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Structure of Glucose and Fructose

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Glucose vs Fructose

Glucose is defined as a monosaccharide and is found in all the primary carbohydrates such as in the table sugar starch. Glucose is also known as grape sugar or blood sugar, and it is represented as a six-membered ring that forms a pyranose ring structure. Glucose is aldohexose and is the primary and the most preferred energy source of the body. It is found in starch.


Fructose is defined as a monosaccharide and is found in fruits and vegetables. In fructose, the glycemic index is lower as compared to glucose. Compared to Glucose, the binding fructose to cellular protein is seven times faster. It is also referred to as D- fructose or fruit sugar and its functional group are known as ketone. Glucose is known to be primarily metabolized in the liver and is not found in starch.

 

Structure of Glucose

Glucose is defined as a group of carbohydrates, a simple sugar having a chemical formula C6H12O6. It is composed of six carbon atoms, including an aldehyde group. Thus, we can refer to this as aldohexose. It exists in two forms, which are either in the open-chain (acyclic) form or ring (cyclic) form. The major source of energy needed for living organisms is given as glucose. Algae and plants prepare glucose during the photosynthesis process with the help of water, carbon dioxide, and sunlight. It is naturally found in honey and fruits. The glycogenolysis process obtains glucose present in animals.


How to draw an Open-Chain Structure of the Glucose Molecule

The required steps to draw an acyclic form of glucose are:


Step 1: Draw six carbon atoms.


Step 2: Draw extended arms for all the carbon atoms.


Step 3: Draw a hydrogen atom to carbon bond in such a way that four will be on one side, one on the other side


Step 4: Fill up the rest of the spaces with an OH group. 


Step 5: Finally, complete both the ends with two single-bonded hydrogen bonds with one double-bonded carbon.


Dry fructose looks like a white coloured crystalline solid, which is odourless and sweet. Fructose is soluble in ether, in water, and also in alcohol.

 

Baeyer showed the open-chain structure of the glucose compound. However, these structures have faced difficulties in explaining why glucose fails to react with Schiff base, sodium bisulphate or the mutarotation process. Haworth introduced the cyclic structure of glucose that confirms the existence of alpha and beta forms of mutarotation, glucose, and more.

 

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Steps to draw the Ring Structure of a Glucose Molecule

Follow the below steps to draw a cyclic form of glucose.

 

Step 1: Firstly, construct a hexagon

 

Step 2: Then, draw carbon atoms at five consecutive edges.

 

Step 3: Thereafter, attach an oxygen atom at the left out edge.

 

Step 4: Now, attach the four carbon atoms with OH and H groups.

 

Step 5: Complete the entire structure by attaching the left out carbon atom to two hydrogen atoms, one carbon atom and OH group.

 

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Glucose Properties

Glucose Chemical Formula

C6H12O6

Glucose Molar mass

180.156 g/mol

IUPAC Name

D-Glucose

 

Fructose Ring Structure

Fructose is described as a monosaccharide which is given as a simple sugar having a chemical formula C6H12O6. It is also known as fruit sugar and was discovered in 1847 by a French chemist named Augustin-Pierre Dubrunfaut. It is composed of a 6-carbon polyhydroxy ketone.

 

Crystalline fructose takes place as a cyclic six-membered structure, which is delinquent to the stability of the internal hydrogen bonding and hemiketal. This form is known as D-fructopyranose. It primarily takes place in vine fruits, honey, most root vegetables, flowers, and berries. It can be obtained Commercially from maize, sugar beets, and sugar cane.

 

Open Chain Structure and Ring Structure of a Fructose Molecule

The cyclic structure of fructose is given below:

 

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The Fischer projections of D-fructose and L-fructose can be given as follows:

 

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Fructose Properties

Dry fructose appears as a white coloured crystalline solid, which is odourless and sweet. It is soluble in ether, water, and alcohol.

 

Properties of Fructose

Fructose Molar mass

180.156 g/mol

Fructose Formula

C6H12O6

Density

1.694 g/cm3

Fructose Chemical Formula

C6H12O6

Melting point

103°C

 

The Difference between the Glucose and Fructose Compounds

Known as constitutional isomers, which means that there is a difference in the bond connectivity. Glucose is an aldehyde, and fructose is a ketone. When they become cyclized by the formation of hemiketal/hemiacetal, glucose becomes a 6-ring sugar and on the other side, fructose becomes a five-ring sugar. Glucose is the most preferred form of energy of the body and every cell in the body is capable of metabolizing the glucose compound. AS far as Fructose is concerned, only the Liver is capable of metabolizing fructose. This is the reason why Fructose intake should be restricted.

 

D-Glucose Chemistry

In the glucose molecule, a hydrogen and oxygen atom group is bonded to the carbon atom. On the other side of the glucose molecule, there exists a double-bonded oxygen atom. Looking at the D-glucose’s Fisher model with the double-bonded oxygen atom, which is pointed down, the hydrogen and oxygen group present at the top of the atom points towards the right.

 

L-Glucose Chemistry

D-glucose and L-glucose are composed of similar atoms. The only difference between these two structures can be displayed using the Fisher model. In the Fisher model, unlike D-glucose, the hydrogen and oxygen group of atoms present in L-glucose points to the left. If these two specific molecules faced each other, they would look like a reflection of one another.

FAQs on Structure of Glucose and Fructose

1. What is the fundamental difference between the structures of glucose and fructose?

The main structural difference lies in their functional groups. Glucose is an aldohexose, which means it has an aldehyde group (-CHO) on its first carbon atom. In contrast, fructose is a ketohexose, featuring a ketone group (C=O) on its second carbon atom. This difference is why they have unique properties, despite sharing the same chemical formula.

2. Why are both glucose and fructose classified as monosaccharides?

They are called monosaccharides (from 'mono' meaning one and 'saccharide' meaning sugar) because they represent the simplest form of carbohydrates. They consist of a single sugar unit and cannot be broken down into smaller sugars. They act as the essential building blocks for more complex carbohydrates like sucrose and starch.

3. How does the ring structure of fructose typically differ from that of glucose?

In aqueous solutions, glucose usually forms a stable six-membered ring called a pyranose ring. Fructose, however, more commonly forms a five-membered ring known as a furanose ring. So, while both are six-carbon sugars, their preferred stable ring structures are different in size.

4. What are the open-chain and cyclic forms of these sugars?

Both glucose and fructose can exist in two main forms:

  • The open-chain form is a straight carbon chain structure, which clearly shows the aldehyde or ketone functional group.
  • The cyclic form is a ring structure that is formed in solutions because it is more chemically stable. This is the most common form found in biological systems.

5. What are anomers of glucose? Can you give an example?

Anomers are specific types of isomers that differ only in the arrangement of the hydroxyl group (-OH) on the first carbon atom (the anomeric carbon) after the sugar forms a ring. For glucose, the two anomers are:

  • α-D-glucose: The -OH group on the first carbon points down.
  • β-D-glucose: The -OH group on the first carbon points up.

6. Why is it important to understand the different structures of glucose and fructose?

Understanding their structures is crucial because structure dictates function. For instance, the slight difference in their structure affects how they are processed and used for energy in the body. This knowledge is vital in fields like biology and nutrition, especially for topics related to metabolism and the sweetness of foods.

7. Even with the same formula (C₆H₁₂O₆), why are glucose and fructose different sugars?

Although they share the same chemical formula, they are considered different sugars because they are structural isomers. This means their atoms are connected in a different order. The position of the carbonyl group (aldehyde in glucose vs. ketone in fructose) fundamentally changes their chemical and physical properties, including how they react and how sweet they taste.