyar and Sudha Goyal https://microbenotes.com/carbohydrates-structure-properties-classification-and-functions/ https://med.libretexts.org/Courses/American_Public_University/APUS%3A_An_Introduction_to_Nutriti on_(Byerley)/Text/03%3A_Carbohydrates/3.02%3A_A_Closer_Look_at_Carbohydrates .