Question

Which of the following monosaccharides are present as five membered cyclic structure (furanose structure)? (A) Ribose (B) Glucose (C) Fructose (D) Galactose

Short answer

The monosaccharides present as five-membered cyclic furanose structures are (A) Ribose and (C) Fructose.

Step-by-step solution

Understand furanose and pyranose cyclic structures

Monosaccharides can form cyclic structures when the carbonyl group (C=O) reacts with a hydroxyl group (-OH) on another carbon atom in the same molecule. This reaction can create hemiacetals or hemiketals. A furanose structure is a five-membered cyclic structure formed from a four-carbon chain with an oxygen atom at one of the vertices. A pyranose structure is a six-membered cyclic structure formed from a five-carbon chain with an oxygen atom at one of the vertices.

Examine ribose

Ribose is a pentose monosaccharide with the formula C5H10O5. The linear structure of ribose is as follows: *(CHOH)4-CH=O* The most stable cyclic structure of ribose is formed when the aldehyde group (C=O) at carbon 1 reacts with the hydroxyl group (-OH) at carbon 4. This reaction forms a five-membered ring consisting of four carbon atoms and an oxygen atom, which is a furanose structure.

Examine glucose

Glucose is a hexose monosaccharide with the formula C6H12O6. The linear structure of glucose is as follows: *CH=O-(CHOH)4-CH2OH* The most stable cyclic structure of glucose is formed when the aldehyde group (C=O) at carbon 1 reacts with the hydroxyl group (-OH) at carbon 5. This reaction forms a six-membered ring consisting of five carbon atoms and an oxygen atom, which is a pyranose structure.

Examine fructose

Fructose is a hexose monosaccharide with the formula C6H12O6. The linear structure of fructose is as follows: *CH2OH-C(=O)C-(CHOH)3-CH2OH* The most stable cyclic structure of fructose is formed when the ketone group (C=O) at carbon 2 reacts with the hydroxyl group (-OH) at carbon 5. This reaction forms a five-membered ring consisting of four carbon atoms and an oxygen atom, which is a furanose structure.

Examine galactose

Galactose is a hexose monosaccharide with the formula C6H12O6. The linear structure of galactose is as follows: *CH=O-CHOH-CHOH-CH(OH)3-CH2OH* The most stable cyclic structure of galactose is formed when the aldehyde group (C=O) at carbon 1 reacts with the hydroxyl group (-OH) at carbon 4. This reaction forms a six-membered ring consisting of five carbon atoms and an oxygen atom, which is a pyranose structure.

Conclusion

Based on the analysis of the cyclic structures of ribose, glucose, fructose, and galactose, we can conclude that ribose and fructose are present as five-membered cyclic furanose structures. So, the correct answer is (A) Ribose and (C) Fructose.

Key concepts

Monosaccharides

Monosaccharides are the simplest form of carbohydrates and are fundamental building blocks for more complex sugars. They are single sugar molecules, mostly composed of carbon, hydrogen, and oxygen, with the general formula \ \(C_n(H_2O)_n\).

Monosaccharides can vary in the number of carbon atoms they contain, with some common examples being:

• Trioses: 3 carbons
• Tetroses: 4 carbons
• Pentoses: 5 carbons, like ribose
• Hexoses: 6 carbons, like glucose and fructose

Their structures can be either linear or cyclic, depending on the chemical reactions they undergo. Understanding these simple sugars is crucial for studying more complex carbohydrate structures, as they are the building blocks for polysaccharides and other important biomolecules.

Cyclic Structures

When it comes to monosaccharides, they often do not remain in their open-chain forms for long. Instead, they can form cyclic structures which are more stable.

This occurs when a hydroxyl group on the sugar undergoes a reaction with the carbonyl group (either an aldehyde or ketone), creating a ring structure. These cyclic forms are prevalent in biological systems and play a key role in their function:

• Furanose: A five-membered ring, as seen in ribose.
• Pyranose: A six-membered ring, as seen in glucose.

The formation of these rings is essential for the functionality of many sugars within cells and are a key aspect of their metabolic activity.

Hemiacetals and Hemiketals

In the formation of cyclic structures from monosaccharides, hemiacetals and hemiketals play an important role. These are chemical structures formed when a carbon atom joins both an alcohol and an ether group.

In detail:

• A **hemiacetal** forms when an aldehyde reacts with an alcohol.
• A **hemiketal** forms when a ketone reacts with an alcohol.

In monosaccharides:

• Ribose and glucose form **hemiacetals**, as they have an aldehyde group.
• Fructose forms a **hemiketal**, as it has a ketone group.

These reactions are crucial for the formation of stable cyclic structures, enabling the monosaccharides to function efficiently in various biological processes.

Organic Chemistry

Organic chemistry is a vast field that covers the study of molecules containing carbon. It includes understanding the structural nuances and reactivity of many bonds and groups, including those found in carbohydrates.

In the context of monosaccharides, the focus is often on reactions that form cyclic structures, including interactions like:

• Carbonyl group reactions: Involve aldehydes and ketones prominently in glucose and fructose.
• Covalent bonds with hydroxyl groups: Important in creating cyclic structures.
• Stereochemistry: Determines the orientation of hydroxyl groups and affects the sugar’s overall function.

Grasping these concepts in organic chemistry enhances our understanding of how small sugar molecules behave, and they provide insight into more complex carbohydrate chemistry.

Pentose and Hexose Sugars

In the world of sugars, pentose and hexose sugars hold a significant place, often seen in biological molecules like nucleic acids and cellular structures.

**Pentose sugars**, like ribose, have five carbon atoms and are essential in nucleic acids, notably RNA. Ribose in its cyclic form is present as a furanose, which is crucial for its role as a sugar in the RNA backbone.

**Hexose sugars**, like glucose and fructose, have six carbon atoms. They are vital energy sources for organisms.

• Glucose primarily exists as a pyranose due to its six-membered ring.
• Fructose can form a furanose structure, enabling versatility in metabolic processes.

These sugars are fundamental not just as energy sources but also as structural components in many biomolecules, playing crucial roles in the biochemistry of living organisms.