Carbohydrates (Bio Molecules)



·         Carbohydrates are a good source of energy. 

·         Carbohydrates are a group of organic compounds that occur in living tissues and foods in the form of sugars, cellulose, and starch.

·         The general formula of carbohydrates is (CH₂O).

·         Example: glucose, fructose, sucrose, maltose, starch, cellulose etc.

·         The carbohydrates are divided into three major classes depending upon whether or not they undergo hydrolysis, and if they do, on the number of products formed.






The monosaccharides are polyhydroxy aldehydes or polyhydroxy ketones which cannot be decomposed by hydrolysis to give simpler carbohydrates. Examples are glucose and fructose, both of which have molecular formula, C6H12O6.


The monosaccharides are the basis of carbohydrate chemistry since all carbohydrates are either monosaccharides or are converted into monosaccharides on hydrolysis. The monosaccharides are polyhydroxy aldehydes or polyhydroxy ketones. There are, therefore, two main classes of monosaccharides.
1.The Aldoses, which contain an aldehyde group

2.The Ketoses, which contain a ketone group

The aldoses and ketoses are further divided into sub-groups on the basis of the number of carbon atoms in their molecules, as trioses, tetroses, pentoses, hexoses, etc. To classify a monosaccharide completely, it is necessary to specify both, the type of the carbonyl group and the number of carbon atoms present in the molecule. Thus monosaccharides are generally referred to as aldotrioses, aldotetroses, aldopentoses, aldohexoses, ketohexoses, etc.

The aldoses and ketoses may be represented by the following general formulas.


Glucose and fructose are specific examples of an aldose and a ketose.



Carbon Atoms

General Terms


























Reducing Sugars

Non-reducing Sugars

Such sugar bears a free aldehyde (-CHO) or ketonic (-CO) group.

These sugars do not have such groups

Reducing sugars have the capacity to reduce cupric ions of Benedict's or Fehling solution to cuprous ions.

Non reducing sugar fails to reduce the cupric ions of Benedict's solution to cuprous ions.


Examples: Maltose, Lactose etc


Example: sucrose




·         It is a monosaccharide’s with molecular formula C6H12O6.

·         It is present in sweet fruits and in honey.

·         It is an aldohexose.

·         It is the monomer of many of the larger carbohydrates, namely starch, cellulose.

·         It is probably the most abundant organic compound on earth.

·         It is also known as dextrose, grape sugar, corn sugar, blood sugar

·         In humans, glucose is one of the most important nutrients for fueling the body.

·         It's especially important for the brain and nervous system, which aren't very good at using other fuel sources.

·         Food sources of glucose: Glucose is found in fruits and vegetables, as well as honey, corn syrup, and high fructose corn syrup. (All plants make glucose, but much of the glucose is used to make starch, fiber, and other nutrients.)


Preparation of Glucose

1. From sucrose (Cane sugar): If sucrose is boiled with dilute HCl or H2SO4 in alcoholic solution, glucose and fructose are obtained in equal amounts.

2. From starch: Commercially glucose is obtained by hydrolysis of starch by boiling it with dilute H2SO4 at 393 K under pressure.

           Starch or cellulose                                                                                         Glucose



Structure of Glucose

 Glucose is an aldohexose and is also known as dextrose. It is the monomer of many of the larger carbohydrates, namely starch, cellulose. It is probably the most abundant organic compound on earth. It was assigned the structure given below on the basis of the following evidences:




1. Its molecular formula was found to be C6H12O6.

2. On prolonged heating with HI, it forms n-hexane, suggesting that all the six carbon atoms are linked in a straight chain.


 3. Glucose reacts with hydroxylamine to form an oxime and adds a molecule of hydrogen cyanide to give cyanohydrin. These reactions confirm the presence of a carbonyl group (>C = O) in glucose.


 4. Glucose gets oxidised to six carbon carboxylic acid (gluconic acid) on reaction with a mild oxidising agent like bromine water. This indicates that the carbonyl group is present as an aldehydic group.


 5. Acetylation of glucose with acetic anhydride gives glucose pentaacetate which confirms the presence of five –OH groups. Since it exists as a stable compound, five –OH groups should be attached to different carbon atoms.


 6. On oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic acid, saccharic acid. This indicates the presence of a primary alcoholic (–OH) group in glucose.


 The exact spatial arrangement of different —OH groups was given by Fischer after studying many other properties. Its configuration is correctly represented as I. So gluconic acid is represented as II and saccharic acid as III.


Glucose is correctly named as D(+)-glucose. ‘D’ before the name of glucose represents the configuration whereas ‘(+)’ represents dextrorotatory nature of the molecule. It should be remembered that ‘D’ and ‘L’ have no relation with the optical activity of the compound. They are also not related to letter ‘d’ and ‘l’ (see Unit 10). The meaning of D– and L– notations is as follows.

 The letters ‘D’ or ‘L’ before the name of any compound indicate the relative configuration of a particular stereoisomer of a compound with respect to configuration of some other compound, configuration of which is known. In the case of carbohydrates, this refers to their relation with a particular isomer of glyceraldehyde. Glyceraldehyde contains one asymmetric carbon atom and exists in two enantiomeric forms as shown below.


(+) Isomer of glyceraldehyde has ‘D’ configuration. It means that when its structural formula is written on paper following specific conventions which you will study in higher classes, the –OH group lies on right hand side in the structure. All those compounds which can be chemically correlated to D (+) isomer of glyceraldehyde are said to have D-configuration whereas those which can be correlated to ‘L’ (–) isomer of glyceraldehyde are said to have L—configuration. In L (–) isomer –OH group is on left hand side as you can see in the structure. For assigning the configuration of monosaccharides, it is the lowest asymmetric carbon atom (as shown below) which is compared. As in (+) glucose, —OH on the lowest asymmetric carbon is on the right side which is comparable to (+) glyceraldehyde, so (+) glucose is assigned D-configuration. Other asymmetric carbon atoms of glucose are not considered for this comparison. Also, the structure of glucose and glyceraldehyde is written in a way that most oxidised carbon (in this case –CHO)is at the top.




Cyclic Structure of Glucose

Ring structure of Glucose explain the properties which are  not explained by  open chain structure because Ring structure has no free aldehydic group, glucose does not respond to certain characteristic tests of aldehydes, like Schiff’s test and addition reaction with sodium- -bisulphite. The cyclic structure is attributed to the formation of hemiacetal the cyclic structure thus formed is a six-membered ring.
This cyclic structure has one chiral centre more than the open chain structure. Therefore, two possible isomeric forms are possible
Fischer projection formula:

These two isomers differ in the configuration of the hydroxyl group at C1.Stereoisomers that differ in the configuration at C1 are known as anomers.
C1 is called the anomeric carbon.
Haworth structures:
The six-membered ring structures of glucose can be called pyranose structures, in analogy with pyran. Hence, the anomers are called alpha D plus gluco pyranose and beta D plus gluco pyranose.



Fructose is defined as a monosaccharide and is found in fruits and vegetables. In fructose, the glycemic index is lower as compared to glucose. Compared to Glucose, the binding fructose to cellular protein is seven times faster. It is also referred to as D- fructose or fruit sugar and its functional group are known as ketone. Glucose is known to be primarily metabolized in the liver and is not found in starch.


Structure of Fructose

Fructose also has the molecular formula C6H12O6 and on the basis of its reactions it was found to contain a ketonic functional group at carbon number 2 and six carbons in straight chain as in the case of glucose. It belongs to D-series and is a laevorotatory compound. It is appropriately written as D-(–)-fructose. Its open chain structure is as shown.



Cyclic structure of Fructose:
Fructose is a ketone and consists of six carbon atoms in a straight chain with the keto functional group at position 2 of the carbon chain.

As the open chain structure fails to explain certain facts like its existence in two isomeric forms and the formation of hydrogen sulphite addition product, the ring structure was established.

The cyclic structure is a five-membered ring. Therefore, there are two possible isomeric forms. The two cyclic forms differ in the configuration of the hydroxyl group at C 2.  These isomers are called anomers.

The five-membered ring structure of fructose is called furanose. The cyclic structures of the two anomers are named alpha D minus fructofuranose and beta D minus fructofuranose.



·         Disaccharides are made of two monosaccharides. 

·         Disaccharides are sweet, crystalline and water-soluble substances. 



Sucrose(table sugar or cane sugar)

Lactose(milk sugar)


Two units of α-D glucose.

α-D glucose and α-D fructose.

β-D glucose and β-D galactose.


α glycosidic linkage between C1 and C4 carbon atoms.

Two monosaccharides are held together by a glycosidic linkage between C1 of α-D-glucose and C2 of β-D-fructose.


β glycosidic linkage.


Reducing sugar

Non reducing sugar and invert sugar

Reducing sugar




It is one of the common disaccharides, which on hydrolysis gives equimolar mixture of D (+) -glucose and D (−) fructose.


Glycosidic Linkage

The ether linkage combining two monosaccharides is known as glycosidic linkage.

For example: In sucrose, the glycosidic linkage is present between glucose and fructose.



maltose is composed of two α-D-glucose units in which C1 of one glucose (I) is linked to C4 of another glucose unit (II). The free aldehyde group can be produced at C1 of second glucose in solution and it shows reducing properties so it is a reducing sugar.



It is more commonly known as milk sugar since this disaccharide is found in milk. It is composed of β-D-galactose and β-D-glucose. The linkage is between C1 of galactose and C4 of glucose. Free aldehyde group may be produced at C-1 of glucose unit, hence it is also a reducing sugar.




·         On hydrolysis polysaccharides yield large number of monosaccharides units.

·         An example of a polysaccharide is starch ,cellulose.




·         It is the main storage polysaccharide of plants

·         The main source is maize, wheat, barley, rice and potatoes.

·         It is a polymer of α-glucose.

·         It consists of two components – Amylose and Amylopectin.

·         It is a non reducing sugar.





It is water soluble fraction for starch

It is water insoluble fraction of starch.

It is 20% of starch

It is 80% of starch.

It is straight chain polymer of D-glucose units.

It is branched chain polymer of D glucose units.

Glucose units are joined by α-1,4  glycosidic linkage

In amylopectin, the glucose units are joined by α -1, 6 glycosidic linkage.

Its molecular mass lies in the range of 10,000 - 50,000

Its molecular mass is in the range of 50,000 - 1,00,000



·        A structural polysaccharide in plants; when consumed, it acts as a dietary fibre.

·        Cellulose is a structural polysaccharide.

  • It is the most abundant carbohydrate in nature.
  • Glucose molecules are linked by beta 1-4 glyosidic linkages.
  • Cellulose is fibrous, tough and water insoluble.
  • It is found in the cell wall of plants.
  • The cotton is almost pure cellulose.
  • However, the digestive tracts of herbivores (and termites) contain symbiotic microorganisms that secrete a series of enzymes, collectively known as cellulases, that can hydrolyze cellulose.

·         Plants use cellulose to support structure

·         It is Difficult to digest therefore, it cannot be consumed by humans

·         Cellulose forms a number of important products, such as:

     o    Cellotape used for packaging.

     o    Viscose rayon used in textile industry.

     o    Gun cotton used as an explosive.




·         The carbohydrates are stored in animal body as glycogen.

·         It is also known as animal starch because its structure is similar to amylopectin and is rather more highly branched.

·         It is present in liver, muscles and brain.

·         When the body needs glucose, enzymes break the glycogen down to glucose.

·         Glycogen is also found in yeast and fungi.


Importance of Carbohydrates

Carbohydrates play a crucial role in the functioning of living organisms, serving as essential sources of energy and providing structural support. Here are some key points about the importance of carbohydrates:

1.     Energy Source: Carbohydrates are a primary source of energy for both plants and animals. They are broken down during digestion into glucose, which is then utilized by cells to generate ATP (adenosine triphosphate), the energy currency of cells.

2.     Food Source: Carbohydrates form a significant portion of our diet. They are abundant in various food sources, including grains, fruits, vegetables, and dairy products. Complex carbohydrates, such as starch and dietary fiber, provide sustained energy and aid in digestion.

3.     Medicinal Use: Honey, which is rich in carbohydrates, has been used in traditional medicine systems like Ayurveda as an instant source of energy. It is believed to have various therapeutic properties and is used for its antioxidant and antibacterial effects.

4.     Storage Molecules: In plants, carbohydrates are stored as starch in structures like roots, tubers, and seeds. Animals store carbohydrates as glycogen in the liver and muscles. These stored carbohydrates can be broken down when energy demands are high, such as during fasting or exercise.

5.     Structural Components: Carbohydrates are essential for the structure of cells and organisms. Cellulose, a complex carbohydrate, forms the cell walls of bacteria and plants, providing rigidity and support. Wood, which is primarily composed of cellulose, is used in the construction of furniture and other wooden products. Cotton, made of cellulose fibers, is used extensively in the textile industry.

6.     Industrial Applications: Carbohydrates serve as raw materials for various industries. They are used in the production of textiles, where cellulose-based fibers like cotton are utilized. Carbohydrates are also used in the paper industry, as a component of lacquers and adhesives, and in the brewing industry for fermentation processes.

7.     Nucleic Acids: Carbohydrates are integral components of nucleic acids, such as DNA and RNA. Two specific aldopentoses, D-ribose, and 2-deoxy-D-ribose, are present in nucleic acids, playing vital roles in genetic information storage and transfer.

8.     Biological Combinations: Carbohydrates are found in conjunction with proteins and lipids in various biological systems. For example, glycoproteins and glycolipids have carbohydrate chains attached to them, which are involved in cell signaling, recognition, and immune responses.

In summary, carbohydrates have diverse functions in living organisms, serving as a major energy source, storage molecules, structural components, and essential components of nucleic acids and biological combinations. They also play a significant role in various industries and have been utilized in traditional medicine for their health benefits.