Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is a non-reducing sugar? (a) glucose (b) sucrose (c) fructose (d) arabinose

Step-by-Step Solution

Answer

Sucrose is the non-reducing sugar.

Step 1: Understand Reducing and Non-Reducing Sugars

Reducing sugars are sugars that have a free aldehyde or ketone group, which allows them to act as reducing agents. Non-reducing sugars do not have this free aldehyde or ketone group.

Step 2: Examine Glucose

Glucose is a monosaccharide with a free aldehyde group, which allows it to act as a reducing sugar. Therefore, glucose is a reducing sugar.

Step 3: Examine Sucrose

Sucrose is a disaccharide composed of glucose and fructose linked by a glycosidic bond. This bond prevents the aldehyde group of glucose and the ketone group of fructose from being free, making sucrose a non-reducing sugar.

Step 4: Examine Fructose

Fructose, like glucose, is a monosaccharide. It contains a ketone group that can act as a reducing agent when its molecular structure opens up. Thus, fructose is a reducing sugar.

Step 5: Examine Arabinose

Arabinose is a monosaccharide with a free aldehyde group. This characteristic makes it capable of acting as a reducing sugar.

Step 6: Determine the Answer

After examining the options, we see that sucrose is the only non-reducing sugar among them, as it does not have a free aldehyde or ketone group due to its glycosidic bond.

Key Concepts

Reducing Sugars

Reducing sugars play a crucial role in biochemical reactions because they contain a free aldehyde or ketone group. This characteristic allows them to act as reducing agents, meaning they can donate electrons to other molecules. Some common reducing sugars include:

• Glucose
• Fructose
• Galactose
• Lactose
• Maltose

These sugars are essential in reactions such as the Benedict's test, which is used to detect the presence of reducing sugars in food and biological samples.
When heated with Benedict's solution, reducing sugars will typically change the solution's color, indicating their presence.
This property is particularly important in metabolic pathways, as well as in various food industry applications.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates, consisting of single sugar molecules.
They are fundamental units of important carbohydrates such as disaccharides and polysaccharides. Common examples of monosaccharides include:

• Glucose
• Fructose
• Galactose
• Ribose
• Arabinose

These sugars can often be classified based on the number of carbon atoms they have, such as triose, tetrose, pentose, or hexose.
In additional categorization, they may be called aldoses or ketoses based on the presence of either an aldehyde or a ketone group respectively.
Monosaccharides are incredibly important for energy production, as glucose is a primary source of energy for cells through glycolysis.
They also serve as building blocks for more complex sugars.

Disaccharides

Disaccharides are carbohydrates formed by the combination of two monosaccharide molecules. They are a significant component of our diet and have a variety of roles in biological processes. Notable disaccharides include:

• Sucrose
• Lactose
• Maltose

While sucrose is commonly found in table sugar, lactose is the sugar present in milk, and maltose is found in malted foods.
Disaccharides can also serve as an energy source when broken down into their constituent monosaccharides.
This breakdown process occurs with the help of specific enzymes, which cleave the glycosidic bonds connecting the sugar units.
In some cases, such as with lactose, insufficient breakdown due to enzyme deficiency (lactase) causes lactose intolerance.

Glycosidic Bond

A glycosidic bond is a type of covalent bond that links sugar molecules together, forming larger carbohydrate structures like disaccharides and polysaccharides.
It is formed through a dehydration synthesis reaction, where a water molecule is removed as two sugar molecules join. In the structure of sucrose, a glycosidic bond connects glucose and fructose, making sucrose a disaccharide.
This bond prevents the free aldehyde or ketone groups of glucose and fructose from participating as reducing agents, classifying sucrose as a non-reducing sugar.
Glycosidic bonds can occur in two forms based on the orientation - 1-E and 2-2. This refers to which carbon atoms of the sugars are connected.
Understanding glycosidic bonds is essential for studying carbohydrate metabolism and functions, as these bonds can influence the digestibility and energy yields of carbohydrates.