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Chemistry > Biomolecule > Monosaccharides
Biomolecule

Monosaccharides

We all consume some form of carbohydrates via our food every day. The biggest source of carbs for us is through sugars. Monosaccharides are the simplest form of sugars found in innumerable common food items such as honey, cane sugar, fruits. Let us have a detailed look at monosaccharides.

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Monosaccharide

We have already learned about carbohydrates and what constitutes a carbohydrate. Now monosaccharides are nothing but the simplest form or classification of carbohydrates. They are hence known as the most basic unit of carbohydrates. They are defined as any carbohydrates (or sugars) that cannot be hydrolyzed any further to give simpler sugars.

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Biomolecule cheat sheet

General Formula of Carbohydrate

Carbohydrates

Types of Monosaccharides

Monosaccharides have two broad classifications on the basis of the functional group present in them. So if they contain an aldehyde group they are known as “aldose”. And if they contain a keto group we call them “ketose”. There is also additional classification on the number of carbon atoms each molecule consists of. This following table will make the names easy to remember

Number of Carbon Atoms Aldehyde Ketone
3 Aldotriose Ketotriose
4 Aldotetrose Ketotetrose
5 Aldopentose Ketopentose
6 Aldohexose Ketohexose
7 Aldoheptose Ketoheptose

Structure of Monosaccharides

The chemical formula that most monosaccharides have is Cx(H2O)y, where generally x≥ 3. The molecule is always formed by three elements and three elements only: Carbon (C), Hydrogen (H) and Oxygen (O). The molecule of monosaccharides is very small and compact in size. This is another reason we call monosaccharides simple sugars.

Glucose

Glucose monosaccharides

The most abundant monosaccharide found in nature is in fact glucose. It is the most abundant organic compound on earth. We can find glucose in varies fruits, honey and even in starch and cane sugar. We obtain a large part of the energy in our bodies from glucose through the foods we eat. It is an aldohexose, which means it has six carbon atoms in its molecule. Its chemical formula is C6H12O6

We obtain glucose mainly from two sources which are starch and sucrose. Let us look at how we can prepare glucose from these sources

  • On a large and commercial scale glucose is prepared from hydrolysis of starch by boiling it with dilute H2SO4. The chemical reaction is as follows

(C6H10O5) + n (H2O) ————-> n ( C6H12O6 )

Starch                                                         Glucose

  • Also, another way of preparing glucose, with fructose as a joint or by-product, is to boil sucrose in dilute HCl or even H2SO4 in an alcoholic solution. This chemical reaction is as follows

C12H22O11 + H2O ————> C6H12O6 + C6H12O6

Sucrose                                          Glucose      +  Fructose

 Fructose

fructose monosaccharides

Fructose is a simple ketonic monosaccharide. We mostly find fructose in plants and their fruits, flowers and root vegetables, hence earning it a moniker of fruit sugar. It is also abundantly present in honey and corn syrup. Generally, fructose bonds with glucose to form a disaccharide we know as sucrose. Fructose was first discovered by a French chemist Augustin – Pierre Debrunfaut.

The chemical formula of fructose is also C6H12O6 but the bonding of fructose is very different than that of glucose. Fructose has a cyclic structure. The structure is an intramolecular hemiacetal. It has its carbonyl group at its number two carbon (its a ketone function group). In its cyclic form, it (generally) forms a five-member ring which we call a Furanose ring.

Solved Example for You

Q: Which one of the following is the reagent used to identify glucose?

  1. neutral FeCl3
  2. CHCl3
  3. Ammonical AGNO3
  4. C2H5ONa

Sol: The correct answer is option “c”. We use Tollen’s reagent to identify Glucose. Glucose contains aldehyde group and is oxidised by ammonical AGNO3.

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