Different Structural Forms of Glucose

Glucose is the most common sugar; a carbohydrate that can be used by any living organism on Earth. It is easily available commercially, too. 

Glucose is a monosaccharide that can conjoin multiple times with other compounds to form a polysaccharide. It is a hexose molecule because it has six carbon atoms. We picture the glucose molecule as a linear chain, but there are different structural forms of glucose. Glucose’s different structural forms mean it has different rotations and confirmations.

Glucose molecule 

• Glucose is a monosaccharide, meaning it has only one monomer unit of sugar molecule.

• It is a 6-carbon compound, and so it is a hexose type of monosaccharide when the monosaccharides are classified based on the number of carbon atoms present. 

• Other examples of hexoses are fructose, mannose, etc. 

• The chemical molecular formula of the glucose molecule is C6H12O6.

• It contains an aldehyde (-CHO) group present on the first carbon (C1). Thus, it is an aldose type when the monosaccharides are classified according to the nature of the functional group.

• Glucose is synthesised in living organisms by breaking down the ingested carbohydrates through the process of glycolysis within the cell. 

• As a consequence of metabolising glucose, the molecule is either stored as glycogen in the body or used up as energy in the form of adenosine triphosphate (ATP).

• The production of glycogen from glucose takes place through a process of glycogenesis.

• The liberation of energy in the form of ATP happens by breaking down glucose through the process of the citric acid cycle or fermentation.

Glucose structure

• Glucose is a sugar-containing six carbon atoms and an aldehyde as its functional group. 

• When the carbohydrate holds a ketone group (-CO) as a functional group, it is called a ketose.

• For example, the hexose monosaccharide fructose is a ketose containing a ketone functional group at the second carbon of the 6-carbon chain.

• The aldehyde containing glucose can convert into a ketone containing fructose because they are isomers. They have equal molecular formulas but differ in their structures and configurations.

• The carbons in the glucose molecules are linked by covalent bonds, which, on breaking, yield energy to an organism.

• The glucose molecule comprises 12 hydrogen and six oxygen atoms bound to the parent carbon chain.

• Generally, the glucose molecule is drawn as a linear chain of 6 carbon atoms for easy understanding.

• But there are different structural forms of glucose. This means that the glucose changes the nature of its chain, its optical rotation, and its confirmations under different conditions.

• Glucose mostly exists as a ring form mimicking a hexagon in the human body.

Different structural forms of glucose:

Fischer projection of glucose structure

• It shows a 2D structure of a linear glucose chain.

• The above structures show two different forms of glucose structure. 

• The D-glucose is dextro glucose, which has an optical rotation towards the right side, meaning it is dextrorotatory.

• The dextro glucose structure turns plane-polarised light in a clockwise direction.

• The L-glucose is levo glucose, which has an optical rotation towards the left side, meaning it is levorotatory.

• The levo glucose twists plane-polarised light in an anti-clockwise direction.

• L and D glucose are mirror images of each other.

• Since they are mirror images of each other, they are called enantiomers. The levo form is the enantiomer of the dextro form of glucose.

Haworth projection of glucose structure

• It demonstrates a 3D structure of glucose molecules in a hexagonal ring format.

• The hexagonal ring of glucose is formed by the interaction between the oxygen atom of the aldehyde group at the C1 and C5 atoms of the molecule.

• The ring form of glucose is chemically named glucopyranose as it has a pyranose ring.

• Based on the optical rotation of the cyclic glucose, the molecule is termed dextro or levo glucose.

• Also, based on the direction of the functional group, the glucose molecule is given prefixes as alpha or beta.

Chair conformation of glucose structure

• It is the most accurate form to represent the structure of cyclohexane. 

• It is also termed ‘boat conformation’ due to the shape of the molecules.

• It clearly shows the positions and tilt of the atoms in the glucose molecules.

• The above structure shows the equatorial arrangement of the atoms, meaning the atoms are seen in the equatorial plane (horizontally).

• When the atoms are seen in a vertical plane, the glucose molecule is structured in the axial configuration.

Conclusion

Glucose is a monosaccharide that is essential for living organisms to produce and store energy for metabolism. It is an aldose containing six carbons with an aldehyde functional group at the first carbon atom. 

Glucose is expressed as a linear chain form. Apart from this, there are different structural forms of glucose. An example of the different forms of glucose structure are the dextro and levo forms, which are enantiomers, as they are mirror images of each other. Dextro glucose is also called dextrose, which is generally more abundant in nature than the levo form.