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Biomolecules - Carbohydrates

Updated on December 26, 2014

Introduction

The following chapter, as in the rest of this hub series, is formatted similar to the notes one may take from their reference books. The content, for ease in terms of reading and retention is provided less in paragraph format, and more in the form of singular points - specifically designed for help for those students preparing for the All India Pre-Medical Test (AIPMT)

Important Terms

  1. Chiral Carbon Atoms - A carbon atom bonded to four different atoms or groups of atoms is known as a chiral carbon atom.
  2. Optical Isomerism - Isomeric molecules differing in three-dimensional arrangement, such that they deflect plane polarised light, are known as optical isomers. They are further explained in the video embedded below.
  3. Dextrorotation - The property of optical isomers by which they deflect plane polarised light in the clockwise direction is known as dextrorotation.
  4. Laevorotation - The property of optical isomers by which they deflect plane polarised light in the anti-clockwise direction is known as laevorotation.
  5. Mutarotation - The property of certain optical isomers due to which optical deflection rotates from clockwise to anti-clockwise or vice versa is known as mutarotation.

A molecule that is involved in the maintenance and metabolic processes of living organisms is known as a biomolecule. Biomolecules in the human body form the organic portion of the cellular pool. Biomolecules can occur in semi-colloidal form, or in the form of lipid membranes and cell walls. The various types of biomolecules are as follows -

  1. Carbohydrates
  2. Amino Acids
  3. Proteins
  4. Lipids
  5. Nucleic Acids


Carbohydrates

AKA Sugars

  • Carbohydrates can be defined as polyhydroxy Aldehydes of Ketones and their derivatives, or substances that yield these upon hydrolysis.
  • Compounds of carbon and hydrogen where hydrogen and oxygen are in the ratio 2:1 and normally carbohydrates. Exceptions to this are - Deoxyribose and Rhamnose, which are carbohydrates that to do not display this ratio, and Acetic Acid and Lactic Acid, which are non-carbohydrates displaying this ratio.
  • Carbohydrates, on the basis of chain length (i.e. compound size) and complexity, can be of three kinds -

1. Monosaccharides - These cannot be hydrolysed further.

2. Oligosaccharides (2-9 Monosaccharide units) - Disaccharides are the most biologically important form of oligosaccharides. Upon hydrolysis, they yield two monosaccharide units.

3. Polysaccharides (Approximately 300 Monosaccharide units) - Upon hydrolysis, they yield multiple monosaccharide units and in the case of mucopolysaccharides (mentioned ahead) some non-sugar units.

Glucose Structure (Haworth Projection)

Molecular structure of glucose in ring form due to glycosidic linkages.
Molecular structure of glucose in ring form due to glycosidic linkages.

Monosaccharides

AKA Simple Sugars

  • These compounds possess a free aldehyde or ketone group and two or more hydroxyl groups.
  • Monosaccharides cannot be hydrated into smaller units.
  • Monosaccharides can be further classified on the basis on the number of carbon atoms present, into trioses (three carbons), tetroses (four carbons), pentoses (five carbons), etc.
  • The can be classified on the basis of the functional group present - aldehyde or ketone - into Aldoses or Ketoses.

Physical Properties

  • Sweet tasting, colourless solids
  • High Melting Point
  • Highly soluble in water; sparingly soluble in alcohol; insoluble in ethers.
  • They char on heating
  • They possess chiral carbon atoms and display optical isomerism. (Chirality and Optical Isomerism are explained in the following video)
  • They may be dextrorotatory or laevorotatory. Optical Rotation may change on standing in water. This is known as Muta-rotation.
  • Largely found in cyclic and ring forms. Straight chains are converted to ring forms known as Haworth Projections by Hemiacetal bonds.

Chemical Properties

  • They are easily oxidised by oxidising agents, and are thus known as reducing sugars.
  • This property is utilised in Benedict's Test and Fehling's Test to detect glucose in urine.

Additional Information - Hexoses

  • They are six carbon sugars
  • Four hexoses are commonly found in biological systems - D-Glucose, D-Fructose, D-Mannose and D-Galactose. The first and second examples are commonly found in dissolved form.
  • The common hexoses show minor differences in molecular structure.
  • D-Glucose, D-fructose and D-mannose differ only in the first two carbon atoms and, in that D-fructose is a Ketose while the other two and Aldoses.
  • D-glucose and D-galactose differ only in the position of the hydroxyl group on the fourth carbon atom.

Khan Academy - Isomerism

Oligosaccharides

  • They contain between two and nine monosaccharide units, linked together.
  • The linkages in oligosaccharides are known as glycosidic bonds. In this, two hydrogen molecules and an oxygen molecule are removed in total from the monosaccharide units - forming an oligosaccharide and a water molecule. The bond between the two monomers is in called an oxygen bridge.
  • Oligosaccharides are classified on the basis of the number of monosaccharide units linked together. They may be -

1. Disaccharides - Sucrose, Lactose, Maltose, Cellobiase

2. Trisaccharides - Raffinose, Gertianose

3. Tetrasaccharides - Stachyose

Glycosidic Linkage

The process of glycosidic link formation; removal of water to form an oxygen bridge
The process of glycosidic link formation; removal of water to form an oxygen bridge | Source

Glycosidic linkage and Non-Reducing Sugars

Sucrose

  • It is the principle disaccharide of higher plants.
  • It is formed by the union of Alpha D-glucose and Beta D-fructose. Linkage is done by glycosidic bond formation.

Physical Properties

  • Colourless crystals
  • Has a melting point of 160 degrees Celsius
  • Soluble in water
  • Dextrorotatory, i.e. it deflects plane polarised light in the clockwork direction.
  • Does not undergo mutarotation.

Chemical Properties

  • Readily hydrolysed by heating with dilute mineral acids
  • It is a non-reducing sugar - therefore it has no effect on Benedict's solution or Fehling's solution.

Benedict's test - concentration of reducing sugar

Source

Maltose

  • It is a reducing sugar
  • Two units of D-glucose combine at their 1st and 4th carbon atoms, by glycosidic linkage to form maltose.
  • It is a degradation product of Starch, preceded by dextrins.

Physical Properties

  • Crystalline, needle-like substance
  • Melting point of 135 degrees Celsius
  • Soluble in water
  • Dextrorotatory, and exhibits mutarotation

Chemical Properties

  • Reduces Fehling's and Benedict's solutions
  • Hydrolysed by Maltose to form two D-glucose units

Lactose

AKA Milk Sugar

  • It is also a reducing disaccharide
  • Composed of glucose and galactose linked by glycosidic bonds at their 4th and 1st carbon atoms respectively

Physical Properties

  • Crystallised into rhombic prisms with one molecule of water
  • It is comparatively less soluble in water.
  • It is dextrorotatory and undergoes mutarotation.

Chemical Properties

  • Reduces Fehling's and Benedict's solutions.
  • Hydrolysed by dilute HCl or the enzyme Lactase to form D-glucose and D-galactose.

Polysaccharides

  • High molecular weight polymers containing multiple monosaccharide units (monomers).
  • Polysaccharides have a high frequency of hexose monomers. These are known as hexosans.
  • Polysaccharides can be classified on the basis of constituent monosaccharides as - Homopolysaccharides and Heteropolysaccharides. The former is composed of the same kind of monomers while the latter is composed of multiple kinds of monomers.

Starch

  • It is the primary storage form of food in plants
  • It is a homopolysaccharide composed of D-glucose units.
  • Starch has two parts - Amylase (15-20%) and Amylopectin (80-85%)
  • The components of starch can be partially separated due to their varying solubility in water.
  • Starch upon hydrolysis produces dextrins, followed by maltose and then finally D-glucose.

Amylose

  • It is a straight chain compound
  • It consists of D-glucose units bonded by Alpha- 1,4 links
  • It is highly soluble in water and stains blue-black due to Iodine.

Amylopectin

  • It is a branched chain compound.
  • It consists of D-glucose units bonded by Alpha 1,4 links as well as Alpha 1,6 links.
  • It is less soluble in water than Amylose.
  • It stains red to purple, due to iodine.

Glucagon

  • It is the reserve carbohydrate form of most cells of the animal body.
  • It is the source of glucose for respiration.
  • It is similar in structure to Amylopectin, but shows more branching (Alpha 1,6 bonds).
  • Forms granules inside cells, and is associated with Smooth Endoplasmic Reticulum.
  • Highly soluble in water; Purple-red colour upon staining due to iodine.

Cellulose

  • It is a straight chain polymeric molecule
  • Composed of Beta D-glucose units with 1,4 links. (Unlike the Alpha 1,4 links in Starch)
  • Most abundant natural product in the world - fundamental component of the plant cell wall
  • It is a white solid that is insoluble in water.

Mucopolysaccharide

  • Refers to a specific group of polysaccharides, that are derivatives of sugars such as amino sugars and uronic sugars.
  • They contain repeating units of D-glucauronic acid and N-acetyl-D-glucosamine.
  • Insoluble in water, sometimes soluble in organic solvents
  • Found in the cell walls of bacteria (Peptidoglycan - A very common mucopolysaccharide)

Chitin

  • It is also a mucopolysaccharide
  • It is found in fungal cell walls - closely related to cellulose in terms of structure
  • It is a polymer of N-acetyl-D-glucosamine
  • Forms D-glucosamine and Acetic Acid on hydrolysis.

Hemicellulose

  • Component of the plant cell wall - less concentration as compared to cellulose
  • It consists of several polymers - Xylans, Glucuronoxylans and Glucomannans

Inulin

  • It is a polymer of D-fructose (Fructosan) - Beta 2,6 linkage
  • Produced primarily by tubers of plants
  • It is readily soluble in water
  • Has between 50 and around 300 D-fructose units.

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