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Carbohydrates are often known as sugars, they are the 'staff of life' for most organisms. They are the most abundant class of biomolecules in nature, based on mass.
Carbohydrates are also known as saccharides, in Greek sakcharon mean sugar or sweetness.

They are widely distributed molecules 
in plant ans animal tissues. In plants, and arthropods, carbohydrates from the skeletal structures, they also serve as food reserves in plants and animals. They are important energy source required for various metabolic activities, the energy is derived by oxidation. Plants are richer in carbohydrates than animals. 


Carbohydrates Definition

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Carbohydrate is a organic compound, it comprises of only oxygen, carbon and hydrogen. The oxygen:hydrogen ratio is usually is 2:1. The empirical formula being Cm(H2O)n (where m can be different from n). Carbohydrates are hydrates of carbon, technically they are polyhydroxy aldehydes and ketones. Carbohydrates are also known as saccharides, the word saccharide comes from Greek word sakkron which means sugar.

Carbohydrates Classification

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Carbohydrates are classified into three groups 

Carbohydrate Classification

Monosaccharides or Monosachoroses
From Greek, mono=one; sakchron=sugar.

Monosaccharides are often called simple sugars, these are compound which possess a free aldehyde or ketone group. They are the simplest sugars and cannot be hydrolyzed. The general formula is C
n(H2O)n or CnH2nOn. The monosaccharides are subdivided into tiroses, tertrose, pentoses, hexoses, heptoses etc., and also as aldoses or ketoses depending upon whether they contian aldehyde or ketone group.

Examples of monosaccharides are Fructose, Erythrulose, Ribulose.

Monosaccharide Fructose

Oligosaccharides or Oligosaccharoses 
In Greek, Oligo means few. 

Oligosaccharides are compound sugars that yield 2 to 10 molecules of the same or different monosaccharides on hydrolysis. 
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-1 and that of trisaccharides is Cn(H2O)n-2 and so on.

Example of disaccharides are sucrose, lactose, maltose etc.
Trisaccharides are Raffinose, Rabinose.

Maltose Disaccharide

Polysaccharides or Polysaccharoses
In Greek, poly means many. 

Polysaccharides are compound sugars and yield more than 10 molecules of monosaccharides on hydrolysis. Theya re further classified depending on they type of molecules produced as a resullt of hydrolysis. They may be homopolysaccharides i.e, monosaccharides of the same type or heteropolysaccharides i.e., monosaccharides of different types. The general formula is (C6H10O5)x.

Example of homopolysaccharides are starch, glycogen, cellulose, pectin.
Heteropolysaccharides are Hyaluronic acid, Chondrotin. 

Glycogen Polysccharide

Properties of Carbohydrates

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General properties of carbohydrates 
  • Carbohydrates act as energy reserves, also stores fuels, and metabolic intermediates. 
  • Ribose and deoxyribose sugars forms the structural frame of the genetic material, RNA and DNA.
  • Polysaccharides like cellulose are the structural elements in the cell walls of bacteria and plants. 
  • Carbohydrates are linked to proteins and lipids that play important roles in cell interactions.
  • Carbohydrates are organic compounds, they are aldehydes or ketones with many hydroxyl groups.
Physical Properties of Carbohydrates 
  • Steroisomerism - Compound shaving same structural formula but they differ in spatial configuration. Example: Glucose has two isomers with respect to penultimate carbon atom. They are D-glucose and L-glucose.
  • Optical Activity - It is the rotation of plane polarized light forming (+) glucose and (-) glucose. 
  • Diastereo isomeers - It the configurational changes with regard to C2, C3, or C4 in glucose. Example: Mannose, galactose.
  • Annomerism - It is the spatial configuration with respect to the first carbon atom in aldoses and second carbon atom in ketoses. 
Chemical Properties of Carbohydrates 
  • Ozazone formation with phenylhydrazine.
  • Benedicts test.
  • Oxidation 
  • Reduction to alcohols

Structure of Carbohydrates

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There are three types of structural representations of carbohydrates:
  • Open chain structure.
  • Hemi-acetal structure.
  • Haworth structure.
Open chain structure - It is the long straight chain form of carbohydrates.

Open Chain Structure

Hemi-acetal structure - Here the 1st carbon of the glucose condenses with the -OH group of the 5th carbon to form a ring structure.

Hemic Acetyl Structure 

Haworth structure - It is the presence of pyranose ring structure.

Haworth Structure

Functions of Carbohydrates

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  • Carbohydrates are chief energy source, in many animals, they are instant source of energy. Glucose is brokedown by glycolysis/ kreb's cycle to yield ATP.
  • Glucose is the source of storage of energy. It is stored as glycogen in animals and starch in plants. 
  • Stored carbohydrates acts as energy source instead of proteins. 
  • Carbohydrates are intermediates in biosynthesis of fats and proteins.  
  • Carbohydrates aid in regulation of nerve tissue and is the energy source for brain. 
  • Carbohydrates gets associated with lipids and proteins to form surface antigens, receptor molecules, vitamins and antibiotics. 
  • They form structural and protective components, like in cell wall of plants and microorganisms. 
  • In animals they are important constituent of connective tissues.
  • They participate in biological transport, cell-cell communication and activation of growth factors.
  • Carbohydrates that are rich in fibre content help to prevent constipation. 
  • Also they help in modulation of immune system.

Types of Carbohydrates

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Like all other biomolecules carbohydrates are made of micromolecules and macromolecules. Micromolecules are the monosaccharides while the macromolecules are the oligosaccharides and polysaccahrides. Actually, the micromolecules polymerize and condense to form macromolecules. 

Carbohydrates basically are of three types - Monosaccharides, Oligosaccharides and Polysaccharides. 

Monosaccharides or simple sugars consists of single polyhydroxy aldehyde or ketone. Most common classes of monosaccharides are 
Aldoses - Aldotiroses, aldotertoses, aldopentoses, aldohextoses and aldoheptoses. 
Ketoses - Ketotrioses, ketotetroses, ketopentoses, ketohextoses, and ketoheptoses. 

Oligosaccharides are polymerized monosaccharides, which contain more two to ten residues on hydrolysis. 
Oligosacharides are classified as disaccharides, trisaccharides and tetrasaccharides. 

Disaccahrides yield two monosaccharides on hydrolysis.
Disaccharides are of two types - Reducing disaccharides (example maltose) and non-reducing disaccharides(example sucrose). 
Trisaccahrides are found in sugar beet and and cotton seed. Example raffinose. 
Tetrasaccharide yield four monosaccharides on hydrolysis. Example: stachyose.

Polysaccharides are polymeric anhyderides of monosaccharides. Polysaccharides are of two types based on their function and composition. 
Based on function, polysaccharides of two types storage and structural.
Storage polysaccharides - starch.
Structural polysaccharide - cellulose.

Different Polysaccharides Structures

Based on composition polysaccharides are of two types homoplysaccharides and heteropolysaccharides.
Homopolysaccharides give single type of monosaccharide on hydrolysis. Example Inulin polymer of fructose.
Heteropolysaccharides example hyaluronic acid, chondroitin sulphate.

Example of Carbohydrates

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Following are common examples of carbohydrates:

Monosaccharides - Glucose, galactose, glycerose, erythrose, ribose, ribulose, fructose.
Oligosaccharides - Maltose, lactose, sucrose, raffinose, stachyose.
Polysaccharides - Starch, glycogen, cellulose, pectin, inulin, hyaluonic acid. 

Foods rich in carbohydrates are referred to as strachy foods. They are found in legumes, starchy vegetables, whole-grain breads and cereals. They also occur naturally with vitamins and minerals in foods like milk, fruits, milk products. They are alsdo found in refined and processed products like candy, carbonated beverages, and table sugar. 

Examples of Polysaccharides

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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
 Callose  Polymer of glucose  Different regions of plant, in sieve tubes of phloem  Formed often as a response to wounds
 Insulin  Polymer of fructose  Iroots and tubers (like Dahlia)   Storage of reserve food
 Cellulose   Polymer of glucose  Plant cell wall   Cell wall matrix
 Pectin  Polymer of galactose and its derivatives   Plant cell wall  Cell wall matrix
 Hemi cellulose   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 cells  Anticoagulant
 Gums and mucilages Polymers of sugars and sugar acids Gums - bark or trees. Mucilages - flower Retain water in dry seasons
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