Monosaccharides

Monosaccharides

Monosaccharides, often known as simple sugars, are the most basic carbohydrates; they are the monomers from which more complex carbs are built. Different types of monosaccharides contain molecular formulae that are multiples of the unit CH2O (from the Greek monos, single, and sacchar, sugar).

Sugar is either an aldose (aldehyde sugar) or a ketose (ketose sugar), depending on where the carbonyl group is located (ketone sugar). Monosaccharides examples such as glucose, for example, is an aldose, while fructose, a glucose isomer, is a ketose (Most sugar names end in -ose).

The size of the carbon skeleton, which ranges from three to seven carbons long, is another parameter for identifying sugars. Hexoses are sugars with six carbons, such as glucose and fructose.

Classification of Monosaccharides

The six-carbon monosaccharides glucose and fructose contain five hydroxyl groups, making them monosaccharides. Chiral centres are common among carbon-hydroxyl group bonds, contributing to the wide variety of sugar stereoisomers seen in nature.

Glucose, galactose and fructose are the three monosaccharides. One asymmetric carbon makes all the difference between glucose and galactose, and that’s all.

Monosaccharides with four, five, six, or seven carbon atoms in their backbones are tetroses, pentoses, hexoses, and heptoses, respectively.

Those having aldehyde groups are called aldoses(like glucose) and those have ketone group are called ketosis (like fructose).

Properties of Monosaccharides

• Colourless, crystalline monosaccharides are readily soluble in water but insoluble in non-polar solvents. It’s safe to say that most of them taste sweet

• Carbon atoms in the backbones of typical monosaccharides are linked together by single-bond unbranched carbon chains

• In addition to the carbonyl group, one of the carbon atoms has been double-bonded to an oxygen atom to form a carbonyl chain

• When the carbonyl group is towards the end of the carbon chain, a monosaccharide is an aldose; otherwise, it is a ketose in an aldehyde group (in a ketone group)

• Basic monosaccharides include glyceraldehyde and dihydroxyacetone (both aldotetroses and ketotriose)

Glucose – C6H12O6

The six-carbon sugar D-glucose, often known as dextrose, is the most prevalent monosaccharide in nature. A carbonyl group (C=O) and several hydroxyl groups (—OH) are present in the molecule.

The word glucose is derived from the Greek glykys, which means “sweet”. Sugars, such as glucose, are more often used energy sources because they are easier to metabolise and store. Glycolysis is metabolising one molecule of glucose through a series of reactions.

Gluconeogenesis is the process of making glucose from noncarbohydrate precursors like pyruvate and lactic acid. Glycogen, the storage form of glucose, is the most prevalent homopolymer in mammalian cells.

Hemiacetal Structure

All monosaccharides with a backbone of five or more carbon atoms form cyclic (ring) structures in an aqueous solution. The carbonyl group creates a covalent bond with the oxygen of a hydroxyl group along the chain.

When alcohols react with aldehydes or ketones, they generate hemiacetals or hemiketals, including an additional asymmetric carbon atom. Therefore, they may exist in two stereoisomeric forms, resulting in these ring configurations.

D-glucose exists in solution as an intramolecular hemiacetal, arising from the reaction of the free hydroxyl group at C-5 with the aldehydic C-1.

On the other hand, Fischer projections show that the hydroxyl groups of both anomeric and farthest-chiral centres are on opposing sides, which contradicts what we observe when we compare them side by side. These six-membered ring compounds are referred to as pyranoses because they are structurally similar to the pyran compound. D-glucose exists in two ring configurations: alpha-D-glucopyranose and beta-D-glucopyranose.

Fructose

C6H12O6

Fructose is a ketohexose, which is an isomer of glucose. Fruits are one source of this sugar derived from the Latin fructus, which means “fruit.”

The hydroxyl group at C-5 (or C-6) combines with the keto group at C-2 to generate a furanose (or pyranose) ring with a hemiketal connection in these molecules. D-fructose readily forms the furanose ring. 

Fructose is the sweetest carbohydrate found in nature. It might be considered either natural or added sugar, depending on the source. It is considered a natural sugar when we consume it straight from whole plant meals. However, when we consume it through packaged foods and beverages that have had fructose-containing sugars (such as crystalline fructose, high fructose corn syrup, or sucrose) added during the production process, it is termed an added sugar.

Difference between Glucose and Fructose

Conclusion

The simplest unit of carbohydrates is the monosaccharide. They’re made up of carbon, hydrogen, and oxygen atoms and can’t be broken down any further because they’re already at their most basic state. Simple monosaccharide carbohydrates like glucose and fructose are absorbed straight into the circulation. Glucose is either used for energy or stored in the form of glycogen. Fructose is either converted to glucose or stored in the form of fat.