Structure of Glucose and Fructose and Functions of carbohydrates

By Anup Pokhrel

Structure of Glucose and Fructose

Glucose and fructose are monosaccharides. There is water soluble and sweet in the test. Their general molecular formula is C6H12O6 and both are functional isomers. Open chain and cycle structures are assigned to them. Some properties are explained by open-chain structure whereas some other properties are explained by the cyclic structure.

  • Open chain structures of glucose and fructose
  1. Glucose: It occurs in ripen grapes, honey and most of the sweet fruits and thus known as grape sugar or dextrose. It is an aldohexose with melting point 146o The elemental analysis proved the molecular formula of glucose to be C6H12O6. It is bifunctional compound i.e. its molecule possesses one aldehyde group and five hydroxyl groups is primary and rest four are secondary in nature. The Fischer’s open chain structures for glucose are given below:



The symbols D and (+) represent the configuration and optical rotation of the molecule respectively. The form of glucose in which the –OH group at C-5, i.e. farthest asymmetric carbon atom from carboxyl carbon, lies on the right, is called as D-glucose and if it lies on the left, the form is called as L-glucose. Glucose is an optically active compound as it possesses four asymmetric carbon atoms or the chiral centers. It may possess two optical isomers, one which rotates the plane-polarized light to the right is called as dextrorotatory and the another which rotates the plane of polarized light to the left is called as levorotatory glucose. Naturally occurring glucose is isomer with D-configuration. Its mirror image will be (-) isomer with L-configuration. Such mirror image isomers are known as enantiomers. The (+) and (-) isomers of glucose are thus enantiomers. If the amount of these isomers is equal, the resulting mixture is called as the racemic mixture. The net optical rotation of a racemic mixture becomes zero.

  • Fructose: It is found in honey and several sweet fruits. Fructose is twice as sweet as ordinary sugar. It is a ketohexose with melting point 102o Fructose is a functional isomer of glucose. The molecular formula is C6H12O6with one ketonic group and five alcoholic groups. The molecule possesses two primary alcoholic groups, three secondary alcoholic groups and one ketonic group at the C-2 atom. The Fischer’s open-chain structures for fructose are given below:


Naturally occurring fructose is (-) isomer with D-configuration. Its mirror image will be (+) isomer with L-configuration. The L (+) from either exists in minor amount along with D (-) from or is synthesized through various chemical reactions. The (+) form is a mirror image to (-) from and hence both are enantiomers. The position or the arrangement of –OH group at c-5 determines whether the configuration is D or L and the action of the isomer on the plane polarized light determines whether the isomer is (+) or (-).

  1. Cyclic structure of glucose and fructose: The open chain structure of glucose and fructose proposed by Baeyer and Fischer in 1876 could explain many of their physical and chemical properties. But failed to explain certain properties of glucose and fructose. These monosaccharides exist in two interconvertible forms i.e. and β formed with different values of specific rotations. These two forms are found to exist in an equilibrium state with the certain value of specific rotation.

Functions of carbohydrates

Carbohydrates constitute a principal class of organic compounds for living beings. The lives can not be assumed without carbohydrates. Some of the major functions of carbohydrates in living organisms are given below:

  • Structural material to plants: The plants and the reservoir of various carbohydrate. They synthesize carbohydrates through the process of photosynthesis and store starch in different forms. Cellulose provides structural support to plants. The aerial and underground plants store starch in different forms.
  • Metabolic materials in plants: All the green plants synthesize carbohydrates in the form of glucose and fructose in the day time. The majority of these monosaccharides are transformed in various polymeric carbohydrates leaving behind some of them to be used as food for plants. Plants use up desired amount of carbohydrates by oxidation in the night time to perform vital activities like growth, flowering, fruiting, etc.
  • The source of energy in animals: Animals take up carbohydrates from plants and use them as the source of energy in their body. The major component of foodstuff for animals in different forms i.e., monomeric as well as polymeric forms. All the complicated carbohydrates undergo enzymatic hydrolysis to be converted into simple sugars i.e. monosaccharides and absorbed into the blood stream where they are broken down through oxidation to generate energy for conducting several vital activities. The excess of simple sugars are stored in the form of glycogen and other within the body of animals which can act as reserved food at the time of need.
  • Components of biomolecules: There are dozens of biomolecules found in animals like amino acids proteins, hormones, nucleic acids, fatty acids etc. These biomolecules mainly possess monosaccharides of different types in their structures. For example, DNA and RNA contain deoxyribose and ribose sugars in their molecule.

Important Questions
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