Carbohydrate: Definition, Classification and Functions

Carbohydrates are the most widely distributed compounds in both plants and animals. Plants can build up carbohydrates from carbon dioxide (CO2) by the process of photosynthesis. The living body contains 1 percent of carbohydrates. Many plants and animals contain large quantities of carbohydrates as reserve food material. They are the compounds which provide energy to the cells of the living organisms. 

Carbohydrate is defined as a neutral compound which is made up of carbon(C), hydrogen (H) and oxygen (O) atom with a ratio of 1:2:1. The general formula of simple carbohydrate is Cn(H2O)n. Carbohydrates are also known as ‘Saccharides’. The term ‘Saccharide’ is derived from the Greek word ‘Sakcharon’ meaning sugar. The main sources of carbohydrates are plants. Higher animals have a trace amount of carbohydrates.

In plants, they are found as cellulose and starch in the stem, fibers, barks, fruits, roots, seeds, sap, etc. while in higher animals, they are stored as glycogen, lactic acid and lactogen in liver, muscle, and milk respectively. Carbohydrates are granular, fibrous or crystal solid substance. They are sweet or tasteless in taste. Most of the carbohydrates are insoluble in water except monosaccharides. In excess heat, they become ash. Carbohydrates form asters combined with acids.

Classification of Carbohydrates

Based on hydrolysis, carbohydrates are of the following types:

1. Monosaccharides (Gr. Mono=single, sacchar=sugar):

It is a simple sugar and the simplest form of carbohydrate that cannot be hydrolyzed into simple sugar. The general formula of monosaccharide is CnH2nOn. Some common examples of monosaccharides are glucose, ribose, etc. It forms the building blocks for more complex carbohydrates. All monosaccharides are water-soluble, sweet and able to form a callus. They possess free aldehyde groups (-CHO) at their carbon 1 or free ketone groups (>C=O) at their carbon 2 position and possess reducing properties. Monosaccharide with aldehyde group is known as aldose while with ketone group is known as a ketose.

Monosaccharides are of the following types based on a number of carbon atoms such as triose, tetrose, hexose, heptose, etc. which contains 3,4,5,6 and 7 carbon atoms respectively.

Most biologically, important monosaccharides are pentoses such as ribose (found in RNA), deoxyribose (found in DNA), and hexoses such as glucose and fructose (fruit sugar), etc.

Triose: It is a simple sugar or monosaccharide which contains three carbon atoms in its primary chain. Triose plays an important role in cellular respiration. In nature, only three possible trioses are available such as dihydroxyacetone, L-glyceraldehyde, and D-glyceraldehyde.

image of D-glyceraldehyde


image of Dihydroxyacetone


Tetrose: It is a four carbon atoms containing monosaccharide in its primary chain. The erythrose (C4H8O4) is a tetrose which contains one aldehyde group. French pharmacist Louis Feux Joseph Garot first isolated the erythrose (C4H8O4) in 1849. Some naturally-occurring tetroses are D-erythrose, D-threose, and D-erythrulose. They have either the aldehyde group in position 1 or a ketone functional group is located in position 2.

image of Tetroses


Pentose: A pentose is a monosaccharide carbohydrate which contains five carbon atoms in its primary chain. The nucleotides are composed by the pentose sugar ribose (C5H10O5) and deoxy-ribose (C5H10O4) and they make up the nucleic acids such as DNA and RNA. Pentoses have a higher metabolic strength than hexoses. Some important other pentoses are ribulose, arabinose, xylulose, lyxose, etc.

Hexose: It is a monosaccharide carbohydrate which contains six carbon atoms in its primary chain with chemical formula is C6H12O6. Hexoses are classified by the functional group into aldohexoses and ketohexoses. In this case, aldohexoses possess an aldehyde (-CHO) group at position 1, while ketohexoses have a ketone (RCR’) group at position 2. The example of hexose sugar is glucose which is the most abundant carbohydrate in nature. It provides energy sources to all living cells. Other important hexose sugars are fructose, mannose, galactose, etc.   

Heptose: It is a monosaccharide carbohydrate which contains seven carbon atoms. Heptoses are classified by the functional group into aldoheptoses and ketoheptoses. In this case,  aldoheptoses possesses either an aldehyde (-CHO) group at position 1, while ketoheptoses have a ketone ( RCR’) group at position 2. Some examples of heptoses are sedoheptulose or D-altro-heptulose and mannoheptulose, etc. 

2.  Oligosaccharides (Gr. Oligo=less; sacchar=sugar):

It is a complex carbohydrate which can be hydrolyzed by acids into 2-10 simple units of monosaccharides. These carbohydrates are soluble in water, sweet in taste and able to form a callus. They can be further divided into various subcategories such as disaccharides, trisaccharides, and tetrasaccharides, etc.

Disaccharides: It is the most important oligosaccharide which contains 2 units of monosaccharides. Sucrose is disaccharides which can be hydrolyzed and gives one molecule of glucose and fructose while maltose gives two molecules of glucose only on hydrolysis.

image of Chemical structure of sucrose

Chemical Structure of sucrose

Trisaccharides: If the carbohydrates give three molecules of monosaccharides on hydrolysis with the same or different, then it is known as trisaccharides. Raffinose is a trisaccharide which is composed of one fructose, one glucose, and one galactose molecules. Other common trisaccharides are maltotriose, maltotriulose, etc.

image of Chemical structure of Raffinose

Chemical structure of Raffinose

Tetrasaccharides: If the carbohydrates give four molecules of monosaccharides on hydrolysis with the same or different, then it is known as tetrasaccharides. The general formula of tetrasaccharide is C24H42O21. Stachyose is an example of tetrasaccharide which gives one molecule of glucose, one molecule of fructose and two molecules of galactose on hydrolysis.

image of Chemical structure of stachyose

Chemical structure of stachyose

3.  Polysaccharides(Gr. Poly=many; sacchar=sugar):

A polysaccharide is a large molecule which is made up of many monosaccharide units. In this case, monosaccharides are bound together by glycosidic linkage. During hydrolysis, polysaccharides are broken down into monosaccharides or oligosaccharides. A polysaccharide is also known as a glycan. A polysaccharide can be a homopolysaccharide when it contains the same molecules of monosaccharides or heteropolysaccharides when it contains varieties monosaccharides.

The structure of polysaccharides may be linear or branched. When polysaccharides contain a straight chain of monosaccharides, then it is called a linear polysaccharide, while polysaccharides have a chain that has arms and it is known as a branched polysaccharide. Polysaccharides perform a wide variety of functions in nature. Some polysaccharides are used to store energy, some act as cellular messengers, and others provide support to cells and tissues. In animals, storage polysaccharide is glycogen and structural polysaccharide is chitin.

Some important Polysaccharides

Starch: It is the reserve food substance in the plant cells. It occurs as grains which may be spherical, oval, lens-shaped or irregular. It is insoluble in water and gives a blue color with iodine. The chemical formula of starch is (C6H10O5)n.

Glycogen: It is known as animal starch as it occurs in the liver and muscle of the animal body. It is soluble in water and gives reddish color with iodine.

image of Chemical structure of glycogen

Chemical structure of glycogen

Dextrin: It is found during partial hydrolysis of starch by acids or amylase enzyme. It consists of a very complex mixture of molecules of different sizes and structures. It is soluble in water and gives reddish-brown color with iodine.

Cellulose: It is the most important structural components of the cell wall of plants. It is also found in a few microorganisms and lower organisms. It forms the bulk or roughage of food but not digested by the man due to the absence of cellulose-digestive enzymes. It is insoluble in water and gives no color with iodine.

image of Chemical structure of cellulose

Chemical structure of cellulose

Chitin: It is a large polysaccharide molecule which is made from chains of modified glucose. It is found in the cell walls of fungi, insect`s exoskeleton, hard parts of other invertebrates and fish. It is not digested by the vertebrate animals. It is a recyclable molecule which can dissolve over time. It has several industrial applications such as surgical thread and binders for dyes and glues.

image of Chemical structure of chitin

Chemical structure of chitin

Based on taste, the carbohydrates are of two types

1. Sugar: They have a sweet taste, with granular structure and soluble in water. Some common examples are glucose (C6H12O6), fructose, sucrose, etc.

image of different Sugar

2. Non-sugar: They are non-sweet, agranular carbohydrates and insoluble in water. Some common examples are cellulose, glycogen, starch, etc.

Some Important Roles of Carbohydrates in Organisms

  • Carbohydrate acts as a bio-fuel and the energy source of the body. Most of the metabolic activities of the cells in all organisms need energy which is derived from the oxidation of carbohydrates.
  • Carbohydrates act as storage of food. All animals get energy from the different types of carbohydrates in their diet.
  • Some polysaccharide such as starch acts as storage of food for plants.
  • It provides energy to the body and regulates blood sugars.
  • Carbohydrates stop skeletal muscle degradation and other tissues such as the liver, kidneys, and heart.
  • It helps to prevent the breakdown of proteins to make energy.
  • Carbohydrates also help to enhance fat metabolism.
  • Carbohydrates form genetic material like DNA and RNA in the form of deoxyribose and ribose sugars.
  • Carbohydrates prevent constipation and help to clear gut.
  • Carbohydrate makes the sweetness of the food.
  • In many industries like textile, paper, lacquers, and breweries, carbohydrates are used as an important component.
  • Some polysaccharides such as agar used in culture media, laxative and food.
  • Carbohydrates form components of biomolecules which play a key role in blood clotting, immunity, fertilization, etc.
  • It helps to make up all the parts of the cell and tissues.
  • Some carbohydrates such as hepatic glycogen help in detoxifying a normal liver.
  • Some structural carbohydrates such as pectin and hemicellulose form the cell walls in the plant.
  • Follicle-stimulating Hormone (FSH) and Luteinizing Hormone (LH) are glycoproteins which help in reproduction.

Concluding Remarks

Protein, lipid, and carbohydrates are the main macronutrients in our diet. Carbohydrates are available in various forms and you can get it from various foods like fruits, vegetables and high-fiber foods like whole grains. It is the most important source of energy for the body. To make a balanced diet, you should always take carbohydrate-rich foods. Carbohydrates provide immediate fuel in every cell in your body in the form of glucose, especially your brain cells.