Download Carbohydrates: Classification, Functions, and Important Types and more Exams Nutrition in PDF only on Docsity! CARBOHYDRATES Carbohydrates are the most abundant organic molecules in nature. They are primarily composed of the elements carbon, hydrogen and oxygen. The name carbohydrate literally means ‘hydrates of carbon.’ Carbohydrates may be defined as polyhydroxy- aldehydes or ketones or compounds which produce them on hydrolysis. The term ‘sugar’ is applied to carbohydrates soluble in water and sweet to taste. Functions of carbohydrates: Carbohydrates participate in a wide range of functions: 1. They are the most abundant dietary source of energy (4 Cal/g) for all organisms. 2. Carbohydrates are precursors for many organic compounds (fats, amino acids). 3. Carbohydrates (as glycoproteins and glycolipids) participate in the structure of cell membrane and cellular functions such as cell growth, adhesion and fertilization. 4. Carbohydrates also serve as the storage form of energy (glycogen) to meet the immediate energy demands of the body. Classification of Carbohydrates: Carbohydrates are often referred to as saccharides (Greek: sakcharon-sugar). They are broadly classified into 3 groups—monosaccharide’s, oligosaccharides and polysaccharides. This categorization is based on the number of sugar units. Mono- and oligosaccharides are sweet to taste, crystalline in character and soluble in water, hence they are commonly known as sugars. Monosaccharide’s: Monosaccharide’s (Greek: mono-one) are the simplest group of carbohydrates and are often referred to as simple sugars. They have the general formula Cn(H2O)n, and they cannot be further hydrolysed. Based on the number of carbon atoms, the monosaccharide’s are regarded as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C). These terms along with functional groups are used while naming monosaccharide’s. For instance, glucose is a aldohexose while fructose is a ketohexose. Oligosaccharides: Oligosaccharides (Greek: oligo-few) contain 2-10 monosaccharide molecules which are liberated on hydrolysis. Based on the number of monosaccharide units present, the oligosaccharides are further subdivided to disaccharides, tri- saccharides etc. Polysaccharides: Polysaccharides (Greek: poly-many) are polymers of monosaccharide units with high molecular weight (up to a million). They are usually tasteless (non-sugars) and form colloids with water. Polysaccharides are of two types—homopoly- saccharides and heteropolysaccharides. Monosaccharide’s: Stereoisomerism is an important character of monosaccharide’s. Stereoisomers are the compounds that have the same structural formulae but differ in their spatial configuration. A carbon is said to be asymmetric when it is attached to four different atoms or groups. The number of asymmetric carbon atoms (n) determines the possible isomers of a given compound which is equal to 2n. Glucose contains 4 asymmetric carbons and thus has 16 isomers. Glyceraldehyde— the reference carbohydrate: Glyceraldehyde (triose) is the simplest monosaccharide with one asymmetric carbon atom. It exists as two stereoisomers, and has been chosen as the reference carbohydrate to represent the structure of all other carbohydrates. D- and L-isomers: The D- and L-isomers are mirror images of each other. The special orientation of —H and —OH groups on the carbon atom (C5 for glucose) that is adjacent to the terminal primary alcohol carbon determines whether the sugar is D- or L-isomer. If the —OH group is on the right side, the sugar is of D-series, and if on the left side, it belongs to L-series. The structures of D- and L-glucose based on the reference monosaccharide, D- and L-glyceraldehyde (glycerose) are depicted in Fig. 65.1
