Carbohydrates

(CH₂O)n is the general empirical structure of carbohydrates. Monosaccharides are simple sugars that serve as fuel molecules and fundamental constituents of living organisms and these are the simplest carbohydrates required as the energy sources. Glucose and fructose are the most commonly known carbohydrates.

Structure of Carbohydrates

Carbohydrates has historically been characterized as compounds with the empirical formula C_n(H₂O)m. Glucose, fructose, and sucrose are popular sugars that suit this formula, however, nowadays, a carbohydrate is defined as a polyhydroxy aldehyde or polyhydroxy ketone with the classical formula, a molecule closely similar to it, or oligomers or polymers of such molecules. Because they are water-soluble and difficult to crystallize, they require a different set of abilities to manage than traditional "natural products" like terpenes, steroids, and alkaloids.

A "monosaccharide" is a carbohydrate derivative having a single carbon chain; "disaccharide" and "trisaccharide" are the compounds, which are made up of either two or three monosaccharide units linked either by acetal or ketal linkages. Larger aggregates with "a few" and "many" monosaccharide units are referred to as "oligosaccharide" and "polysaccharide," respectively. The divide between "few" and "many" appears to be established at around 10 units in current usage.

By the middle of the nineteenth century, chemists in Europe, particularly in Germany, had discovered a variety of relatively pure carbohydrates such as sucrose, cotton cellulose, starch, glucose, fructose, mannose, and lactose. Emil Fischer synthesized phenylhydrazine for his University of Munich thesis in 1878. In 1884, he discovered that when two phenyl hydrazines reacted with the aldehyde group and the carbon next to it, carbohydrates formed crystalline phenylosazone.

Types of Carbohydrates

The saccharides or carbohydrates are simply defined as ketones or polyhydroxy aldehydes or the compounds that produce units of such type on hydrolysis.

Few common types of carbohydrates are milk, bread, popcorn, potatoes, maze, etc.

The carbohydrates are largely distributed in both plant and animal tissues. Carbohydrates occur mainly in the form of glycogen and glucose in animal cells and as cellulose and starch in the plant cells.

Classification of carbohydrates

Generally, carbohydrates are classified into three major groups. They are as follows: 

1. Monosaccharides

Monosaccharides are simple sugars. They cannot be hydrolyzed into a simpler form. The simplest carbohydrates are the three-carbon dihydroxyacetone and trioses glyceraldehyde. They are further classified into glucose, fructose, galactose, and mannose.

Glucose

Glucose can be seen generally in the fruit juices and formed in the body by hydrolysis of cane sugar, starch, lactose, and maltose. Glucose is said to be the sugar of the body. Glucose structure can be depicted in the form of a ring or chain. It is found in blood, fruits, honey and under abnormal conditions, in urine.

Fructose

Fructose can be seen naturally in honey, tomatoes, and apples. Hydrolysis of cane sugar in the body can also give up fructose. C₆H₁₂O₆ is the molecular formula for fructose. Generally, fructose is the sweetest monosaccharide and is prepared by sucrose hydrolysis.

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Galactose

An element of glycoproteins and glycolipids is galactose. It is produced in the mammary glands and hydrolyzed to make the lactose of milk.

Mannose

On the hydrolysis of plant gums and mannosans, mannose is obtained. A constituent of the prosthetic polysaccharide of albumins, mucoproteins, and globulins is a mannose. Hexoses and pentoses exist in both ring and open chain forms.

According to the number of carbon atoms they possess, simple sugars might be further divided into tetroses, trioses, hexoses or heptoses, pentoses and as ketoses or aldoses based on whether the ketone or aldehyde groups are present.

For example, let us look at the below table:

1. Trioses

Trioses are formed throughout the metabolic breakdown of the hexoses in the body. Example: dihydroxyacetone and glyceraldehydes.

2. Pentoses

Pentoses are vital constituents of many coenzymes and nucleic acids. They are also formed as transitional throughout certain metabolic processes. Example: nucleic acids and coenzymes NAD, Ribose that is a structural element of ATP and flavor proteins: Arabinose, ribulose, and xylose.

2. Disaccharides

Disaccharides are comprised of two monosaccharides connected by a glycosidic linkage (C-O-C). Cn (H₂O)_n-1is the general formula for disaccharides. The most common disaccharides forms are lactose, sucrose, and maltose.

Maltose

Maltose is formed as a transitional product of the action of amylases on starch and it contains two glucose residues in 1, 4 linkages. It can be seen in a detectable amount in many germinating tissues and seeds where starch is being broken down.

Lactose

Lactose can be found in milk. On hydrolysis, it produces D-galactose and D-glucose.it is a reducing disaccharide, as it has a free anomeric carbon on the glucose residue.

Sucrose

Cane sugar or sucrose is a disaccharide of fructose and glucose. The hydrolysis of sucrose to D-glucose and D-fructose is often known as inversion as it is accompanied by a net change in optical rotation from dextro to levo as the equimolar mixture of fructose and glucose is formed and this mixture is known as invert sugar. Certain enzymes like invertases catalyze this reaction. Sucrose is tremendously abundant in plants and is commonly known as table sugar.

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Trehalose

Trehalose possesses two D-glucose residues and it is a non-reducing disaccharide like that of sucrose. It is the main sugar that can be seen in many of the insects' hemolymph.

3. Trisaccharides

Plenty of trisaccharides occur free in nature. Raffinose can be seen abundantly in many higher plants and sugar beets. Melezitose can be seen in the sap of a few coniferous trees.

4. Polysaccharides

Majority of the carbohydrates that can be found in nature take place as polysaccharides of high molecular weight. Polysaccharides are complex carbohydrates that are formed by the method of polymerization of a huge number of monosaccharide monomers. The other name for polysaccharides is also known as glycans.

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They are lengthy which might be unbranched or branched. After the completion of hydrolysis with specific enzymes and acid, the polysaccharides give up simple monosaccharide derivatives and/ or monosaccharides. Depending upon the composition, polysaccharides can be classified into two types: Homopolysaccharides and heteropolysaccharides.

1. Homoglycans or homopolysaccharides are the complex carbohydrates that are formed by the method of polymerization of only one type of monosaccharide monomer. Example: glycogen, starch, and cellulose are composed of a single type of monosaccharide known as glucose.

On the basis of monosaccharide unit, the polysaccharide is known as glucan which is made of glucose, xylan which is having xylose, fructan that is made of fructose, galactan that is formed of galactose, Araban that is made of arabinose, etc.

2. Heteroglycans or heteropolysaccharides are complex carbohydrates that are made by the process of condensation of either one type of monosaccharide monomer or monosaccharide derivatives.

Example: agar, chitin, arabinogalactans, peptidoglycan, arabinoxylans, etc.

Examples of Carbohydrates

A few examples of Carbohydrates across different product types are given below.

• Fruits - Whole fruit, fruit juice

• Legumus - Beans, plant-based proteins

• Dairy Products - Ice cream, yogurt, milk

• Starchy Vegetables - Potatoes, corn

• Grain - Wheat, cereal, rice