Structure and Classification of Carbohydrates

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Classification of Carbohydrates Biochemistry

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 (CH₂Oₙ). Just as in water, the ratio of oxygen and hydrogen is fixed in carbohydrates, which is 2:1. It generally breaks down to release energy in humans and animals. Today, we are going to learn about the classification of carbohydrates and their structures.

Carbohydrates Classification

Given below is the classification of carbohydrates in biochemistry.

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Types of Carbohydrates – Simple Carbohydrates

The basic type of carbohydrates is simple carbohydrates that are found in natural sugars present in fruits, vegetables, milk, and honey. These carbohydrates are much simpler to study since they have a less complex structure.

Simple carbohydrates consist of only units of monosaccharides, which is why they are the smallest and simplest of all the other types of carbohydrates. Their smaller size plays a vital role in metabolism and digestion in the gastrointestinal tract.

Types of Carbohydrates – Complex Carbohydrates

Complex carbohydrates are an essential source of energy for our body. They give us the sustained fuel that our body needs for carrying out the day to day activities, for working out, and for even taking rest. Complex carbohydrates often comprise different units of monosaccharides bound together and provide us with long-lasting energy. The complex carbohydrates are classified depending on their hydrolysis behaviour. They are divided into three groups as follows.

  1. Monosaccharides

  2. Disaccharides

  3. Polysaccharides

1. Monosaccharides

Monosaccharides are carbohydrates that cannot be hydrolyzed further for giving simpler units of either a polyhydroxy aldehyde or a ketone. If a monosaccharide consists of an aldehyde group, it is referred to as aldose and if it consists of a keto group then it is referred to as a ketose.

Structure of Carbohydrates – Glucose

Glucose is amongst the most important monosaccharides. The two commonly used methods to prepare glucose are as follows.

  1. From Sucrose: When sucrose is boiled with dilute acid in an alcohol solution, glucose and fructose are obtained.

  2. From Starch: Glucose can also be obtained by the hydrolysis of starch and boiling it with dilute sulphuric acid, at a temperature of 393K, under high pressure.

Glucose is also known as dextrose and aldohexose and is plentily available on earth.

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The cyclic structure of glucose is as follows:

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Structure of Carbohydrates – Fructose

Fructose is an essential ketohexose having a molecular formula C₆H₁₂O₆. It consists of a ketone functional group situated at carbon number 2 and contains six carbon atoms in the form of a straight chain. The ring member of fructose is analogous to the compound called Furan and is therefore termed as furanose. The cyclic structure of fructose is as follows:

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2. Disaccharides

After the process of hydrolysis, disaccharides tend to yield either two molecules of the same or the different monosaccharides.

  1. Two units of monosaccharide are joined by an oxide linkage that is formed when there is the loss of water molecule, and this linkage is referred to as glycosidic linkage.

  2. Sucrose is amongst the most common disaccharides that give both glucose and fructose on hydrolysis.

  3. Maltose and lactose often referred to as milk sugar, are also the two kinds of important disaccharides.

  4. Maltose contains two α-D-glucose whereas lactose consists of two β-D-glucose that are connected through an oxide bond.

3. Polysaccharides

  1. Polysaccharides consist of longer monosaccharide units that are joined together by glycosidic bonds.

  2. Most of these polysaccharides act as storage for food, such as starch. Starch is known to be an important storage polysaccharide in plants.

  3. Starch is a polymer of α glucose and has two components, that are amylose and amylopectin.

  4. Cellulose is also an essential polysaccharide that is found mostly in plants.

  5. It comprises β-D- glucose units that are joined by a glycosidic bond between the C1 of one glucose unit and the C4 of another glucose unit.

FAQ (Frequently Asked Questions)

1. What are Carbohydrates? How are they Classified?

Answer: Carbohydrates are, chemically, polyhydroxy aldehydes or ketones, their polymers, or their simpler derivatives. The term hydrates or carbohydrate is derived from its basic formula which has a carbon atom joined to oxygen and hydrogen in the same ratio as that of water.

They are mainly classified into simple and complex carbohydrates depending on their structures. Complex carbohydrates, however, are further divided into monosaccharides, disaccharides, or polysaccharides, depending on the process of hydrolysis. Most of the sugars end their names with -ose, which depicts their nature of being a carbohydrate. Hence, we often call table sugar as sucrose, principle blood sugar like glucose, and malt sugar as maltose.

2. Describe the Classification of Carbohydrates.

Answer: The classification of carbohydrates depends on the process of hydrolysis, which is as follows.

  1. Monosaccharides: They are the simplest carbohydrate form which cannot be hydrolyzed further and have a generic formula (CH₂O)ₙ. Glucose and ribose are examples of monosaccharides.

  2. Disaccharides: Disaccharides give two units of either the same or different kinds of monosaccharides on hydrolyzing.

  3. Trisaccharides: Trisaccharides yield three molecules of either the same or different monosaccharides in the process of hydrolysis.

  4. Polysaccharides: Polysaccharides are the carbohydrates that yield a larger number of monosaccharides after undergoing hydrolysis. They do not have a sweet taste and therefore, are often called non-sugars as well.