Biochemistry Notes: Carbohydrates

Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones having general formula (CH₂O) _n. Carbohydrates are the most found compounds in nature and the important structural parts of all living organisms. Plants also produce carbohydrate that is glucose as a result of photosynthesis, and the excess food is stored in the form of another carbohydrate that is starch.

Definition

Carbohydrates are a group of compounds containing carbon, hydrogen, and oxygen or their derivatives that yield carbohydrates on hydrolysis. These are saccharides because of their sweet taste. These compounds can be found everywhere around us. 

Structure of Carbohydrates

The carbohydrates contain three elements in their structures. These are:

• Oxygen 
• Carbon
• Hydrogen

This empirical formula for carbohydrates is (CH₂O) _n. Here n varies from three to many thousands. These compounds are present in chains or rings with an aldehyde or ketone group attached to them. The simplest form of carbohydrates is a monosaccharide that is further linked together through glycosidic bonds to form large and complex structures.

There are three types of structures of Carbohydrates.

Open Chain Structure

In this type, carbon atoms are linked together to form long chains.

Closed or Ring Structure

The carbohydrate ring is formed when its first carbon atom condenses with the hydroxyl group present on the fifth carbon.

Haworth Structure

In this structure, a pyranose ring is present.

Properties of Carbohydrates

Two kinds of properties are found in this compound.

1. Physical properties
2. Chemical properties

Physical Properties

• These are mostly crystalline compounds. Some are Amorous.
• All the carbohydrates show optical activity.
• Stereoisomerism is an important physical property of carbohydrates in which the compounds have the same structural formula, but they differ in their configuration.

For Example, Glucose has two isomers: D glucose and L glucose.

• Except for polysaccharides and a few disaccharides, they all reduce sugars.
• The molecular weight of these compounds varies according to the number of carbon atoms present in their structures.
• Carbohydrates can store energy.

Chemical properties 

• Carbohydrates react with phenylhydrazine to form osazone.
• These compounds show positive benedict’s test. In benedict’s test, a brick red colour is produced when the benedict’s reagent and reducing sugars are heated together.
• Upon oxidation, carbohydrates are converted to carboxylic acid.

Classification of carbohydrates

The carbohydrates are classified into three groups.

1. Monosaccharides
2. Oligosaccharides
3. Polysaccharides

Monosaccharides

These are the simplest forms of carbohydrates in which the value of n varies from 3 to 7. Monosaccharides are trioses, tetroses, pentoses, hexoses, or heptoses based on the number of carbon atoms present in their structures. If the structure has an aldehyde group, it is called aldose, or if it has a ketone group, it is called ketose sugar.

Properties

• These are the simplest sugars that cannot be hydrolyzed.
• These are crystalline and colourless.
• Monosaccharides are sweeter in taste and soluble in water.
• They are the building blocks of complex carbohydrates.
• Examples: Glucose is aldohexose, and ribose is ketohexose.

Examples of monosaccharides

An important example of a monosaccharide is glucose, the major energy source in approximately all organisms. In cellular respiration, glucose is metabolized for the production of ATP using various pathways. Plants produce glucose, and animals feed on them to get them.

Other examples of monosaccharides are galactose and fructose. Galactose is found as milk sugar, whereas fructose is present in figs and other fruits.

Oligosaccharides

Oligosaccharides are produced due to glycosidic linkage between two to ten monosaccharides units. The most common oligosaccharides are disaccharides, in which there is the condensation of two monosaccharide units. 

In the condensation process, the hydroxyl group of one monosaccharide is linked with the hydrogen of another monosaccharide, releasing a water molecule. A covalent bond is formed between two monosaccharide units called glycosidic bonds.

There are two types of glycosidic bonds.

Alpha bond

It is formed when the hydroxyl group of first carbon is below the plane of the carbohydrate ring.

Beta Bond

It is formed when the hydroxyl group on the first carbon is above the plane of the ring of carbohydrates.

Properties

• These are less sweet than the monosaccharides.
• Oligosaccharides are sparingly soluble in water.
• They yield two to ten monosaccharide units on the hydrolysis.
• These are called disaccharides, trisaccharides, and so on, based on the number of the monosaccharide units linked.
• A glycosidic bond is formed to link monosaccharide units.

Examples of oligosaccharides

The common examples of oligosaccharides are sucrose, lactose, maltose, etc. sucrose is the form in which carbohydrates are mostly transferred from plants to the animals that feed on them. Sucrose is formed on the condensation of glucose and a molecule of fructose. The glycosidic bond is formed between carbon one and carbon 2 of the glucose and sucrose, respectively.

Lactose is found in milk and formed from the glycosidic linkage of glucose and galactose on releasing a water molecule. Other oligosaccharides are raffinose and rabinose.

Polysaccharides

Polysaccharides, also known as glycans, contain more than ten monosaccharide units. They form long chains that may be branched or unbranched. These are the most complex carbohydrates and often act as the structure building units in plants and animals. Polysaccharides are of two types.

1. Homoglycans contain only one kind of Monosaccharide unit.
2. Heteropolysaccharides contain more than one type of monosaccharide unit in their structure.

Properties

• Polysaccharides are complex and can be hydrolyzed.
• These are tasteless compounds found in approximately all the species.
• There can be hundreds of monosaccharide units linked together to form polysaccharides.
• These are the important structural units and a means to store excess amounts of energy.
• Polysaccharides are non-reducing sugars.

Examples

Glycogen is present in animals as storage of excess glucose just like the starch, which is also a polysaccharide, in the case of plants. So, they are called animal starch. It is a homoglycan formed by linking glucose molecules. There are two forms of starch.

Amylose

It is the starch in which glucose molecules are linked together to form unbranched chains.

Amylopectin

It is the starch in which glucose molecules are linked together to form branched chains.

The other example is chitin, a part of the cell wall of fungi and the exoskeletons of insects and crustaceans.

Functions of Carbohydrates 

Some major functions of the carbohydrates are

1. Carbohydrates provide 4kcal of energy per gram to living organisms.
2. These are the basic units to form many proteins and other components.
3. They act as an energy source and storage for utilization in various processes.
4. These are the structural components found mainly in animals’ cell walls and exoskeleton.
5. Carbohydrates are essential for the proper functioning of immune systems.

Frequently Asked Questions

References

1. Self-Assessment and review of biochemistry by Rebecca James Perumcheril 2^nd Edition.
2. Lippincott Illustrated Reviews: Biochemistry Sixth Edition
3. Harper’s Illustrated Biochemistry 30^th Edition
4. https://microbenotes.com/carbohydrates-structure-properties-classification-and-functions/
5. https://courses.lumenlearning.com/wm-biology1/chapter/reading-types-of-carbohydrates/