CBSE Class 12 Chemistry Chapter 14: Biomolecules NCERT Solutions

Solution: Monosaccharides are the simplest forms of carbohydrates. They are defined as carbohydrates that cannot be further hydrolyzed to yield simpler units of polyhydroxy aldehydes or ketones. These fundamental units form the building blocks for more complex carbohydrates.

Monosaccharides are categorized based on two main criteria: the number of carbon atoms they contain and the type of functional group present. If the functional group is an aldehyde, the monosaccharide is called an aldose. If the functional group is a ketone, it is called a ketose. Further classification is based on the carbon count: trioses (3 carbons), tetroses (4 carbons), pentoses (5 carbons), hexoses (6 carbons), and heptoses (7 carbons). For instance, a carbohydrate with three carbon atoms and an aldehyde group is an aldotriose, while one with three carbon atoms and a keto group is a ketotriose.

Solution: Reducing sugars are a class of carbohydrates characterized by their ability to reduce certain oxidizing agents. Specifically, they can reduce Fehling's solution (a deep blue alkaline solution of copper(II) tartrate complex) and Tollen's reagent (an alkaline solution of ammoniacal silver nitrate). This reducing property arises from the presence of a free aldehyde group or a free keto group, or the ability to form such groups in alkaline solution.

It is important to note that all monosaccharides, whether aldoses or ketoses, are reducing sugars. Many disaccharides are also reducing sugars. However, some disaccharides, like sucrose, and all polysaccharides are typically non-reducing sugars because they lack a free aldehyde or keto group.

Solution: Carbohydrates play crucial roles in the life of plants. Two of their main functions are:

1. Energy Storage: Plants store excess glucose, produced during photosynthesis, in the form of starch. Starch acts as a reserve food material, which can be broken down to release energy when needed by the plant.
2. Structural Support: Cellulose, a complex polysaccharide, is a major structural component of plant cell walls. It provides rigidity and mechanical strength to the plant tissues, enabling them to maintain their shape and support the plant body.

Solution: Carbohydrates can be classified based on the number of monosaccharide units they contain. Monosaccharides are simple sugars that cannot be hydrolyzed further, while disaccharides are composed of two monosaccharide units linked together.

Based on this classification, the given compounds can be categorized as follows:

Monosaccharides: These are simple sugars.

Ribose

2-deoxyribose

Galactose

Fructose

Disaccharides: These are formed by the combination of two monosaccharide units.

Maltose (composed of two glucose units)

Lactose (composed of glucose and galactose units)

Solution: A glycosidic linkage, also known as a glycosidic bond, is a type of covalent bond that connects a carbohydrate molecule to another group. In the context of carbohydrates, it specifically refers to the linkage formed between two monosaccharide units through an oxygen atom. This bond is created via a dehydration reaction, where a molecule of water ($H\_2O$) is eliminated.

For example, in the disaccharide sucrose, the monosaccharide units of \alpha-glucose and \beta-fructose are joined together by a glycosidic linkage. The linkage involves the hydroxyl group of one monosaccharide and the hydroxyl group of another, forming an ether bridge (-C-O-C-) and releasing water.

The structure can be visualized as:

CH_2OH

HOH_2C ↓ UL Glycosidic OH, CH_2OH linkage H OH OH H ↓ - D - glucose - D - fructose

Surcose

Solution: Glycogen is a complex carbohydrate, specifically a polysaccharide, that serves as the primary form of glucose storage in animals, fungi, and some bacteria. It is often referred to as 'animal starch' due to its structural similarity to starch.

Starch, on the other hand, is the main storage polysaccharide found in plants. It is a mixture of two components: amylose (typically 15-20%) and amylopectin (typically 80-85%).

The key differences between glycogen and starch are:

• Location: Glycogen is found in animals and fungi, while starch is found in plants.
• Structure: Both glycogen and amylopectin (a component of starch) are branched polysaccharides. However, glycogen is generally more highly branched than amylopectin. The branching in glycogen occurs more frequently, leading to a more compact structure.
• Components: Starch is composed of two distinct polysaccharides (amylose and amylopectin), whereas glycogen is considered a single type of polysaccharide structurally similar to amylopectin.

Solution: Hydrolysis of disaccharides breaks them down into their constituent monosaccharide units. This process typically occurs in the presence of an acid catalyst or specific enzymes.

(i) Hydrolysis of Sucrose: Sucrose is a non-reducing disaccharide composed of \alpha-glucose and \beta-fructose units linked by a glycosidic bond. Upon hydrolysis, sucrose yields one molecule of glucose and one molecule of fructose. The reaction is represented as:

C_{12}H_{22}O_{11} \quad \xrightarrow{H^+ \text{ or enzyme}} \quad C_6H_{12}O_6 \quad + \quad C_6H_{12}O_6

Where the products are glucose and fructose.

(ii) Hydrolysis of Lactose: Lactose, also known as milk sugar, is a disaccharide composed of \beta-galactose and \alpha-glucose units linked by a \beta1→4 glycosidic bond. Hydrolysis of lactose yields one molecule of galactose and one molecule of glucose. The reaction is:

C_{12}H_{22}O_{11} \quad \xrightarrow{H^+ \text{ or enzyme}} \quad C_6H_{12}O_6 \quad + \quad C_6H_{12}O_6

Where the products are galactose and glucose.