Classification; Aldoses and Ketoses

Introduction

Carbohydrates are organic compounds which consist of carbon, hydrogen and oxygen, usually with a hydrogen:oxygen atom ratio of 2:1. They are naturally occurring carbon compounds that also contain several hydroxyl groups. Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that can be hydrolyzed to them. Carbonyl compounds of carbohydrates include ketones and aldehydes. Carbohydrates are considered as the most abundant organic molecules in nature. They are also referred to as saccharides. Carbohydrates are technically hydrates of carbon. The empirical formula is Cn(H2O)n. Carbohydrates are also considered as the most abundant biomolecules in terms of mass in our body.

Sources of Carbohydrates

Plants are the main sources of carbohydrate. Some of the other sources of carbohydrate are as follows:

• Grain products

The leading source of carbohydrate includes green products in our diet. They mainly have a high concentration of starch which breaks down into sugar and acts as a source of carbohydrate in our body. Common grains include wheat, oats, rice and barley.

• Starchy vegetables

Starchy vegetables include potatoes and corn. They have high levels of complex carbohydrates which are converted into sugar in our body.

• Fruits

Almost all the fruits that we eat contain carbohydrates in the form of natural sugars.

• Beverages

Among beverages, milk is one of the important sources of carbohydrate that is derived from animals.

• Sweets and added sugars

Sweets are also a source of carbohydrate. Carbohydrates are added to many processed foods.

Classification of Carbohydrates

Carbohydrates are classified as Monosaccharides, Disaccharides and Polysaccharides.

Monosaccharides

Monosaccharides are the simplest form of sugar. They cannot be hydrolysed into smaller units. They are also called simple sugars. They possess a free aldehyde or Ketone and two or more hydroxyl groups. Monosaccharides having an aldehyde group as the carbonyl group are called aldoses. Monosaccharides having a Ketone group as the carbonyl group are called ketoses.

Disaccharides

Disaccharides are the form of carbohydrates that are composed of two monosaccharides. Disaccharides can be of two types- reducing disaccharides and non reducing disaccharides.

Polysaccharides

Polysaccharides are the form of carbohydrates that contain more than ten monosaccharides bonded together by glycosidic bonds. They are also called glycans. They are a long chain of monosaccharides.

Aldoses

Monosaccharides having an aldehyde group as the carbonyl group are called aldoses. Their formula is the same as most carbohydrates, Cn(H2O)n.

Structure of Aldoses

• They have an asymmetrical carbon center that is why all aldoses exhibit stereoisomerism.
• Depending on the chirality of the asymmetric carbon, they can exist in either L-form or D-form.
• The determination of the D-form or L-form is done on the chirality of the asymmetric carbon furthest from the aldehyde end.
• The simplest possible aldose is the triose glyceraldehyde. It only contains three carbon atoms.
• Usually formaldehyde and glycolaldehyde are not considered as carbohydrates, hence we cannot consider them as aldose.
• Examples of aldoses are – glyceraldehyde, ribose, glucose, erythrose and galactose.

Nomenclature of aldoses

The number of carbons in the main chain help us to differentiate different aldoses. The only aldotriose is glyceraldehyde.

Glyceraldehyde

• Glyceraldehyde is commonly known as glyceral. It is a triose monosaccharide.
• It has C3H6O3 as a chemical formula and it is the simplest form of aldose.
• It occurs as a sweet, colourless and crystalline form in nature.
• It is an aldotriose which comprises propanal and have hydroxyl groups at the 2- and 3- positions.
• Glyceraldehyde plays an important role in the formation of advanced glycation end-products.

Erythrose

• Erythrose is a part of the aldose family and it is a tetrose saccharide which has one aldehyde group.
• It has a chemical formula C4H8O4.
• The natural isomer of erythrose is D-erythrose.

Ribose

• Ribose is a simple sugar which is included under monosaccharide. Its molecular formula is C5H10O5.
• Ribose has five carbons hence they are called pentose sugar. It is one of the most important pentoses present in living organisms.
• It is sweet in taste and a polar compound that dissolves in water.
• Ribose is not present in free form in nature. They are found along with sugar molecules.

Glucose

• Glucose is a simple sugar. It has six carbon atoms with one aldehyde group. It has a chemical formula of C6H12O6.
• Glucose is known as aldohexose and dextrose.
• Glucose can be present in two forms: ring or open-chain structure.
• In plants it is present in fruits and in animals it is synthesized in the Kidneys and liver.

Ketose

• Monosaccharide containing one Ketone group per molecule is known as ketose.
• Dihydroxyacetone is the simplest form of ketone, containing only three carbon atoms and it is the only ketose with no optical activity.
• Ketoses are generally classified on the basis of the number of carbons in the main chain.
• The examples of ketoses are: dihydroxyacetone, erythrulose, ribulose, xylulose, fructose, sorbose, nanose.

Dihydroxyacetone

• It is a type of ketose which consists of acetone bearing hydroxy substituents.
• It can be obtained from glycerol through microbial fermentation.
• It is also known as DHA. It is the simplest ketone form of sugars and can be used in self-tanning products.

Ribulose

• Ribulose is an organic substance which occurs in all domains of life. It is colourless anion and involved in photosynthesis.
• It is a type of ketones containing sugar with five carbon atoms.

Fructose

• Fructose is the only naturally occurring ketohexose. It is a simple ketonic monosaccharide and is also known as fruit sugar.
• The formula of fructose is C6H12O6.
• It occurs as a white crystalline solid and is highly soluble when compared to other sugars.
• It has a cyclic structure and forms intramolecular hemiacetal. It also absorbs moisture quickly and hence becomes sticky.
• Fructose can be used in various aspects in our life. It is used in plastics, in food like nutrition bars and cookies. It is also used as a food for diabetics.

Xylulose

• Xylulose is a monosaccharide which contains five carbon atoms and includes a ketone functional group. Hence it is called ketopentose.
• It has the chemical formula C5H10O5.

Erythrulose

• It is a type of carbohydrate with one ketone group. It’s chemical formula is C4H8O4.
• It is a part of ketose family and can be used in self-tanning cosmetics.

Conclusion

Aldose and ketose are monosaccharides which can be differentiated very easily on the basis of the group they contain. Aldoses are the monosaccharides having an aldehyde group as the carbonyl group and their formula is the same as most of the carbohydrates. Aldoses have an asymmetrical carbon center and that is why all aldoses exhibit stereoisomerism depending on the chirality of the asymmetric carbon they exist in either L-form or D-form. Some of the common aldoses include glyceraldehyde, ribose, glucose, erythrose and galactose. Glyceraldehyde is a triose monosaccharide. It is the simplest form of aldose and occurs as a sweet, colourless and crystalline form in nature. Erythrose is a part of the aldose family and it is a tetrose saccharide which has one aldehyde group and the natural isomer of erythrose is D-erythrose. Ribose is a simple sugar which is included under monosaccharide and has five carbons hence they are called pentose sugar. It is one of the most important pentoses present in living organisms.

Similarly ketoses are also monosaccharides but they contain one Ketone group on every molecule. Dihydroxyacetone is the simplest type of ketose. Ketoses are generally classified on the basis of the number of carbons in the main chain. The examples of ketoses are dihydroxyacetone, erythrulose, ribulose, xylulose, fructose, sorbose, nanose. Dihydroxyacetone is a type of ketose which consists of acetone bearing hydroxyl substituents and can be obtained from glycerol through microbial fermentation. Fructose is the only naturally occurring ketohexose. It is a simple ketonic monosaccharide and is also known as fruit sugar. Xylulose is a monosaccharide which contains five carbon atoms and includes a ketone functional group. Hence it is called ketopentose.

Aldose and ketose differ from each other in various aspects, such as:

• Aldose consists of aldehyde group while ketose consists of ketone group.
• All those are mainly found in plants whereas ketose are mainly found in processed food items.
• Aldoses can isomerize into ketoses but the ketoses can isomerize into aldoses only in the presence of reducing sugars.
• The Seliwanoff’s test gives light pink of aldoses whereas it gives deep cherry red colour of ketoses.