Classification of Carbohydrates

Carbohydrates are classified on the basis of their behaviour on hydrolysis. They have been broadly divided into following three groups:

· Monosaccharides

· Oligosaccharides

· Polysaccharides

Monosaccharides:

A carbohydrate that cannot be hydrolysed further to give simpler unit of polyhydroxy aldehyde or ketone is called a monosaccharide. About 20 monosaccharides are known to occur in nature. Some common examples are glucose, fructose, ribose, etc.

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 C_x(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

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

· 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

D.L. configuration

The "D" and "L" specifications in the names of D-glucose and L-glucose are used to differentiate between two different shapes of the glucose molecule. D-glucose and L-glucose are enantiomers, meaning that their molecular structures are mirror images of each other. The structural difference between these two molecules is best described in terms of the Fisher projection model, which is one way of drawing organic molecules.

§ if the OH on the bottom chiral centre points to the right, it is referred to as D-

• if the OH on the bottom chiral centre points to the left, it is referred to as L- .

Fructose

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.

Oligosaccharides:

Carbohydrates that yield two to ten monosaccharide units, on hydrolysis, are called oligosaccharides. They are further classified as:

· disaccharides

· trisaccharides

· tetrasaccharides, etc.,

depending upon the number of monosaccharides, they provide on hydrolysis. Amongst these the most common are disaccharides. The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different. For example, one molecule of sucrose on hydrolysis gives one molecule of glucose and one molecule of fructose whereas maltose gives two molecules of only glucose.

Sucrose

This is the most important disaccharide. It is popularly known as table sugar. Sucrose is found in all photosynthetic plants. It is commercially obtained from sugarcane and sugar beets via an industrial process. Let us take a look at some chemical properties of sucrose

· The molecular formula of sucrose is C₁₂H₂₂O₁₁.

· If sucrose goes through acid catalysed hydrolysis it will give one mole of D-Glucose and one mole of D-Fructose.

· The chemical structure of sucrose comprises of α form of glucose and β form of fructose

· The glycosidic linkage is α linkage because the molecule formation is in α orientation

· Sucrose is a non-reducing sugar. As you can see from the structure it is combined (linked) at the hemiacetal oxygen and does not have a free hemiacetal hydroxide

· Since has no free hemiacetal hydroxide it does not show mutarotation (α to β conversion). Sucrose also does not form osazones for the same reason.

· We can prove the structural formula of sucrose by hydrolysing it with α-glycosidase enzymes which only hydrolyses α glucose. This test is positive for sucrose.

Lactose

This is a disaccharide you may already be familiar with. Lactose is the primary ingredient found in the milk of all mammals. Unlike the majority of saccharides, lactose is not sweet to taste. Lactose consists of one galactose carbohydrate and one glucose carbohydrate. These are bound together by a 1-4 glycosidic bond in a beta orientation.

If you look at the structure of lactose you will see that there is one significant difference between galactose and glucose. Galactose’s fourth carbon has a different orientation in galactose than in sucrose. If it was not so the resulting molecule would have just been sucrose (glucose+glucose) instead of lactose.

Also from the structure, we can notice that lactose is a reacting sugar since it has one free hemiacetal hydroxide. So when we react Lactose with bromine water it will give monocarboxylic acid.

Maltose

Maltose is another disaccharide commonly found. It has two monosaccharide glucose molecules bound together, the link is between the first carbon atom of glucose and the fourth carbon of another glucose molecule. This, as you know, is the one-four glycosidic linkage. Few of its properties are

· On acid catalysed hydrolysis one mole of maltose gives two moles of D-glucose.

· Maltose has a free hemiacetal hydroxide, hence it undergoes mutarotation. It exists as both α-Maltose and also β-Maltose

· For the same reasons it also gives a positive test with Benedicts and Tollens reagent.

Polysaccharides:

Carbohydrates which yield a large number of monosaccharide units on hydrolysis are called polysaccharides. Some common examples are starch, cellulose, glycogen, gums, etc. Polysaccharides are not sweet in taste, hence they are also called non-sugars.

Starch

Starch is an element present in all photosynthetic plants. We generally find starch in the plant’s roots and seeds. All plants when they synthesize glucose, the extra glucose is stored in the form of starch.

Starch is a glucan, meaning it only consists of glucose molecules all linked together. The general molecular formula for starch is (C₆H₁₀O₅)n. The ‘n’ denotes the number of molecules linked together.

We find starch in the seeds of plants as granules. On heating these granules in the water we form a colloidal suspension. We obtain two components from this process. These two components are Amylose and Amylopectin.

Amylose

· Amylose themselves are also polysaccharides.

· Constitute about 10-20 % of a starch molecule

· They are made up of D-glucose units that connect with each other with the help of a α-glycosidic linkage.

· One glucose unit  connects to another glucose unit from the one-four position i.e. { α (1-40 }

· Amylose has the same basic structure of maltose, multiplied by ‘n’ number of times.

· In a basic amylose structure, there are almost 1000 upwards glucose molecules forming a link

· Although they are a big molecule they are very compact in size because they form an alpha-helical structure.

· Amylose molecules exist in form of a helix

Amylopectin

· They have the same basic structure that Amylose does which is D-glucose units combining in a { α (1-40 } form

· Constituent about 80-90% of a starch molecule

· They have a very interesting structure. They have a main branch similar to Amylose, but then also have branches.

· Branching in amylopectin occurs between C₆ – C₁, which means the sixth carbon in the chain connects with the first carbon of the branch.

· And the branching occurs every twenty to twenty-five glucose units.

Glycogen

Glycogen is also a Glucon i.e., it is made up exclusively of D-glucose units. It is a reserved carbohydrate source for animals as well as plants. Let us now see the structure and the functions of Glycogen.

Cellulose

Cellulose is an important structural element of the cell walls of all photosynthetic plants. It is a fibrous kind of polysaccharide which is highly insoluble in water. Here again, Cellulose is a glucan. The D-glucose units connect in (1 → 4) fashion.

The connection though is different from starch and glycogen, it is a beta linkage. So the linkage is β-glucosidic linkage. The structure is not helical since the beta linkage confines the polysaccharide to a straight-chain form.

In the structure of cellulose -OH groups point outside the chain structure. Whenever two chains come close to each other they tend to form a stack on each other due to hydrogen bonding between these hydroxyl groups. As a result, we get a fibrous insoluble structure which is suitable for the functions of cellulose in the cell walls.