Classification of Carbohydrates and its Structure


What are Carbohydrates?

It is a group of organic compounds occurring in living tissues and foods in the form of starch, cellulose, and sugars. The ratio of oxygen and hydrogen in carbohydrates is the same as in water i.e. 2:1. It typically breaks down in the animal body to release energy.

Carbohydrates Formula

The general formula for carbohydrates is Cx(H2O)y.

Definition of Carbohydrates in Chemistry

Chemically, carbohydrates are defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce units of such type on hydrolysis. Carbohydrates are also called saccharides which is a Greek word and it means sugar because almost all the carbohydrates have a sweet taste.

Sources of Carbohydrates

We know carbohydrates are an important part of any humans diet. Some common sources of carbohydrates are:

  • Potatoes
  • Maze
  • Milk
  • Popcorn
  • Bread

Classification of Carbohydrates

Classification of Carbohydrates

Classification of Carbohydrates

The different types of carbohydrates can be classified on the basis of their behavior on hydrolysis. They are mainly classified into three groups:

  • Monosaccharides
  • Disaccharides
  • Polysaccharides

Monosaccharide carbohydrates are those carbohydrates that cannot be hydrolyzed further to give simpler units of polyhydroxy aldehyde or ketone. If a monosaccharide contains an aldehyde group then it is called aldose and on the other hand, if it contains keto group then it is called as a ketose.

Structure of Carbohydrates – Glucose

One of the most important monosaccharides is glucose. The two commonly used methods for the preparation of glucose are

  • From Sucrose: If sucrose is boiled with dilute acid in an alcoholic solution then we obtain glucose and fructose.
  • From Starch: We can obtain glucose by hydrolysis of starch and by boiling it with dilute H2SO4 at 393K under elevated pressure.

Glucose is also called aldohexose and dextrose and is abundant on earth.

Simple Carbohydrate - Glucose

Simple Carbohydrate – Glucose

Glucose is named as D (+)-glucose, D represents the configuration whereas (+) represents the dextrorotatory nature of the molecule.

The ring structure of glucose can explain many properties of glucose which cannot be figured by open chain structure.

The two cyclic structures differ in the configuration of the hydroxyl group at C1 called as anomeric carbon. Such isomers i.e. α and β form are known as anomers. The cyclic structure is also called pyranose structure due to its analogy with pyran. The cyclic structure of glucose is given below:

Cyclic Structure of Carbohydrates - Glucose

Cyclic Structure of Carbohydrates – Glucose

Structure of Carbohydrates – Fructose

It is an important ketohexose. The molecular formula of fructose is C6H12O6 and contains ketonic functional group at carbon number 2 and has six carbon atoms in a straight chain. The ring member of fructose is in analogy to the compound Furan and is named as furanose. The cyclic structure of fructose is shown below:

Carbohydrate Classification - Fructose

Carbohydrate Classification – Fructose

Examples of Carbohydrates

Here are a few examples of where you’ll find the most carbs:

  • Dairy Products – Yogurt, Milk, Ice cream
  • Fruits – Fruit juice or Whole fruit
  • Grains – Cereal, Bread, Wheat, Rice
  • Legumes – Plant-based proteins, Beans
  • Starchy Vegetables – Corn, Potatoes

Disaccharides

On hydrolysis, disaccharides yield two molecules of either same or different monosaccharide. The two monosaccharide units are joined by oxide linkage which is formed by the loss of water molecule and this linkage is called glycosidic linkage. Sucrose is one of the most common disaccharides which on hydrolysis gives glucose and fructose.

Maltose and Lactose (also known as milk sugar) are other two important disaccharides. In maltose, there are two α-D-glucose and in lactose, there are two β-D-glucose which are connected by oxide bond.

Polysaccharides

Polysaccharides contain long monosaccharide units joined together by glycosidic linkage. Most of them act as food storage for e.g. Starch. Starch is the main storage polysaccharide for plants. It is a polymer of α glucose and consists of two components-Amylose and Amylopectin.

Cellulose is also one of the polysaccharides that are mostly found in plants. It is composed of β-D- glucose units joined by a glycosidic linkage between C1 of one glucose unit and C4 of the next glucose unit.

Glycogen: These carbohydrates are stored mainly in the animal body. It is present in the liver, muscles, and brain. When the body needs glucose, enzymes break the glycogen.


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