Glucose and fructose are two of the most important simple sugars we study in chemistry and biology. They are the building blocks of many carbonates we eat every day, like fruits, sweets, and even bread. Learning their structure helps understanding how these sugars work in our body and how they are different from each other. For exams and practicals, and knowing then open-chain and in ring forms of glucose and fructose is very helpful. This topic is often asked in board exams, NEET, and JEE, so having a clearer idea of their structure is a must.

What are Glucose and Fructose?

Glucose and fructose are simple sugars, also called monosaccharides. They both have the same chemical formula C6H12O6 but their structures are different. Glucose is an aldose sugar because it has an aldehyde group, while fructose is a ketone sugar as it contains a ketone group. These sugars are naturally found in fruits, honey, and many other foods. They give energy to our body and are important in many biological processes. Understanding their basic nature is the first step to learning their structure.

Chemistry Notes Free PDFs

Glucose and fructose both have the molecular formula C6H12O6, meaning they contain the same number of carbon, hydrogen, and oxygen atoms. But the key difference lies in their functional groups:

• Glucose has an aldehyde group (CHO), so it’s called an aldose
• Fructose has a ketone group (C=O), so it’s known as a ketose.

This difference affects their structure and how they react in chemical tests. 

Structure of Glucose

Glucose has molecular formula C6H12O6. It contains an aldehyde functional group and six carbon atoms. Therefore it is known as an aldohexose. Glucose has two structures- open chain structure and ring structure (cyclic structure). The process of production of glucose is known as glycogenesis, whereas the process of breakage of glucose is known as glycolysis.

Let us now study how to draw the structure of glucose.

Steps to Draw the Open Chain Structure of Glucose:

To draw an open chain structure or acyclic structure of glucose, we need to follow these steps-

Step I- Draw 6 carbon atoms bonded to each other by a single bond in a straight chain. 

Step II- Extend the remaining two bonds of four carbon atoms in the middle and attach hydrogen to one side of these carbon atoms.

Step III- For the remaining four bonds of middle carbons, attach hydroxyl groups.

Step IV- For the remaining two carbon atoms, attach two hydrogens and hydrogen to one terminal carbon.

Step V- For another terminal carbon, attach an aldehyde to the carbon atom. 

Steps to Draw Ring Structure of Glucose:

In order to draw a ring structure or the cyclic structure of glucose, follow undermentioned steps-

Step I- Draw a hexagon because glucose is formed of 6 carbon atoms. 

Step II- At five vertices, put one carbon atom at each vertical. 

Step III- On the remaining one vertice, put an oxygen atom. 

Step IV- To the four carbon atoms, now attach hydrogen and hydroxyl groups, respectively.

Step V- To the remaining one carbon, attach a carbon atom bonded with hydrogen and hydroxyl groups, respectively. 

Pyranose Structure of Glucose

Pyranose means a monosaccharide in the form of a cyclic hemiacetal containing a pyran ring. Glucose is mostly found in pyranose form. The pyranose structure of glucose contains a six-membered ring. The ring structure is formed by 5 carbons and an oxygen atom. There are no double bonds present in this structure of glucose.

Haworth Structure of Glucose

Haworth structure proposes a structure containing a pyran ring. The two anomeric forms, α and β of D (+) glucose have been assigned pyranose ring structures by Haworth. Pyran is a six-membered heterocyclic ring containing an oxygen atom. For glucose, the two cyclic structures are -

Structure of Fructose

Fructose has a molecular formula C6H12O6. It is made up of 6 carbon-membered polyhydroxy ketones. It is found in fruits, honey, flowers and berries. It has two structures- an open chain structure and a ring structure (cyclic structure).

Ring and Open Chain Structure of Fructose

The ring structure of fructose is in the shape of a pentagon. The ring is made up of four carbon and oxygen atoms. The other two carbon atoms are attached to carbons bonded with oxygen atoms.

The open chain structure, also known as the fischer projection, is formed by 6 carbon-membered straight chains.

Haworth Structure of Fructose

For fructose, Haworth proposed furanose ring structures for the two anomeric forms of D (-) fructose. Furan is a structure containing a five-membered heterocyclic ring with an oxygen atom. It forms the basis of the two cyclic structures of D (-) fructose known as α – D (-) fructofuranose and β – D (-) fructofuranose.

Difference Between Glucose and Fructose

Isomers in Glucose and Fructose

Glucose and fructose show functional isomerism. Both the compounds have the same molecular formula, i.e. C6H12O6, but there are functional group differences that can be observed in the structure of the two compounds. Glucose contains an aldehyde group, whereas fructose contains a ketone group. 

Importance in Biochemistry

Glucose and fructose play a major role in how our body gets and uses energy. Glucose is the main source of energy for our cells. It is used in processes like glycolysis, where it is broken down to release energy. Fructose, though not the primary energy source, is also converted into useful forms in the body.

These sugars are also important in food science and health. For example, glucose is used in energy drinks, while fructose adds sweetness to fruits and processes foods. Understanding their role helps in topics like metabolism, digestion, and even in managing health issues like diabetes.

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