Carbohydrates-Classification-structures-functions of carbohydrates

Carbohydrates are naturally occurring organic molecules. Carbohydrates are the basic source of energy in our bodies. They are composed of carbon, hydrogen, and oxygen. The basic formula of carbohydrates is Cx (H2O)y.  

The word carbohydrate indicates the “hydrate of carbon”. Carbohydrates are further classified into many classes like monosaccharides, disaccharides, and polysaccharides. Carbohydrates are complex macromolecules. After ingestion, carbohydrates must be breakdown into simple units through the process of digestion.

Classification of carbohydrates

Carbohydrates are classified into major three classes depending upon the number of units present. The three classes of carbohydrates are:

A monosaccharide is the smallest unit of carbohydrates. Oligosaccharides and polysaccharides consist of many units of monosaccharides. Oligosaccharides and polysaccharides yield the units of monosaccharides on hydrolysis (reaction with water).


Monosaccharides are also called “simple sugars”. Monosaccharides are the simplest form of carbohydrates. It is the smallest unit of carbohydrates; therefore it cannot be further hydrolyzed. The general formula of monosaccharide is Cn (H2O).

Monosaccharides are further classified into aldoses and ketoses based on the functional group present. They are known as aldoses when the functional group is aldehyde (CHO) For example, glucose.

image showing structures of glucose and fructose

Monosaccharides are known as ketoses when the functional group is ketone (CO). For example, fructose

Monosaccharides are also classified based on the number of carbon atoms present in them like they are classified as trioses (3C), tetroses (4C), pentoses (5C), and hexoses (6C).


Oligosaccharides are made of 2-10 units of monosaccharides.

For example, sucrose, lactose, and maltose.

Monosaccharides are linked through a glycosidic bond to form oligosaccharides. A glycosidic bond is formed between the hydroxyl groups of two monosaccharides.

Oligosaccharides are further classified as disaccharides, trisaccharides, tetrasaccharides, pentasaccharides, hexasaccharide, and heptasacchrides based on the number of monosaccharides units present.

image showing structures of sucrose and lactose
image showing structure of maltose


Polysaccharides are high molecular weight carbohydrates with many units of monosaccharides linked through the glycosidic bond.

For example, starch, cellulose, and glycogen.

On hydrolysis, polysaccharides yield many units of monosaccharides and oligosaccharides.

Polysaccharides are further classified into two classes, homopolysaccharides, and heteropolysaccharides, depending upon the type of monosaccharides present. When a single type of monosaccharide repeats to form the chain of polysaccharides, it is classified as a homopolysaccharide. When different types of monosaccharides are linked through the glycosidic bond, they are classified as heteropolysaccharides.

image showing structure of starch and cellulose

Functions of carbohydrates


Carbohydrates provide most of the energy in a cell. The most basic function of glucose is to provide energy in the cell and body. Glucose mainly provides energy to the brain, muscles, and lungs. Glucose also assists in respiration.

Glucose carries a large amount of ATP which is provided to cells as the source of energy. Glucose transfer ATP to cells on oxidation. Glucose also helps in the contraction of the muscle. There are so many functions of glucose besides these.


Fructose is used as an alternative to glucose. When a high supply of energy is required in a cell, glucose alone cannot fulfill all the requirements so in this case fructose is used as an energy supplier. Fructose also provides ATP on complete oxidation.


Sucrose on complete hydroxylation with the help of enzyme ‘sucrase’ yields glucose and fructose which in turn are then utilized as energy transporters.

Sucrose is the sweetest among all the carbohydrates, therefore, use as a sweetening agent in food industries. The other name of sucrose is ‘table sugar.


Maltose is also used in energy production. Our body can absorb maltose. When the enzyme ‘maltase’ catalyzes maltose it breaks into glucose and fructose which are used as energy suppliers.

The other major role of maltose is in beverage industries to produce alcohol.


Lactose is also called ‘milk sugar’. Lactose is used as lactating agent. It is the basic source of energy for infants. The enzyme ‘lactase’ splits the lactose into glucose and galactose.


Starch is made up of two polysaccharides amylose and amylopectin. Starch is mainly used in plants to store energy. The excess energy of plants is stored as starch. Starch is also used in animals to provide energy.


Cellulose is responsible for the shape of cellulose. It assists to become cell rigid.

Cellulose only plays its role in plants. It is absent in animals.


Glycogen is also called ‘animal starch’. The main function of glycogen is to provide in the absence of glucose. During fasting conditions, when we are not taking glucose through diet, glycogen acts as a glucose supplier. Glycogen is responsible for maintaining glucose concentration in blood.

So next time when you are asked a question about carbohydrates, I hope you can answer. If you still have any questions let me know in the comment section.